Carburetter? mixture problem.

[Ian Anderson]

I have an ongoing problem with my Jupiter which I have been unable to cure. I have posted a request on the Jupiter page in the past but many Jupiters seem to have been modified with Dellorto Carbs and external balance pipes etc so I thought I would try the Javelin page as my car has standard (Zenith 30VM?) carbs which were professionally refurbished, and no external balance pipe, so my problem may have occurred on somebody's Javelin.
The engine was rebuilt by Mike Smailes some years ago, but has only done a couple of hundred miles since then as it developed this problem which means I have been unable to use the car for some years. It used to start and run very well, then one year when I tried to wake it from Winter hibernation, it would not start. New petrol seemed to have cured the problem, but the following year it developed a new problem. If I remove the plugs, they are all black. If I clean them and replace them, the engine will start and run, but as it warms up, it starts to run roughly with slight black smoke on tickover. eventually the engine stops and the plugs are sooted up again. Up to now I have tried the following -

1) Removed the float chambers and checked for blocked jets (though I assume this would have the opposite effect)
2) Removed the Air filters to check for blockages (but they are only standard wire gauze filters and would be difficult to block)
3) Checked that the chokes are opening/closing correctly. (the only way the car will start is no choke until it fires then about 1/3 choke to start)
4) Out of desperation I have reinstalled the 60 year old points capacitor as it had a bigger "kick" on the meter than the modern replacement.

I am now at a loss, as surely this has to be something that has GONE wrong rather than been wrong since I rebuilt the car. One thing I have not been able to check is the pintle valve as I don't know what it looks like and I cannot see anything inside the oil filler cap except a cross washer and bolt head well below the breather pipe. The manual is rather vague about its effect other than it can case carburration problems.
I would welcome any suggestions from the Javelin fraternity.

Ian Anderson

[Keith Clements]

Hi Ian,

Since this is something affecting all cylinders we should rule out a carb issue. But this poor car has hardly been used so lots of things could have gummed up.
Since you say it was working OK and I assume nobody 'adjusted' anything in the meantime. it is something the car has done to itself.

Check choke mechanism make sure both choke flaps are open. Tick.

Check the petrol pressure and make sure float needle valves are both closing. It is quite common for the wrong petrol pump to be fitted. No petrol dripping inside carb. No petrol outside carb. Carefully remove bowl and check level of petrol which should be two thirds up. Is float floating? Shake float and make sure no petrol inside. It is very common for a holed float!

Check valves are closing by (assuming you have an electric petrol pump) first by listening to the pump. It should stop ticking with the ignition switched on and car not started. If not, or if the tick is more than once every 30 seconds, check each needle valve in turn. Remove bowl from one carb, stick finger on needle plunger and hold valve closed. switch on ignition, and see if valve leaks.


Check filters and float bowls and blow out with compressed air to clear jets. Put clean new petrol in the bowls, just in case the petrol has been contaminated. Partly tick. Note that old petrol,( i.e. petrol more than 3 months old) can cause problems. Empty tank of all petrol and refil with new. You can use the old petrol again if you wish by adding in stages to the new petrol as you use the car. A carb cleaner may remove gum deposits.

Check the filter bowl gasket is making a good seal. If it is not the petrol metering system will not work correctly.

Check air cleaners or temporarily remove. Tick.

Check ignition timing both static and with strobe.

Check the pintle valve is working. Search for 'pintel' or 'pintle' in JowettTalk. Best done with vacuum gauge although finger over hole is reasonable indication . Also check for leaks in feed to valve.

Page 32 0f Jav Jup maintenance manual has section on breather valve. 12" of upper cylinder lubricant in a manometer is their test. Check the breather felts are not blocked.

Be brave , with the filler cap off and engine running (cannot do this if it is not) stick third finger into oil filler tube, there is a hole two inches down through which air is sucked. I very much doubt a failing breather valve would stop the car starting. I suspect the problem is in the float chamber. Is the car on level ground when doing these checks?

I cannot find anything on checking or repairing the pintle valve, although I have a faint recollection that somebody has posted something in JowettTalk or JowettGallery or it may be in a Jowetteer, The Javelin or Flat Four somewhere. Oh for our Legacy Project to start up again!

May be balance pipe seals have gone although this is usually accompanied by oil smoke from exhaust.

Finally it could be that both gaskets in the carbs have been affected by ethanol or gum . This involves removing the plate on the filter bowl (Part 34 below) that houses the metering channels. You may need to get a new gasket if you remove this plate! See Tech Notes for detail.

