Holley Off Idle Stumble

[ProJunk]

I've had good success with the blue pump cam in many street driven applications for curing an off-idle stumble. Here's a couple of other thoughts:

1) You could try enlarging the idle feed restrictors in the metering block. This would increase available fuel flow at tip-in and might cure the stumble.

2) Assuming the carb is jetted correctly, try a higher rated power valve. Those carbs come equipped with a 6.5. Depending on where your idle vacuum is at in gear, you might want to try either a 8.5 or even a 10.5 power valve. This would bring in earlier fuel enrichment through the p.v. circuit in relation to throttle opening/vacuum drop.

In my experience, a squirter change isn't always the answer, and isn't needed all that often unless you have a really mismatched combination on the converter stall speed and/or rear axle ratio. If the squirter "fixes" the stumble, it may just be masking the real issue.

 

[Truckedup]

Depending on engine tune the main jets don't come on line until maybe 1800-2000 rpm.Try fiddling with the accelerater pump as mentioned.
On some engines that go slightly lean off idle,the idle restriction "jet" is too small. This orfice supplies an atomized fuel-air mixture to the idle adjustment screws and the off idle tranfer slots.Adjusting the idle mixture screws changes the amout of pre mixed air-fuel mixture.It does not actually control the fuel only.Anyways,it can be that the off idle circuit is just too lean.If so it's difficult to mess with as the Holley metering block had drilled holes for the idle restrictions.I have drilled these restrictions out a little,.002 at most,they are generally in the .028-.032 range.It's a last resort because once you drill them is expensive to make them smaller.

 

[slimbo]

Depending on engine tune the main jets don't come on line until maybe 1800-2000 rpm.Try fiddling with the accelerater pump as mentioned.
On some engines that go slightly lean off idle,the idle restriction "jet" is too small. This orfice supplies an atomized fuel-air mixture to the idle adjustment screws and the off idle tranfer slots.Adjusting the idle mixture screws changes the amout of pre mixed air-fuel mixture.It does not actually control the fuel only.Anyways,it can be that the off idle circuit is just too lean.If so it's difficult to mess with as the Holley metering block had drilled holes for the idle restrictions.I have drilled these restrictions out a little,.002 at most,they are generally in the .028-.032 range.It's a last resort because once you drill them is expensive to make them smaller.

i drilled and tapped all mine for brass set screws and drilled theset screws till i was too big and then replaced and drilled to the right size. i would also suggest a different pump cam. you could also try richening up the idle a little. the idle has to feed enough fuel to feed the engine until the transfer slots come in. if you pop the carb and look underneath can you see any transfer slot stickin out under the primary blades? you should see what appears as a small square of slot under the front blades. if the primaries are closed too much and there is no slot showing it would have a horrible off idle stumble. as far as going up on the primary jet ...the jets arent flowing till around 2000 and up, before that its fed off the transfer slots. and for the guy who said maybe its the vac secondary spring.....that has nothing to do with an off idle stumble.