acceleration stumble

[Jerry G.]

Ran the race car in a local race this weekend. Used a new holly clone carb( qwickfire) . went with standard jetting for this elevation but ran into a stumble at 2/3 throttle. Up to that point it was fine and at wide open throttle it was fine. Looked at plugs and burn looked normal. I suspect the accelerator pumps on this unit are a little small. Anyone else have an idea? Jerry

[Clem Z.]

Re: acceleration stumble

Ran the race car in a local race this weekend. Used a new holly clone carb( qwickfire) . went with standard jetting for this elevation but ran into a stumble at 2/3 throttle. Up to that point it was fine and at wide open throttle it was fine. Looked at plugs and burn looked normal. I suspect the accelerator pumps on this unit are a little small. Anyone else have an idea? Jerry

does the secondary have a power valve and if it does remove it and jet up to compensate. calculate the area of the jet and do the same for the area of the power valve restriction add the 2 together and put in a jet equal in area. a lot of road race carbs use a power valve to get better fuel mileage and if that is needed i would try a 10.5 power valve so it opens sooner

[Jerry G.]

Re: acceleration stumble

Hi Clem. The secondary does not have a power valve. The primary does. I use a 10 jet number spread between front and back. Smaller in front. Would I do better removing the PV on the primary? Jerry

[Clem Z.]

Re: acceleration stumble

Hi Clem. The secondary does not have a power valve. The primary does. I use a 10 jet number spread between front and back. Smaller in front. Would I do better removing the PV on the primary? Jerry

no leave in the primary but just going up 10 steps in jetting is not the correct way to get close to what you did have with a power valve in the secondary. the way i posted is the way to do it. it may come out 10 steps or it may come out 8 steps.

[Joe C.]

Re: acceleration stumble

Ran the race car in a local race this weekend. Used a new holly clone carb( qwickfire) . went with standard jetting for this elevation but ran into a stumble at 2/3 throttle. Up to that point it was fine and at wide open throttle it was fine. Looked at plugs and burn looked normal. I suspect the accelerator pumps on this unit are a little small. Anyone else have an idea? Jerry

You mean QuickFuel. Damn high quality carburetor. Cast aluminum and runs very cool. Refresh my memory about your engine size, IDLE and CRUISE vacuum, cam, max RPM, header primary length and size, intake manifold.

I run a QFT750 on my 331. Came out of the box with 72/82 main jets, 6.5 in-hg primary PV, no secondary PV. It is a double squirter with no choke horn. The plugs are a little bit dark, and I haven't yet started to dial it in, but I anticipate going to 68/78 jets for starters. I am spoiled already because the slight rich mixture makes a cold engine start immediately and run without dying when cold (no choke). The thing pulls like a freight train with no stumbles or flat spots anywhere. Engine had slight detonation at cruise tip-in with the old 2818......but no more, with timing set for full power. 93 pump octane, no additives necessary.

[Timothy B.]

Re: acceleration stumble

Jerry,

From reading your post I assume the carburetor did not come with a secondary power valve.

When the secondary butterflys are opened in the last third of throttle opening, could you be running out of acellerator pump shot. Nozzle size two big, pump cam duration not long enough. Sounds like a gulp of air with nothing to smooth change over to secondary main circuit.

[Clem Z.]

Re: acceleration stumble

is this carb a double pumper or a vacuum secondary ???

[Jerry G.]

Re: acceleration stumble

Double pumper. Mechanical secondary. Engine requires 700CFM which carb manufacterer say this carb will do.

[Clem Z.]

Re: acceleration stumble

Double pumper. Mechanical secondary. Engine requires 700CFM which carb manufacterer say this carb will do.

check to make sure the secondary pump is squirting as soon as the secondary butterflies are starting to open.

[Jerry G.]

Re: acceleration stumble

Will do. I think it may be too small. Any ideas on how to determine correct accelerator pump size?

[Dick W.]

Re: acceleration stumble

Will do. I think it may be too small. Any ideas on how to determine correct accelerator pump size?

If it is set properly and you still do not get enough fuel, you can install the "Reo" pump body to get more fuel. It is quite a bit larger than the stock

[Clem Z.]

Re: acceleration stumble

check to make sure the secondary pump arm is no bottoming out on the intake manifold. some stock chevy intakes were not clearanced for double pumper carb pumps.