[David Kemp]

Though a Bradford man,Jupiters & Bradfords have same carb. So my two bobs worth, check the float valve for crud, check the fuel pump for crud. Check the air filter for blockage insect or mouse nests.
Finally check the idle mixture , if too rich it will start well but cause a very rich mixture later.Check your ignition timing & points, if the car has been sitting they can corrode.

[ChrisCole]

The Zenith carbs are built with 1 mm thick washers under the brass float valves. At this thickness my Javelin runs well but the plugs are always sooty. I have just fitted 1,5 mm thick washers and the plugs were cleaner. At 2 mm washers the plugs are improved again.

If the engine is running, and you put a finger over the pintle valve inside the oil filler tube, then the engine should slow if not stall, and you should be able to feel the engine sucking on the end of your finger. If there is no change in revs then the balance pipe seals may be leaking. Chris

[Keith Clements]

Did you ever resolve your issues Ian?

A number of people, including Jaak Jacobs in Belgium, have been suffering carb problems, so I sorted through my 50 spare (scrap) carbs at the weekend. I was surprised to find so many supposedly non Jowett carbs in the hoard. (C1074,C1344, C1343 , C855A, C1461, CS 1194.) They all did come off Jowetts so I guess they all must have worked.

I would be interested to know if anybody else has Zenith carbs with 'C' numbers diiferent from this official list.

Javelin Zenith Contract Sheet Number C1084 30VM-4, C1130 30VM-5, C1161 30VM-5, C1316 30VM.

Jupiter Zenith Contract Sheet Number C1245 30VIG-5, C1316 30VM.

Are any of the non-Jowett bodies and float chambers suitable in a Jowett? Or indeed would any actually be better with modern petrol and tuning?

Whilst doing this survey it would also help if you checked the jets in the carbs as they may have also changed over the years (they will have worn as well of course!)
See http://jowett.net/Parts/TechNotes-Part18-Zenith.htm for details of how they left the factory.

You may also like to see other Zenith carbs here. http://www.stationary-engine.co.uk/Zeni ... thCarb.htm and tell me if you find and equivalent.

Jowett.rtf

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[Keith Clements]

I have just looked up on http://zenithcarb.co.uk/cf/vehicle/list ... le=Jupiter
Jowett Jupiter 1950 to 1951 C1245
Jowett Jupiter 1950 to 1953 C1343
Jowett Javelin 1948 to 1954 C1334
So I guess that explains some of the ones I have in stock.
I have not found what jet sizes these variants had though!

[Keith Clements]

For competition the following settings were recommended. The main difference is the larger choke size, up from 25mm. If the engine is balanced to give higher revs then an even larger choke size should be used, requiring a different body. The 30VM C1343 which was fitted to many of the Javelin engines I acquired had 110 main and 65 compensating jets. It is likely such changes may have resulted from further tests done after the Tuning Notes were published, or maybe they gave a more economical setting. Any ideas? The vent or capacity orifice was also fitted with a different size, either 2.5 or 2.6. Understanding how all these orifices interract is indeed a study for another day. Since I never used the Zenith in competition it has not been done by me before. But has anybody else...but will they tell?

Somehow a pair of 30VM C1627 were fitted to Amy's SC. So sometime soon I am going to investigate what is in them. I suspect they may not be correct. No idea what they came off.

From Competition Tuning Notes...
Carburettors – Special 30 VM. Carburettors with the following settings:
27 mm Choke; 110 Main Jet; 50 Compensating; 2.2 Vent over Capacity; 45 Slow Running; 120 Progression; 1.5 mm Needle Seating (1 mm Washer) should be fitted in place of the Standard VM. 4 or 5. These Carburettors are interchangeable and carry our Part- numbers ¬LH – 1.53732; RH – 1.53733.
The Throttle Spindle operation is in the reverse direction, and the Throttle Cable must consequently be rearranged. A suggested method is to fit a Cable approximately 9”.(25 cm) longer than standard, securing the Inner Cable to the Bracket on the left hand Tappet Cover, and the Outer Cable to the Throttle Rod. The Throttle Re-turn Spring should be connected to the Clip to which the original Outer Cable was secured on the right hand Tappet Cover.
k) NOTE: The 30 VM Carburettor (not to be confused with 30VM-n which were confusingly earlier) were fitted to other cars (Commer, Hillman...), but 'the jets will have to be altered and pairs will not be available –- only one side. This will mean that the controls on one side will be on the outside; however controls from an old carburettor could be substituted. The main point to watch is the position of the hole for the Advance/Retard Fitting to the Distributor. Some carburettors use this fitting with a hole to the underside of the butterfly for a vacuum brake booster unit and if this is fitted to a Distributor will give full Advance on starting up. The hole should be on or slightly above the butterfly.

