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34 PICT Mixture issue

Hi, I'm running a Formula Vee in Australia that uses a Solex 34 PICT-3 carby on a 1600 Engine. On the dyno, the mixtures are not looking right, basically, it is running too lean at low RPM, and rich at high RPM. Using the main jet and/or the air-correction, I can tune the mixture correctly for either 5000 and above, or for 3000-5000, but I can't get both looking right. What we are seeing at wide open throttle (WOT) is RPM Mixture

3500 14.8 4000 14.5 4500 14.0 5000 13.8 5500 13.4

What I'd like to see is the exact opposite, leaning out at high RPM, this is what my 32 PICT used to look like (when making good HP and torque) RPM Mixture

3500 12.6 4000 12.8 4500 13.0 5000 13.5 5500 14.0

As we are a controlled class, I have to run the 34 PICT. I have spent many, many hours on the dyno, and not been able to get the mixture slope to reverse. This has included going to silly sized jets (like

2.3mm main and 1.7 air)... and modifying the emulsion tube Currently I'm running around 1.85 main, 1.45 air To get the mixture slope slightly OK, I ran a pilot jet bored out to 1.2 mm... which did make the mixtures sort of better, but of course the car won't really idle (not that it matters much with a race car)

So I have two main questions

1) Has anyone seen this, if so, how was it fixed 2) What exactly does the auxiliary jet do ? it is the only one that I don't really know when it's meant to make an impact on the mixture

Cheers David

Reply to
dfrith
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Reply to
Peppe

Peppe, can you help me identify exactly what are the enritchment circuits ? are they the two brass pipes running at angles into the top of the carby, or do you mean the "auxiliary jet"... (the small one that screws out, with a grub scre covering it - no one seems to know what it does) thanks david

Reply to
dfrith

the brass angled pipes pressed in the top of carb, just above the chike plate these start to work at high RPM and full throttle, but on stronger than stock engines with more vacuum them cause problems to the carburetion. them works almost always, so giving a non linear enrichment. you can solder them shut. those have an air passage and a drilled gasoline jet, but are difficult to adjust. i advice you using a larger intake valve, 170 instead of the stock 150

Reply to
Peppe

David,

Jim of SR Racing here... We build hundreds of these things... (US FST 1600)

Your mixtures are not that bad. I would simply fatten the main jet by .20 or so. For optimum HP and a cool running engine you want it to be about

12.9 to 13.5 from 4500 up. 12.8 at the high end is acceptable. 12.6 is also acceptable at 3500. YOu are never running that low of RPM on the track anyways so it don't matter much. Plus a slightly richer mixture will help to eliminate a dead sport that often occurs arounf there. Your engine produces max HP in the 12.9 to 13.5 area. Anything leaner and you will be running hot valves/heads etc. Anything fatter and you will lose HP. (But don't worry about down low in the RPM)
Reply to
sracing

Good advise Jim!!

Reply to
chircoestore

Hi Don.. If you agree, it must be good advice.

Jim

Reply to
JIM

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