SU carbs

[David Kemp]

Thanks everyone, I have been reading the web & the more I read the more it seems ,the best carb is the one you know the best.
But everyone seems to think if you are going to drive your car , junk the Zenth's.
It is looking like webers for me as they are available unlike downdraft SU's.
Where did you find 30 Webers, from what I see only 34's are now made.

[Keith Clements]

There are a few people who have good results with Zeniths once they have been rebuilt, so do not junk them too soon. You need to consider the period look and resale value of cars with visible modifications.
There will also be issues with competition regulations in concourse, race and rally. Some people even accept the challenge of getting them working properly as it is just part of the old machinery.

[aj4cks]

The Webbers will not be detrimental to resale values and look authenticas well as improving performance.

[jowettgeoff]

I agree with Keith that a good set of Zeniths, properly adjusted, are fine. But if the engine has been tuned, it's probably better to go for Dels, because there is a wide selection of jets and emulsion tubes available, so they can be accurately set up for different engine specs (eg larger capacity, reworked heads, different camshafts, modified exhausts etc). Not sure what the jet availability is for Webers, but I'm sure Dennis would have a good idea. Agreed, the Webers do look a bit more 'period' but the Dels are perhaps somewhat more sophisticated when it comes to accurate tuning.
G McA.

[David Kemp]

Looking at carbs again it seems cheaper to by a progressive twin throat carb than two single throat carbs. I think it may be simpler to tune as well. any comments on this idea?

[Forumadmin]

Do you mean using a single carb like a Bradford with long inlet tract (like your earlier post suggested) or using two of them, perhaps even separating the throat in the head? Whichever the Jav/Jup engine firing order does not make it easy to balance the load on the carb or any air filtration system. The ideal is a separate carb on each cylinder, anything else is a compromise. But then you have to know the max power revs you are aiming for and the efficiency of your gas flow design to get the best out of the arrangement.

I am just doing some carbs for Bob Wall but discovered one of his 'pair' was stamped 10HP (no C number) and had different jets and emulsion block to the other which was a C1161M. Anyhow I will try to match them up or will find another C1161M so at least he will have a pair. This is not the first Jowett I have seen with non matching carbs.

Interesting fault on Chris Spencer's car on Monday after battling with the refurbished dizzie I thought one carb was being starved, so took the bowl off. Put it back and then when the car started the carb was flooding. The problem was the float was getting stuck in the bowl. Perhaps it had been dropped and distorted the bowl, anyhow some fettling restored operation.

[David Kemp]

I was thinking more of a twin throat progressive carb similar to , cortina Subaru .
Why do you think twins are the way to go given other flat fours (subaru) used both twins & singles , but settled on a mid mounted twin throat carb on a long manifold?

[PJGD]

Having a bank-to-bank manifold makes sense on other flat four engines that don't have siamesed ports.

Bridging cylinders 1 & 2 and 3 & 4 with separate tracts with either two separate carburettors or a single with two barrels makes a lot of sense; that way the suction pulses are evenly spaced in each tract.
In our case with siamesed ports and a bridging manifold, you will get two pulses on way, then two the other way, and because air has less inertia than fuel droplets, the first of the two cylinders will end up a little lean of stoichiometric and the second cylinder will be a little on the rich side. Thats not the end of the world, but it is not optimum when tuning for power, fuel consumption, or durability. Also, as pointed out in my earlier post in this thread, I would not expect SU carburettors on each bank to take kindly to this breathing characteristic, particularly at low speed.

Philip

[David Kemp]

Thanks that makes some sense , a Reece fish or Minnow was meant to be a good choice for siamese ports , most notably Mini & BMC A series engines. Any one had experience with these?
New ones are being made but seem expensive to me , Webers would be cheaper or possibly fuel injection.

[BobCulver]

Almost anything of appropriate size can be adapted. Presumably larger venturi for more top power the aim. But optimising jets can be a long and tedious process unless info available (or an oxy sensor) I had endless trouble with pocketed valves, now realise due mainly to weak mixtures. Finding a similar capacity 4 cyl car with similar timing, split manifold etc a guide to jets (ie for Solex, refer Singer etc) Proven set ups ideal. With Zeniths the innards of the emulsion block vary from application to application.
Apart from the unstreamlined venturi, what is wrong with the Zenith? Do alternatives all have pumps and benefit from? Are the earlier VIG carbs smoother? Venturi for these can be changed. I have machined venturis on Austin Seven carbs but a lot of careful work for an extra 2mm when carbs with replaceable venturis available.
For some reason now forgotten I ran my Javelin for years on Solex 32PBIC with 23 mm venturi ,equiv a larger zenith. With 25mm venturi the throttle seemed to lose progressiveness. All the pump functions were eventually blocked off. The simple single choke Webers seem similar.
Can be a lot of work in adaptor pieces, linkage reversal etc
The pursuit of power can lead to a car not pleasant to drive normally.
On other cars have found SUs veru smooth. But dd SUs uncommon. Without info tuning would be an exercisse.
Recommendations for hot siamese port VWs may be of interest.

[george garside]

Have only just looked through this thread as tend to only look regularly at the pre war and Bradford bits.

The one and only Jupiter I owned in the sixties came to me fitted with 2 downdraft SU carbs . The result was an extremely smooth engine that would manage 40 mpg on fast trips eg Manchester to Bristol in 3 hours . It would also see off MG a's on the mootrway without any trouble.

There were 2 snags,
1. the choke (jet lifter) would only function on one carb as the sideways facing lifter arm fouled the dynamo on the other. This made winter cold starting a problem sometimes resorting to stuffing a rag down the top of the offending carb.

2. Experimentation with needles was probably needed as you could have either smooth fast performance with 40 mpg and unreliable tickover , or richen it up to get a tickover but lose top end performance.

I didn't persue the matter as I sold the Jup to buy an immaculate 1938 Jowett 8!!

george

[David Kemp]

Thanks, reading a bit more on downdraft SU's it seems due to the position of the slide, it gets worn on the bottom causing idle & tuning problems. Sounds like the problem you experienced.
Most of these downdrafts appear to be pre war , on Singers etc , I think finding an unworn one would be very difficult.