Dizzies

[Forumadmin]

Ok I am now off on another path looking at advance curves and vacuum advance.

What got me thinking was this...

Now I am on the hunt for programmable distributor but first need to understand what I am doing.
Do modern petrols need more max advance than the pool petrol used when the DM2 40317 curve was chosen?
Does a balanced Jowett with ported heads that can rev up to 6000rpm need a different curve?
Does the vacuum advance need to come from the manifold or the port and how much vacuum advance should be added?

I do not understand the difference between a CW and a CCW distributor especially when the list http://www.starchak.ca/tech/pdfs/lucas.pdf shows a CCW 40795A 25D4 as a replacement for a CW 40317A DM2P4 . I would assume the weights would not operate correctly if you swapped.

I need to find out if the 25d4 body can be used because they are now remanufactured and cheap at £30.

[Forumadmin]

Right. The direction of all Jav/Jup dizzies is CCW so not sure why the list shows them as CW. I have 25D4 that I think I have used on the Jav. It is a 41419H and brand new for a 1275 Mini and others.

Its curve is 2500 10 1400 0 400 0 compared to 40795A 25D4 with 1200 10 600 6 350 1 and the 40317A DM2P4 with 1300 10 650 4.5 500 2. So no wonder I took it off ! There is no advance below 1400 revs when the original was already at max advance.

Searching through the list the MGB 41288A 25D4 with 1500 10 800 9 300 3 is fairly close. Interesting that other MGBs with slightly different model numbers have quite different curves. These may have been tuned engines.
The reason for going for the 25d4 is more is known about it . e.g. http://www.custompistols.com/cars/artic ... as25d4.htm

The search continues.

The Lucas 25D4 distributor was used in the MGB, and other, automobiles from 1962 until late 1974 . Each distributor, while retaining the basic model number, will also have a "specification number" stamped into the side to identify the exact application it was intended for. There is also a date code stamped under the specification number which gives the week and year of production (e.g. 5 68 or 49-72). The internals of all of this model distributor are interchangable. Hence, it is possible to have an early specification distributor with later specification internals. Thus, each distributor must be individually examined to determine whether it is suitable for the intended application.

A second problem area is the centrifugal advance mechanism. This is composed of two weights which swing outward as the speed of the distributor shaft increases and moves the ignition timing forward, thus advancing it. The weights are returned to a stationary position and their rate of advance controlled by two springs. These springs may be either identical or of different specification depending on the particular advance curve desired. Exact replacements for the original springs do not appear to be available to the general public from any known source. They may be made available to certain rebuilders, but this has not been verified. We do know, however, that rebuilt distributors are available, but have not been able to test them on a distributor test machine to see how well they perform against the original specifications.

Several distributors, believed to be fully original, have been purchased and/or examined and the markings on the centrifugal advance system noted. Further, the springs have been examined in an attempt to begin to locate suitable replacements. Not all of the distributors listed as being used in the MGB have been obtained. The ones found were taken from several running vehicles and found in an MG wrecking yard. They appear to be the ones most commonly encountered in the US. The following information has been obtained:

--------------------------------------------------------------------------------
40897A 25D4 CCW 1500 10 450 4.5 300 1.5 200 1962 MG
Distributor Specification Number 40897 (11-64)

Weights: Not marked
Cam Arm: 10 degrees

Springs: Number 1 Number 2
Diameter of wire: .020 .030"
Number of turns: 12 9
Spring diameter: .185" .211"
Coiled length: .260" .250"
Overall length: .632" .661"

--------------------------------------------------------------------------------

Distributor Specification Number 40897E (10-66)

Weights: not marked
Cam Arm: 10 degrees

Springs: Number 1 Number 2
Diameter of wire: .021" .039"
Number of turns: 12 6
Spring diameter .186" .203"
Coiled length .256" .255"
Overall length .625" .764"

--------------------------------------------------------------------------------
41400 25D4 2100 18 1350 12 950 7.5 10/15/5 1973 MG 1275cc Midget MkIII LC
Distributor Specification Number 41400 (49-72)

Weights: not marked
Cam Arm: 18 degrees

Springs: Number 1 Number 2
Diameter of wire: .031" .033"
Number of turns 5 5
Spring diameter .244" .243"
Coiled length .278" .285"
Overall length .695" .696"

--------------------------------------------------------------------------------
41491 25D4 2400 19.5 1900 16 1000 8 10/15/5 1973-74 MG MGB 18V 672Z 673Z LC
Distributor Specification Number 41491 (not marked)

Weights: 19 degrees
Cam Arm: not marked

Springs: Number 1 Number 2
Diameter of wire: .030" .030"
Number of turns: 7 7
Spring diameter .204" .204"
Coiled length: .320" .323"
Overall length: .735" .734"

--------------------------------------------------------------------------------

CONCLUSIONS:

It may be possible to have these spring produced on a custom basis to be sold as sets to be used in the rebuilding of the Lucas 25D4 distributor. Whether such custom made springs will work properly will only be determined by having them produced, then installing them in several test distributors. The test distributors will then have to be tested on a distributor test machine to see how closely the new springs mimic the factory supplied springs. Timing tolerances are available for the later production distributors, but not the early production versions. Hence, some form of tolerance must be developed to determine whether the newly produced springs can be marketed. These tolerances should be developed along with an article covering the rebuild of the distributor and such article included with the distributor rebuild packages.

