Carb stumble
Thread Starter
Joined: Jan 2007
Posts: 11,941
Likes: 33
From: Conroe, Texas
Carb stumble I have a 750 vacuum secondary(temporary) that is idleing fine, when i step into it, she slightly begins to rev then cuts out for a half secont to the rocket to life, anyone got any ideas? I love the way my new motor is running, but this is annoying. Got any ideas?
Is it a Holley, Carter, Rochester???? Generally a stumble indicated an issue with the air/fuel. How is it jetted? Does it run rich/lean? Most stumble issues are due to the enrichment stage. The accelerator pump (or similiar function depending on carb) may not be sufficient causing a lean stumble, or you may have the vacuum secondaries set to light which would allow for a lean mixture as the engine tries to catch up to the air. Is the stumble only when you open the secondaries, or is it everytime it comes off idle. A little more info please!
Thread Starter
Joined: Jan 2007
Posts: 11,941
Likes: 33
From: Conroe, Texas
It is a holley, it does it right off idle when you stab the throttle to rev it or just get on it in a straight line. I backed the jest off for a 350 i had it on that was stock, i cannot remember the jetting, i am going to stick my pro form on it tomorrow and see how it does, i thought the pro form was messed up but at the time i tried to use it my pressure was set at 9 PSI, i have it at 5.5 now. I also did raise the float level to find no help, it will stumble and then rocket to life like it is no tomorrow.
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The Holley off idle stumble is usually fixed with adding the larger 50cc accelerator pump diaphrams over the stock 30cc. However if your float level or jetting is not correct, replacing the acc pump will do nothing.
it can be a combo of things,
what model holley, what jets, power valve, and secondary spring you running?
best i can suggest based on the car in gear jabbing the throttle off idle (1500-3000 rpm) is
first verify your float level good like was said, that's always step #1. if anything err it on the high side, a lot of times a stumble can be caused by a low fuel level resulting in a lean out in the transition circuit.
From there, you need to run the correct power valve, the correct spring running vacuum secondaries, and right accelerator pump cam and squirter size. Those are the 3, or 4, things you juggle.
what can happen is,
if you have too light a secondary spring, the secondaries open too soon and cause a lean out.
or if you have too high a power valve # it may be dumping too much fuel too soon causing an over rich condition,
or you accelerator pump can be squirting too little or too much fuel when you give the throttle a quick stab, depending on the cam or the squirter.
If you can accelerate easily and steadily (sort of fast but not that fast) and don't get the stumble then you can be more certain you've got the fuel level, power and accelerator circuits dialed in ok and the problem is being caused by the accelerator pump circuit (that only has an influence when you stab the throttle and there's a sudden change/loss in vacuum).
But, if you get the stumble pretty much all the time then one or more of the other circuits are not adjusted right, then any adjustments on the accelerator pump circuit you make is only a band aid fix and the real problem lies in the fuel level/power valve/ and possibly secondaries.
here's a good article on accel pump, hope this helps.
http://www.stockcarracing.com/techar...ent/index.html
what model holley, what jets, power valve, and secondary spring you running?
best i can suggest based on the car in gear jabbing the throttle off idle (1500-3000 rpm) is
first verify your float level good like was said, that's always step #1. if anything err it on the high side, a lot of times a stumble can be caused by a low fuel level resulting in a lean out in the transition circuit.
From there, you need to run the correct power valve, the correct spring running vacuum secondaries, and right accelerator pump cam and squirter size. Those are the 3, or 4, things you juggle.
what can happen is,
if you have too light a secondary spring, the secondaries open too soon and cause a lean out.
or if you have too high a power valve # it may be dumping too much fuel too soon causing an over rich condition,
or you accelerator pump can be squirting too little or too much fuel when you give the throttle a quick stab, depending on the cam or the squirter.
If you can accelerate easily and steadily (sort of fast but not that fast) and don't get the stumble then you can be more certain you've got the fuel level, power and accelerator circuits dialed in ok and the problem is being caused by the accelerator pump circuit (that only has an influence when you stab the throttle and there's a sudden change/loss in vacuum).
But, if you get the stumble pretty much all the time then one or more of the other circuits are not adjusted right, then any adjustments on the accelerator pump circuit you make is only a band aid fix and the real problem lies in the fuel level/power valve/ and possibly secondaries.
here's a good article on accel pump, hope this helps.
http://www.stockcarracing.com/techar...ent/index.html