throttle.jpg (10 KiB) Viewed 324 times

Figure 6. Throttle cable arrangement

[Keith Clements]

I have spent a couple of days renovating three C1161 carbs as fitted to later Javs. These have 90-50-110-45-23 set up. All butterflies close well and the spindles are not lose in the fitted inserts. I will trial tune them with Peter's engine later.
I also cleaned up a few C1343s which are the same as the C1316 fitted to the very late Jav and Jup with a 120-65-120-45-27 set up.

The jig used for redrilling of the throttle spindle holes may need some modification as I am contemplating how best to accurately align the chuck. Ideally I would fit the new spindle and butterfly and then align the drill with the centre of the spindle. But the current jig does not allow removal of the butterfly once set up. I may have to make a new jig with a hole in the side.

On inspection of the many carb bodies, I noticed that some of the butterflies had worn the barrels in such a way that the new butterflies could not be inserted.
I checked the dimensions of the butterflies as accurately as I could. New ones are eliptical 30.30mm at right angles to spindle axis and 29.94mm on spindle axis. I will do some more measurements to see how this changes on butterflies removed from the carbs. Looking down the barrel at a strong light you can see if there is any gap around the butterfly when it is closed.

Most of the wear occurs on the brass spindle not the body, but some bodies have a lot of wear as well. A new spindle is 6.96mm. Some of my worn ones were down to 6.01mm.
there was sometimes corresponding elliptical wear on the housing to give a wiggle of over 2 mm, so that would let a lot of air in, but it would also cause the butterfly to wear unevenly!

I am waiting for some new measuring kit which has an alleged resolution of 1 micron, so next week we may see exactly what is what and look at whether the jig needs modification.

[Keith Clements]

In the Tech Notes Mike Allfrey says this.

A dramatic improvement was found to be achieved by replacing progression jet, Item 24, with a smaller one. If no smaller jet is available try filling the original hole with solder or 'Easyfile) and redrilling with a 1 mm drill. This gives a jet orifice of 0.040" as compared with genuine jets – Number 110 1.15 mm (0.045"), Number 120 1.19 mm (0. 047").

Does anybody know the corresponding drill sizes for jet 65?

I need to make 120 jets and 65 jets to match up with the Jup carbs I am now renovating.

Answered my own question here

jetsize thou.ins. nearest drill sizes

40 016 1/64 or 0.40mm
45 018 77 or 0.45mm
50 020 76 or 0.50mm
55 022 74 or 0.55mm
60 024 73 or 0.60mm
65 026 71 or 0.65mm
70 028 70 or 0.70mm
75 030 69 or 0.75mm
80 032 67 or 0.80mm
85 034 77 or 0.85mm
90 036 64 or 0.90mm
95 038 62 or 0.95mm
100 040 60 or 1.00mm
105 042 58 or 1.05mm
110 044 57 or 1.10mm
115 046 56 or 1.15mm
120 048 3/64 or 1.20mm

[Forumadmin]

Just ordered a couple of sets of 20 drills in .05 mm increments from .3mm to 1.6mm so can tune the carbs how we like now.

[Keith Clements]

Found out why Chris' Javelin's carb was flooding. -- float full of petrol!
But also the carbs are not a pair with different jets in them, the vacuum pipe had a stripped thread, and one has very worn throttle and choke spindles.
The choke throttle advance was so worn it did not actually work.
So another couple for the production line.

[BobCulver]

As far as I know jets are adjusted for flow and formed with taper reamers so size is somewhat nominal. Drilled is OK for progression but a bit haphazard for mains, esp if mixed with commercial jet.
I use needles, safety pins etc to gauge bores.

bob Culver

[Keith Clements]

That is a good idea Bob, the taper on the needle would be a good gauge as you could easily measure the distance it enters the oriface.
Hopefully I can get the hole diameter about right (or at least within .025mm) and then try to see what shape the oriface is on a new jet. Some microscopic fettling might produce an approximate jet engine shape. Sorry cannot remember what the mathematical curve is from my fluid mechanics course, probably a hyperbola!