--------------------------------------------------------------------------------

ADDENDUM:

Anglo Parts Spring Kit for the Lucas 25D4 Distributor

Dia. Of Wire .020" .027" .030" .0305" .031"
No. Of Turns 6 6 7 6 6
Spring Dia .190" .198" .195" .200" .196"
Coiled Length .281" .286" .301" .305" .289"
Overall Length .658" .682" .710" .711" .689"



ADDENDUM II

Auto Zone rebuilt Lucas 25D4 distributor for 1968 MGB part number 31-402
41234 25D4 3000 15 2250 14 1800 11 1100 6 300 MG MGB 18V HC
Specification number 41234 H (15-73)

Weights: Not Marked
Cam Arm: 19 deg

Springs: Number 1 Number 2
Diameter of wire .026" .027"
Number of turns 7 9
Spring diameter .226" .196"
Coiled length .185" .296"
Overall length .630" .695"

[Keith Andrews]

Now I am on the hunt for programmable distributor but first need to understand what I am doing.
Do modern petrols need more max advance than the pool petrol used when the DM2 40317 curve was chosen?
Does a balanced Jowett with ported heads that can rev up to 6000rpm need a different curve?
Does the vacuum advance need to come from the manifold or the port and how much vacuum advance should be added?

1st one must understand what influnences advance, and why, and this applies to all petrol and most internal combustion engines regardles of age make chainsaw or vechilce
1/ higher compression, less advance, fast burn time from ignition to max explosion... cyclinder pressure
2/higher octane, more advance... and with modern unleaded fuels with more volitile solvents slightly more advance
3/ leaner mixtures, more advance...idle and cruise circuits carb should be delivering around mid 15s AFR and full power high 11s mid 12s

The piont of max cyclinder pressue/ explosion will be at a given piont between 12 and 18 degs ATDC.. this is generally termianed mainly by bore to piston ratio.. plus a few other things...
if one has a short block, and pushes down on a piston at TDC, it will not move. The lower it gets in the cylinder the easier it moves. As the piston gets lower the compression/ cylinde pressure drops...there fore there is a trtade off between piston too hard to move at TDC and firing with little compression further down....this 'premuim trade off is between , at a give piont for any given engine, 12 and 18 degs.
This is why when one 'tunes by ear' with an over advanced engine it sound stronger .. no resistance, but actually produces less power.

There are 3 components in a dizzy... the intial, the cent and the VA.
And there are 2 systems... ported vaccuum.. taken above the carb butterflies , and manifold vaccuum for the VA.
post early mid 70s polution stuff/ EGR etc was added, and this has a very different dizzy config which runs on ported vaccuum.. we will not deal with that here
No EGR manifold vaccuum
Intial is kept low... 2 to 14 degrees to keep loads off starters, armitures and batteries.
idle is intial +VA.. and will be between 8 and 16 degs depending on cam section to get a stable idle... wilder the cam the more idle advance...Important.. the VA must be all in no more than 1" above the idle rpm (temp set) with no VA attached.
So u fire up on say 8 degrees, the VA pulls all the way in , say 10 degs in the VA, idle advance would = 18 degs
The total (Intial + Cent)
All petrol engines have a total between 30 and 36 degs, this varies , as mentioned above on chambers, fuels, compression, cams etc... And all in between 2800 and 4000 rpms depewnding on vecchile weight, rear end ratios etc.
Then there is the all up, intial+cent + VA.. this can be as high as 50+degrees...BUT unless one has data loggers knock sensors etc DO NOT exceed 42 degrees, and make 40 the max.
Dialing in..
1/ the carb HAS to be ball park tuned in .. idle circuit and cruise circuits giving around mid 15 AFR, and WOT mid 11s to mid 12s.
2/ the HT leads, plugs condesor, pionts etc MUST be in top conditon.
3/ establish the idle by fixing the cent with rubber bands, disconect VA and turn till find the lowest advance at the idle rpms u want...say its 14 degrees... Rem idle = intial+VA.
4/ now establish the total (intial+cent)... this is done at wide open throttle .. best up a hill, over a given distance, for the best time...between about 2500 +rpms and higher...Start over advanced about 38 degs and retard back in 2 deg increments.
There will be a piont within a couple degrees where power drops off very suddenly...not this piont and add 2 to 3 max degrees... this is your total... lets say 35 degrees.
5/ the speed at which the cent advances...this is determined by the counterweight weights and spring sizes....never have the weights advance any earier than 300 rpms above idle speed, and should be all in around the 3000 rpms...If too fast u can get detonation at some piont within that advance at full or part loads with no VA attached.
6/ OK we have established an idle (intial+VA) of say 14 degs, a total (intial+cent) of 35 degs and looking for about 40 degs all up (intial+cent+ VA) lets say the cent has 26 degs, that means for a total of 35 we need an intial of 9 degs which leaves 5 in the VA for the 14 deg idle....and a all in of 40 .
But things rearly work out quite like that..
Some VA are adjustable by changing the tension on the diapham spring... but this also changes the distance the arm moves and therefore the number of degrees in it... OR one can change the amounrt of movement in the arm by filling out one end of the slot, and/ or adding a stop, mounted on the VA mounting screws.. this way one can change the vaccuum range it works in AND the number of dregrres.
Changing the degrees in the cent can be a little more tricky....work out what limits the amount the counter weight move... this may slots, or even the shape of the counter weight tails...what ever, one can then by filling a little here and there or dropping shims over pins in slots.

Trying to figure out what will be right for YOUR engine by comparing to another engine, forget it. Sort out what your engine needs, then eith mod your dizzy curves, or if real lucky may find another dizzy that has what u need...which is very unlikely.
Simply boring , reconditioning a block or skiming a head or deck will change the requirements of advance for your engine.

