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Carburetor stumble

Aron Gleason

Well-Known Member
Local time
12:38 PM
Aug 5, 2020
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I recently had my car dyno tuned. It runs alot cleaner, but still has a bad stumble. When cruising down the road I can stab the throttle and it will fall on its face. The carburetor is a modified 950 quick fuel/thumper carburetor. I've had everything from a 750 vacuum secondary carb to the 950. About 5 different carburetors. And they all have had the same stumble. What am I overlooking? I was told by a local tuner to put a 10.5 power valve in it, and to lock the distributor.
What is in this car?

if they all stumble then it’s probably not a carb problem.
284 484 purple shaft cam
Ported 906 heads
msd pro billet 8546 distributor..msd 6a box..hvc coil...msd plug wires.
4500 stall
4.57 gears. Car made power up to 6300 rpms
And I'm thinking it's something else other than the carburetor causing the stumble.
It could use a LOT more initial timing for optimum performance, but you should still be tunable with 20*.

You probably need to increase the PVCR size [ Power Valve channel Restrictions ].
The PV controls when extra fuel is added, the PVCR controls the quantity.

Try increasing the m/jets by 3-4 sizes. If this helps then it is a good bet the PVCR needs increasing.
I should mention I've updated the fuel system with #10 braided hose from the fuel cell to regulator, and a #10 return line. And # 8 braided from regulator to carburetor. Fuel pressure is at a constant 6lbs. When I stab the throttle my afr gauge will go to 15+.
The guy that dynoed the car took out about 3 jet sizes. The stumble was the same before and after the jet change. Would I go with a bigger number pvcr?
When I stab the throttle my afr gauge will go to 15+.
Does it stay there or is this only the initial reading?
Hitting the throttle will always give a short lean signal but should drop back very shortly after, ideally to around 12.8 - 13.0.
If it stays that lean it needs more juice, it could go pig rich and you need to go the other way.
When it stumbles I let off the gas. I've never stayed in it when it stumbled.
I am not sure you are understanding the function of the PV & m/jet. The m/jets control the cruise mixture; the PV adds fuel when the load is increased beyond cruising.
If jet size was reduced to get the cruise mixture correct, then less fuel is in the main system to take the increased engine load that the PVCR adds. So it is very likely that the PVCR needs to be increased, by at least the area of the 3 jet sizes that it was jetted down.
up the squirter/shooter size a few steps and more aggressive throttle cams,,when stabbing the throttle without more gas it will stumble and/or backfire thru carb,,,,,Been there.
And I'm thinking it's something else other than the carburetor causing the stumble.
Now you are on to something! The BIG clue it it does the same thing with 5 different carburetors.

What compression ratio?

how much power and torque on the dyno? 4.5 gears and 4500 converter are too much for 1/4 mile and that tiny camshaft.
whats your vacuum reading at idle? If you have a 10.5 PV and say 9" engine vac that is probably too high and it wont open. Rule of thumb is half the idle vacuum (example 4.5 power valve) but that is just a starting point, have to tune it while driving.
I hope the lobes are hanging in there.
It looks like you have been chasing this issue for a while. It might be time to get a good, wideband AFR meter to give you a better idea of what's going on.

A stumble like you describe is usually too much or too little fuel. Either condition can usually be resolved with a little carb tuning, but it's hard to tell which one you're dealing with. The AFR meter just lets you cheat a little. If the meter shows 9-1 when you stumble, the carb is too rich, if it shows 15 or 16-1 then the carb is too lean.
He says he has an afr gauge on a post.

how much is this car street driven or raced? Freeway or idle around at the cruise night like In Enumclaw? does it overheat? Lean overheats easier.