Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

dirt track carb problem

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glntom:
First a little back ground:
My buddy and I each have a "purestock" car which we run on 1/4 mile dirt tracks. Both cars typically run in the top 3 on any given race night.
They both are on 6.80 rear gear, both are 355 cid SBC, both on .450 lift camshafts, I236/E240@ 0.050. Both of us have been running holley 4412's with no problem. Both cars timed at 34 degrees total timing.
Last weekend we ran at a track that allowed Qjets, so we put our identically prepared 750cfm carbs on .
Both cars were on VP113 fuel.
Both cars ran fine packing the track and hot lapping, but had the same problem in the heat races. About the flag stand they acted like the secondarys cut out, and lost power(around 6500rpm) , then coming out of the corner the secondarys didn't seem to come in and both cars went to the back.
My driver reported sputtering and missing, his driver said it was just like the air valve had been pulled shut by the vacuum dashpot.
 I know some holleys will shut the powervalves due to increasing vacuum under these conditions, could our Qjets be pulling the air valves shut or are we just running the float bowls dry?or something else?

His car has both a holley electric pump at the tank and a mechanical pump on the motor, mine has just a mechanical pump. Both have 1/2 fuel lines to the pumps(mechanical) and then feed though 3/8 line and 6AN fittings.

Hopefully I have explained the problem enough to be clear, if not I will try to provide any additional information needed.

glntom:
Carbs are SMI stage III
Starts great, idles well, good low rpm(under 2500-3000rpm) performance

Cliff Ruggles:
Most likely running out of fuel or venting issues, or both......Cliff

glntom:
Venting issues? tank or carb?
What capacity pump do you recommend?

Cliff Ruggles:
All racing applications require carburetor venting modifications to keep the fuel in the carb under all conditions.

They also need to have larger fuel inlet seat assemblies (and other specific modifications) installed with enough fuel delivery to the carburetor for the power level.

Since I don't know the carburetor numbers, or any specifics about how they were set-up, difficult if not impossible to make any recomendations, etc.

I will say that about 30 percent of my business is setting up carburetors that were prepped someplace else for some sort of "racing" or "high performance" application......Cliff

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