[robert lintott]

A few miles from where I live is a man called Martin Jay, the "Distributor Doctor " I have good reports of him and sometime will take my Javelin to him ,I am sure the timing is off. He has a good web site, google " distributor doctor somerset" and would appear to have answers to many questions being raised. Bob

[Keith Andrews]

We have a similar Dizzie specialist near us, excelent reputaion in the automotive industry, and hosted a club rech evening a while back.
He will rebuild any dizzy , VA what ever...."to the specs the customer requires"
or put another way..the specs the customer establishes for his vechile, be it by dialing in the engine advance and carb, or simply selecting a factory spec out of a book.

What most do not realise is the prescion of CORRECT timing at a given load with in 1 or degrees at a given rpm for a particualr engine....build 2 identical engines and they will have different advance requirements... IF dialed in
Change a jet, plane a head, change or grind a cam, bore a block, change a fuel.

How far can timing go?.. well on just needs to look at modern technoly with very accruate sensors thru a compter, running 12 and 13:1 compression ratios on pump gas mixtures of 40:1 (!!!!!!) and octanes of 91/ 96.

And the old myths of valve rescetion without leaded fuels....all propaganga BS put out by a world wide monopoly company that produced the lead .. biggest rort of the 20th century....and all based because people would simily tune timing by ear, simply up intials, and therefore the whole advace ... high chamber temps, and cause the rescition.

Just because we have basicaly 1912 technoly engines, should not mean that we treat them with the disprect of the past.
We use modern rubbers in our bushes, modern paint and filler systems, modern allies in our pistons, and do not reconise the tools now availbe at reasonable prices to dial engine specs in at far greater accrucay than major manfactures could not do 30/40 yrs ago.

To clarify dialing in as against "tuning"
"dialing in" is establishing the the workshop manual specs one on sets/ tunes ones engine to.

[Forumadmin]

Thanks Keith A. for your input. Although I struggle sometimes with what you were trying to say I think I get the main points.
Your point 3/

lowest advance at the idle rpms u want...

On a Jav/Jup engine the idle rpm is about 850 rpm although some will idle at 500rpm. So when you say 'lowest advance' would you turn distributor (or micrometer adjustment) clockwise to retard until the idle was unstable, or what? I assume here the initial is the static timing which on a JAv/Jup is supposed to be TDC to 3/8inch after TDC.
Now if a properly adjusted 40317 distributor is fitted there should be no centrifugal advance below 250rpm (500rpm at the crankshaft) but the springs could be weak or worn and it actually comes in earlier. Hence lock the 'cent' as you say with a rubber band, or I will have a look and see if it can be stopped from the top without taking the points base plate off.

Your point 4/
Is this done with the vacuum disconnected?
The 40317 has a max of 10 deg (crankshaft 20 deg) if it is not worn. So this with say the recommended initial of TDC or 0 would give a max of 20 deg which I think is too low for modern petrol which is considerably higher octane and more volatile than 'pool' petrol, and my non-standard Dellorto carb giving a leaner mixture. My gas flowed head with higher compression and faster revving engine than standard may negate some of that required advance, but if you look at the way curves changed in the 60s and 70's in most cases they were advanced up as you say to over 30 degrees. The problem I see here is that the stock 40317 has no way of advancing that far. To get there would require a static advance of 10deg or 1 inch Before Top Dead Centre. The car then may not start or idle. Any ideas?

Your point 6/
Your idle timing says that this is the initial(static) plus vacuum, but my recollection on the Jav/Jup is that the vacuum does not operate at idle. Am I wrong?

The ignition is advanced so you reach peak cylinder pressure right after TDC (top dead center). If the spark comes too soon the burning fuel will try to push the piston back down the cylinder before it reaches TDC, resulting in a loss of power and possible engine damage. If the spark is too late you will not reach full power and get poor gas mileage. There are two major factors that effect how much advance is required, engine speed and load. You increase advance with rpm and decrease advance with engine load. You don't need a distributor test stand to curve a distributor. All you need are some basic hand tools, a timing light, and a tach.

The first thing you should do is find a shop manual for your particular car. Read about the advance mechanism and make yourself familiar with the different components. Before doing too much you should check the condition of your advance mechanism. If your vacuum diaphragm is bad replace it. Lube all the components and make sure they are moving freely.

You probably won't have enough timing marks to test total advance. You can buy stickers for the harmonic balancer or you can make one. I made some on my computer. If you are really cheap you could just draw lines on a piece of tape. Trace them from the timing marks you already have to get the right spacing. The easiest way is to buy a timing light with an advance dial. They are kind of spendy, about $60, but a big time saver.

The mechanical, or centrifugal, advance adjusts the timing based on engine speed. The faster the motor spins the more it will advance the timing. The factory advance curve is very conservative. By using a more aggressive advance curve you can greatly improve your engines performance. To setup your mechanical advance you first need to disconnect your vacuum advance line. Next you should hook up a tach and timing light and see what your timing is set at. Simply watch the timing marks with the light. The reading at idle is your initial advance. Rev the motor up until the distributor stops advancing and note what speed it maxes out. This is your total advance.

Total advance is the most critical setting. Short of a dyno the best way to find what total advance you need is some track tuning. The MPH reading at the end of the track is your best indicator of engine output. Make a run or two to get a baseline then increase your total advance and make another run. If the MPH increases advance it some more and run again. Continue advancing the ignition until the MPH starts falling off then pull it back to the point where you had the highest trap speed. It may take quite a few passes until you find the optimal setting. Total advance is initial advance plus mechanical advance. There are two ways to change the total, adjust the initial advance by turning the distributor or adjust the mechanical advance mechanism. The easiest, and cheapest way is to just turn the distributor until you get the total you want then just leave the initial wherever it ends up. If you want more control over the initial then you need to adjust the advance mechanism. If you have a total of 36° and want to run 12° initial then you need 24° mechanical advance.

Initial advance isn't very critical. Usually you just set the total where it needs to be and leave the initial wherever it ends up. Once you found the best total advance setting you can play with the initial. Basically you want to run as much as you can before the motor cranks over hard. With the motor warm pull the timing way up and try to start the motor. If it cranks real slow then pull it back until it spins normally. Lets say that is 20° and you ran best with a total of 36°, that means you need 16° mechanical advance. Any time you adjust the initial you need to adjust the mechanical advance so the total stays the same. Now if you're running overdrive and/or a heavy car you will probably encounter some sort of pinging if you stand on it with that much advance. If it happens at real low engine speeds then you need to pull the initial timing back. If it happens at moderate RPM levels then you can probably fix it with the advance springs.

The springs adjust the advance rate. Where the distributor is bolted down determines the initial advance, the amount of travel in the mechanical advance determines the total advance, and the springs determine what RPM total advanced is reached. Lighter springs allow the mechanical advance to move more easily so you will reach total advance at a lower RPM. Stiffer spring will delay the total advance. Most distributors use two springs, a small one and big one. The big one will usually have a bit of lash so the small one does all the work at low speeds. This allows the advance to come up quickly off idle. Once the lash on the big spring is used up the weights will be trying to pull both springs so the advance rate will slow down. The chart below shows a typical stock advance "curve". It is the dual springs that give it the curve. If there were only one spring the chart would just be a straight line.

curve.gif (2.61 KiB) Viewed 324 times

In this example it idles around 500 rpm and has 6°initial advance. At this point only the small spring is holding back the timing. It is a light weight spring so the advance rises fairly quickly until it hits 18° at 1800 rpm. Once you hit 18° the lash on the big spring is taken up so the advance rate levels off a bit. If you increased the lash on the big spring the advance will go further than 18° degrees before hitting the big spring, less lash will hit the big spring sooner. To adjust what rpm you hit the big spring you would change the small spring. A lighter small spring will cause you to hit the big spring before 1800 rpm and a heavier small spring will delay it until after 1800 rpm. In this example you hit total advance (28°) at 4000 rpm. As mentioned above the total is limited by the mechanical advance mechanism. The rpm total advance is reached is determined by the big spring. A heavier big spring will delay total until after 4000 rpm and a lighter big spring will allow total to come in before 4000 rpm.

If you experience pinging just off idle you should lower your initial advance. If your motor pings around the point you reach total advance you have two options; lower the total advance or put in a heavier big spring to delay the total. If its pinging well above this point you will need to pull back the total. If your motor is fine at low and high speed but pings in the mid range then you need to either reduce the lash in the big spring to lower the mid advance point or install a heavier small spring to delay the mid advance point. Making these fine adjustments can be a pain in the neck because adjusting one element often changes the others. If you want to bypass this whole mess you could use a programmable digital ignition. It allows you to adjust any of these points independent of the others and from the comfort of the drivers seat.

The mechanical advance is adjusted for high load wide open throttle conditions. Under light load, part throttle conditions the manifold pressure is lower so volumetric efficiency is lower so the cylinder pressure is lower so the fuel mixture burns more slowly. This means you need to light the mixture sooner so you reach peak cylinder pressure at the ideal time. This is the purpose of the vacuum advance. The lower the load is the more it will advance the timing. Vacuum advance will improve gas mileage and drivability of a street driven car. A lot of guys think a vacuum advance hurts performance, this is not true. The vacuum advance is entirely independent of the mechanical advance. They are two separate systems that perform two separate functions. The mechanical adjust timing based on RPM where the vacuum adjusts timing based on load. Under high load, wide open throttle, performance conditions there is almost no manifold vacuum so the vacuum advance does nothing. The only time the vacuum advance adds timing is at part throttle, low load conditions when there is manifold vacuum. So unless you race at half throttle a vacuum advance will have no effect on performance. It will however improve part throttle drivability so unless your car is a 100% race car I would recommend running a vacuum advance.

You're probably thinking, "Sure there is no manifold vacuum at wide open throttle but aren't I supposed to use ported vacuum for the vacuum advance." Hold onto your hat, THEY ARE THE SAME THING! Except ported is shut off at idle. There are a lot of misconceptions when it comes to the ported vacuum source. After hearing 20 different theories I decided to hook up two vacuum gauges, one to manifold and one to ported, then drive my car and watch it. I found out they are the same, except the ported is shut off when the throttle is closed. Even then I had a hard time convincing guys so I hooked up a couple MAP sensors and a throttle position sensor to a data logger and recorded them while driving then dumped it into a spreadsheet and made a chart. As you can see, there is a direct relationship between throttle position and vacuum. When the throttle is closed vacuum is high, when the throttle is open vacuum is low, and ported vacuum is the same as manifold except when the throttle is closed. So which one do you want to hook it to? I prefer manifold vacuum. This pulls in more timing at idle which is good since there is virtually no load. Your motor will idle smoother and cooler with the extra timing. One night I was at the drags and my car was running hot in the staging lanes, I swapped the vacuum advance from ported to manifold then it would idle all night at 175°. Believe it or not the purpose of ported vacuum is to raise the temperature at idle, to lower NOx emissions. If you're like most hotrodders that is of no concern to you. If you have a big cam with a choppy idle then a vacuum advance hooked to manifold vacuum can really help. It will idle smoother and requires less throttle to maintain speed. Often a big cam requires you to open the throttle so far that the curb idle adjustment needles won't work. Hooking the vacuum advance to manifold vacuum will allow you to close the throttle some which may be enough for the idle mixture screws to work. Someone told me he noticed less dynamic braking with the vacuum advance hooked to manifold. I didn't notice it on my car but it makes sense. If the motor is running more efficiently with the added advance it will make a less effective brake. So which should you use? Try both and see which you like best.

Once you have the mechanical advance setup to give you the most power, and no pinging, at WOT then you should setup the vacuum advance. A stock vacuum advance will pull in 20° or more. If your car is pinging or running rough after hooking up your vacuum advance then you need to turn it down. Most vacuum canisters are adjusted by sticking an allen wrench in the vacuum tube. Turning the wrench counterclockwise will reduce the timing. Just turn it down a bit at a time until the problem goes away. I had to turn my vacuum advance down until it only pulled in 5°.

[Keith Andrews]

Your point 3/

lowest advance at the idle rpms u want...
On a Jav/Jup engine the idle rpm is about 850 rpm although some will idle at 500rpm. So when you say 'lowest advance' would you turn distributor (or micrometer adjustment) clockwise to retard until the idle was unstable, or what? I assume here the initial is the static timing which on a JAv/Jup is supposed to be TDC to 3/8inch after TDC.
Now if a properly adjusted 40317 distributor is fitted there should be no centrifugal advance below 250rpm (500rpm at the crankshaft) but the springs could be weak or worn and it actually comes in earlier. Hence lock the 'cent' as you say with a rubber band, or I will have a look and see if it can be stopped from the top without taking the points base plate off.

The rubber bands are to lock the cent advance ...just because the factory spec is 850 or 400 what ever means squat...that is for an orginbal engine, no rebuild boring, skim heads, cam profile, running on the British pool fuel of that period. No one meets any of that critia today... which means the engine has to be re dialed in to extablish ALL tuning specs...
So forget what the book says, the book requires re writting for modern fuels and engine mods/ machining.

The problem I see here is that the stock 40317 has no way of advancing that far. To get there would require a static advance of 10deg or 1 inch Before Top Dead Centre. The car then may not start or idle. Any ideas?

The examples Im using above I use the word "say..." a hypothetical numer... untill the idle, power curves(cent) VA deg requirements and vacc range it works in, are all established, there is no problem untill these estblished tuning pionts need to be put in the dizzy.....and as I mentioned above , this may require rounding off counterweight tails, filling out slots or even dropping shims over pins...depending on the type and manufacture of the dizzy...or even using different compatable parts from other dizzies....
If it is a problem, .. or cant be done for a jowett, then this whole discousion is frutile...I have yet to see a dizzie that cant be modded.....

Your idle timing says that this is the initial(static) plus vacuum, but my recollection on the Jav/Jup is that the vacuum does not operate at idle. Am I wrong?

The timing curves, even the cams of British cars up into the early 60s was archaic technology even for that era...to say the least...take the bore to stroke ratios, the rest of the world had moved on, cams with square profiles that relied on tappet clearance for overlap....ad the crap fuels, and reduculously low compressions, engine rpm ranges....
basically what is needed is to have a low intial for easy start, low starter loads, once the engine fires up, the VA comes in increasing the idle advance (espec with the slower burn time of modern fuel) to give clean stable idle , low NOx and good AFR mixtures. (note over advance too far and NOx increases dramatically.)
Then start to move off the manifold vac drops, and the car accelertes on the power advance (cent) reach cruise, the manifold vaccuum increases and gives more advance for leaner mixtures.

The articule quoted... a guy who knows what he is on about.. good pionts, marking the balancer a MUST out to 50 deg in 5 deg increments... a taylors tape to measure circumferance, divide by 36 and mark these increaments WITH the TAyLORS tape then a coal chitle to stamp each 10 deg and a dot punch inbetween.... then paint with white paint, wiping excess off leaving paint in the stamps... If have a set of number stamps its very helpful in practical terms to stamp the 10 deg marks.
And make use of graph paper , lots of it.

Ported and manfold vaccuum VA......Even then I had a hard time convincing guys so I hooked up a couple MAP sensors and a throttle position sensor to a data logger and recorded them while driving then dumped it into a spreadsheet and made a chart. As you can see, there is a direct relationship between throttle position and vacuum. When the throttle is closed vacuum is high, when the throttle is open vacuum is low, and ported vacuum is the same as manifold except when the throttle is closed. So which one do you want to hook it to? I prefer manifold vacuum. This pulls in more timing at idle which is good since there is virtually no load. Your motor will idle smoother and cooler with the extra timing.

Ported Dizzies and non ported, be pre 60s or post mid 70s, are very different cent and VA specs....expect to have mods, increasing the cent, decreasing VA and change maybe in the VA operating vaccuum.

A stock vacuum advance will pull in 20° or more

This statment says a lot... this guy is working on American post mid 70s dizzy.....A manfold VA generally has between 7 and 12 degs max starting around 5 to 7 " and all in around 10 to 12". The 10 to 12" go back to my post above about establishing the top end of the range the VA is all in at......taking the intial,(NOT the established idle advance intial+VA) tie the cent counterwerights....re adjust the idle rpms and mixtures temp and no more than 1 " above MAX..is the vaccuum VA has to be all in at.
Another thing that says he is working on post 70s ported vaccuum dizzies descigned for EGR is the curve example (graph)
These dizzies .. being American, need approx 1mm rounded off the tail of the cent weights to increase the cent degs, to ball park specs I describe in top post, lighter springs for faster adavance and change the VA specs ... And he is working on HEI dizzies.
Good artcule.

Once you have the mechanical advance setup to give you the most power, and no pinging, at WOT then you should setup the vacuum advance. A stock vacuum advance will pull in 20° or more. If your car is pinging or running rough after hooking up your vacuum advance then you need to turn it down. Most vacuum canisters are adjusted by sticking an allen wrench in the vacuum tube. Turning the wrench counterclockwise will reduce the timing. Just turn it down a bit at a time until the problem goes away. I had to turn my vacuum advance down until it only pulled in 5°.

Again refer back to my post re adjutable VAs screwing the adjuster in ALSO reduces the number of degs.. it tightens up on the spring, but also reduces the distance the diapham moves... hence my comment about changing the postion the arm operates in and the distance with maybe filling out the slot and/ or stops mounted to the VA mounting screws.

What this guy doesnt say is that AFRs must be in ball park...dial in the parameters, re check the AFRs and if need be, fine tune the curves... mixture/ AFR makes a huge difference to the speed of the flame to max explosion across the chamber.

A note of caution when using data loggers/ 02 sensors, the slightest miss fire will give lean AFR readings... NOT rich , even thu u have unburnt fuel... the O2 sensor measures O2 not fuel.. the unburnt O2 increases.

If one is ACTUALLY serious about dialing in carbs and timing curves
Tools.
AFR... at least a O2 gauge and sensor in the exhaust ....preferbly a data logger like the Innovate LM-1 or LM-2... and that preferbly thu not required to be able to log RPMS, AFR, IMAP (manifold vac), throtlle postion and knock sensors.
Taylors tape to mark off the balancer.
Vaccuum gauge
Stop watch.. far better than the mph in the articule...different cam profiles, final gear ratios, can have faster times at less mph.
Graph paper
files drills, screw drivers and spanners
And establish 1 tuning piont at a time... never more than 1.

Think of getting that max pressure of explosion that piont between 12 and 18 degs ATDC to shooting a deer on the run with a bit of a cross wind...nail it right the deer drops in its tracks...bit off got a walk of a couple 100 yrds... further off haver to track the thing down for the next few hrs.....yes all these the deer dies... the car runs....way off, have a deer wouned , cant track it, may or may not die, bad pain....no deer...car either runs in a manner that cant drive it or doesnt run at all.
99% of classic, hot rods , modded and reconditioned cars have huge potentuals for more power, better economy and excelent emissions.....
We send a lot of money on oil pumps, pistons, brakes, paint trim...and get it right....and use modern sewing machines tools etc
but when it comes to the engine, getting that right...the thought of using, getting the approiate tools, 99% of enthsitist fail horribly... result old classic cars get bad rap when comes to emissions economy and even power.

Another note:
do not over advance... tune power or idle by ear....there is that retarded point of no power, and a couple degs above full power... from there the engine sounds stronger because of fieing lower in the cylinder, less resstance... But it is like an an optical illusion.. dont fall for it.

[Forumadmin]

Thanks Keith A. again.
But we (or I) need to rein in the possibilities here. If we stick with the existing Lucas DM2 40317 body and innards, the things we can do are:
1. Open up the hole for the max advance stop
2. Alter the springs (wire diameter, number of coils and length)
3. Possibly change the weights (although as far as I know they were always the same).
4. Play around with spring or slackness of the vacuum unit (If someone can find an adjustable replacement that will fit , then great)
5. Possibly move where the vacuum is taken from which currently is near the throttle butterfly. Note I must check where it is on the Delllorto and the Zenith!!
6. add a smoothing chamber (about 1 cm diameter x 2 cm long) in the vacuum take-off tube as I have seen on some Jowetts.

What is becoming fairly obvious is that all Jowetts would benefit from a new distributor profile and rebuilding to the original spec may actually make it worse. This is because a worn dizzie tends to advance (bring in early at lower speeds) the centrifugal due to weak springs and worn posts and give a large max advance due to a worn hole and worn stop. Thus this takes the profile close to where it needs to be just by accident. perhaps Jowetts had a crystal ball and designed the systems to cope with future petrol.

That does not mean a worn dizzie is good as the wear in the shaft causes erratic firing and the car may be difficult to start as the initial advance is too great.

What we need here is someone to say they have had a dizzie rebuilt by the Distributor Doctor and the car went faster and pulled better as a result. And then for them to tell us what their max advance is set to, and at what rpm it reaches max advance.

Yes I know each engine is different, but all Jav/jups on the whole run on the same petrol, most have had their compression ratio increased and most have had some form of combustion chamber work done. So I would be happy to start from that baseline.

Here is another quote to all those people over the years who ask why I run a vacuum advance as I race and rally my car..

Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

taken from this quite good treatise on a Corvette forum. The rest of this article is another easy to read take on the subject.

[Keith Andrews]

the things we can do are:
1. Open up the hole for the max advance stop
2. Alter the springs (wire diameter, number of coils and length)
3. Possibly change the weights (although as far as I know they were always the same).
4. Play around with spring or slackness of the vacuum unit (If someone can find an adjustable replacement that will fit , then great)
5. Possibly move where the vacuum is taken from which currently is near the throttle butterfly. Note I must check where it is on the Delllorto and the Zenith!!
6. add a smoothing chamber (about 1 cm diameter x 2 cm long) in the vacuum take-off tube as I have seen on some Jowetts.

couple things for further investigation... do not take
1. Open up the hole for the max advance stop.
Do not assume that that hole is what limits the number of degrees/ throw out... I have been fooled into the obious more than once re curving dizzies... check very close it is not the shape of the counter weight tail and anvail it works on, or how a rotor sits...be 100% sure what limits BEFORE cutting anything
2. Alter the springs (wire diameter, number of coils and length)
yes rate of advance.. rules of thumb, have cent come off about 300 to 500 rpms above the idle rpms and all in Total..cent +intial about the 34 degs @around 3000/3200 rpms ball park start.
Dont count spring diameters or coils... hang them up and hang a weight on them see how far they stretch .. distnce similar to what they do in the dizzy , measure with verniers.
3. Possibly change the weights (although as far as I know they were always the same).
Unlikely.. generally the curve will need increasing and faster, played with heavierweights yes doner that but fae easier to get a hand full of springs off different cars from a wrecking yards...yes modern cars too
5. Possibly move where the vacuum is taken from which currently is near the throttle butterfly. Note I must check where it is on the Delllorto and the Zenith!!
Manfold vacuum should be consistant thruogh out the manfold...regardless , belowq the butterflies... and if inconsidtant would not matter.. higher the better position, but even at a lower piont that piont will alter as per all pionts and if VA tuned to that area does the same thing...
6. add a smoothing chamber (about 1 cm diameter x 2 cm long) in the vacuum take-off tube as I have seen on some Jowetts
NO..the timing needs fast response to any change in engine loads.. not having so can cause detonation if VA hangs in too long overadvanced...

What is becoming fairly obvious is that all Jowetts would benefit from a new distributor profile and rebuilding to the original spec may actually make it worse. This is because a worn dizzie tends to advance (bring in early at lower speeds) the centrifugal due to weak springs and worn posts and give a large max advance due to a worn hole and worn stop. Thus this takes the profile close to where it needs to be just by accident. perhaps Jowetts had a crystal ball and designed the systems to cope with future petrol.

yes...and worn counter weight pins, shaft bushes etcjust give inconsidtant timing.. worn pins can be sorted by driiling out the counter weight holes (also worn.. and adding brass or hard plastic bushes....again look at dizzies in wrecking yards for bushes that can adapt....worn shaft biushes.. this is another subject.. synted bushes dont reem to size.. if not familar with replacing these get done professionally... also shafts wear far more than the bushes, often replace a shaft is all that is required.

What we need here is someone to say they have had a dizzie rebuilt by the Distributor Doctor and the car went faster and pulled better as a result. And then for them to tell us what their max advance is set to, and at what rpm it reaches max advance.

Yes I know each engine is different, but all Jav/jups on the whole run on the same petrol, most have had their compression ratio increased and most have had some form of combustion chamber work done. So I would be happy to start from that baseline.

Sort of..like I have said previoysly, get 2 identical engines and they will dial in different, often quite different...
Going back to comments made in your quoted articule...memory ok...about factory curves are conservative, this prevented at identical engine, amonst other things that need more advance ending up being returned because of detonation or to lesser degree burnt valves at an early age...
A fuerther note: it was the thing to do to up the factory curve, increase the intial for more power... which it gave, but that over advanced cruise and WOT.. leading to burnt valves etc...the Correct way was to lighten one or both of the springs AND maybe a couple degrees only on the intial.

Here is another quote to all those people over the years who ask why I run a vacuum advance as I race and rally my car..

Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Hell yeah and ANYONE who says different for a street track car doent know basic principles of timing or how a dizzie works.
Any street / track car should be drivable on the street and enable that bit of extra advance when the foot is lifted on the track for the leaner mixtures....The exception is a dedicated track car that has a cam so damn wild it will not idle no matter what under 1200+ rpms... and in this case they generally dont have a cent curve at all... a locked dizzie set permentally at the total, even as intial... have huge aftermarket or remote starters.
That Advance 101, yes I know John, have for many years.. He is , like me 1st Generation Chev Camaro person.
That is the polite verson, not the orginal verson where he talks about so called expert mechanics through to enthuists who have no REAL idea about timing or why or how.

I have been messing with timing / curves seriously since the early 80s, but goes back to the mid 70s.. not just petrol but other fuels including high octane petrol, LPG and CNG.

[PJGD]

Having some limited expertise in this area (and having been a Lucas employee, albeit on the diesel injection side) I would say that those parts of Keith A's postings I was able to understand on distributor tuning is correct, therefore I am guessing that the rest that I didn't comprehend is correct too. On the whole, I would be surprised if the ignition curve established by Jowett back in circa 1950 could be used for anything other than a rough guide today. That is to say, that a recently rebuilt engine fitted with an original specification distributer will run acceptably well, but will be quite a way off from optimum in terms of performance and fuel consumption.

For example, most Jowett engines in running cars have been rebuilt multiple times and many will have had modifications made to the combustion chamber such as polished ports and non-standard pistons with non-standard squish characteristics due to a different compression height. These modifications will change the bulk turbulence in the chamber which will in turn change the burn rate and thus the amount of advance required. While the whole point of mass production is that every engine will be the same and thus a single distributer calibration should work equally well for all engines, we are now in a situation where every engine is highly likely to to be different from each other and certainly operating under different conditions from the original realities of 1950 as mentioned previously with respect to fuel (so whatever happened to Cleveland Discol and National Benzole?).

Philip

[Keith Clements]

Thanks guys but I am still after that ball park starting point on the max advance. In 1996 my Jup was on the rolling road.

[Keith Clements]

So the question is what has changed since then? Petrol certainly, wear on engine and I must try to remember what I have done to it since then!

[David Morris]

Hi there,

Faced with a very worn original distrubutor on our Javelin, I opted a year or tow ago for one of the new 25D4's available at under £30. I selected one for the early MGB, as it looked reasonably compatible with the original Jowett one's. Ok, so it might not be an exact match, but I bet it was better than the original clapped-out one we had fitted!

My experience has been excellent. She has a new distributor at minimal cost and all the previous slack in the bearings has been overcome. Fitting the Jowett drive shaft was no problem.

Why not give it a go?

All the best,

David

[Forumadmin]

Just come back from Watford Classics / Accuspark in Hemel. They replaced the unit I bought last week and I had a long chat with the owner who was busy working on a couple of Midgets (MG type).
They do the 25D4 complete for MGB 1964 to 1972 as well as kits for the DM2. But he said the advance curve was a generic one for the MGB since there were many different types. What was the spec of the dizzie you bought, David? Do you know the max advance and vacuum advance of your unit? What do you set the static timing at?

Word of warning on the electronic units ...they do not like it if there is persistent overvoltage >15v (perhaps caused by regulator fault or battery disconnected or high resistance battery leads) or if you even momentarily connect them to positive earth (unless you have the positive earth one that is). Also you cannot use them with copper spark leads as the HF EMR (high frequency electro magnetic radiation) blows them up.

He also said that the E10 petrol we are about to get here and that we experienced on our European trip will cause havoc on cars with fixed advance curves. Hence the reason for looking at a programmable unit.

Anyhow make sure your vacuum advance is working as this might contribute to overheating and poor fuel economy!

MGB dizzies
40897A 25D4 CCW 1500 10 450 4.5 300 1.5 200 1962 MG MGB and GT
40916 25D4 2200 12 1500 9 500 4.5 400 4 300 4/10/12 MG MGB 18G, 18GA, 18GB, 18GD, 18GG LC
41155A 25D4 CCW 1500 10 450 4.5 300 1.5 200 5/13/10 (z) screw MG MGB and MGB GT
41156A 25D4 CCW 2500 12 500 12 300 4 200 4/12/8 (z) screw MG MGB and MGB GT low octane
41210A 25D4 CCW 1800 15 600 8.75 350 1.5 250 6/13/5 (z) screw MG MGB and MGB GT USA
41234 25D4 3000 15 2250 14 1800 11 1100 6 300 MG MGB 18V HC
41264A 25D4 CCW 2600 15 1150 10 500 1.5 350 5/11/8 (y) push Austin "America" (Automatic) late, 1969 MGB
41288A 25D4 CCW 1500 10 800 9 300 3 200 5/13/10 (y) push 1966-70 MG MGB 18V581F,18V582F,18V583F,18V581Y,18V582Y,18V583Y (HC)
41290 25D4 2200 12 1500 9 500 4.5 400 4 300 4/12/8 (y) push 1967 MG MGB 18V LC
41290A 25D4 CCW 2500 12 1050 6 300 1 200 4/8/5 (y) push 1967 MG MGB LC
41339A 25D4 CCW 3000 10 1900 9 900 4.5 400 0 400 push 1971 MG MGB HC
41370B 25D4 CCW 2800 15 1400 10 1200 8.5 400 0 400 7/13/3 (y) push 1972 MG MGB HC
41391A 25D4 3000 15 2250 14 1800 11 1100 6 600 4/12/8 (y) push 1972-77 MG, Reliant MGB 18V LC, Reliant TW9B and TW9C
41394A 35D8 CW 2800 14 2000 12 900 5.75 300 5/17/8 (y) push 1972-76 Range Rover, MG MGB GT V8
41427 45D4 CCW 2000 9 5/13/10 1963-70 MG and Reliance Tractors 1963-70 MGB HC, 1974 onward Reliance Tractors RM Trois Mille, 1974-77 RM 200 and RM 300
41491 25D4 2400 19.5 1900 16 1000 8 10/15/5 1973-74 MG MGB 18V 672Z 673Z LC

Then they go to the 45D4

41599E 45D4 CCW 2500 18 10/15/5 (y) push 1976 Austin, MG 1976 Austin Marina and 1975 Federal MGB 75-76 MG with 18V engine
41601 47DE4 MG MGB

[David Morris]

Hi Keith,

Thanks for your reply. I will try to answer your questions :-

It was a while ago, but I believe the new distributor I purchased, for around £27.50 then, was a 40897A, marked as suitable for a 1962 MGB.

The original Jowet distributor was, according to my info and excluding the earlier DKY type, was the 40317A, which apparently was then superceeded by the 40571.

These distributors have spec. figures in my Lucas data sheet as :-

Advance 1 = 10 degrees, Advance 2 = 4.5 degrees and Advance 3 = 2 degrees.

The replacement 40897A has the following figures in my data sheet as :-

Advance 1 = 10 degrees, Advance 2 = 4.5 degrees and Advance 3 = 1.5 degrees.

I reckon this was a reasonable match?

I set the static timing at TDC.

Having played about with and built my own electronic ignitions over about 50 years, I have given them up! For me, it's back to straight-forward points. When they go wrong, you stand a good chance of getting going again at the roadside, if you have a spare capacitor and points in the tool box, plus a coil if you are lucky.

I have converted the electrics to negative earth and regularily make sure the vacuum advance is working by using the timing light on the front pulley. Interestingly, my flywheel has been lightened in the past, to the extent that all the timing marks have been completely erased! Our Javelin was owned by your predecessor as Competition Secretary, Ken Lees, so I guess he was seeking improved acceleration? Anyway, it doesn't seem to have affected the slow running, she is still smooth. I have no idea what the weight of the flywheel is now, but it must be a fraction of the original. There is only just enough meat to hold the ring gear on!

All the best,

David