[Adam G]

I had my Holley 780 rebuilt a few months ago and I've noticed the car has developed somewhat of a bog, or stumble off idle as I'm letting out the clutch. After a second or two, the engine catches and the revs go up.

Any idea what might be causing this? Carb idle mixture screws or accelerator pump?

 

[made2cut]

Look at the accelerator pump first. Take off the air cleaner and using a flash light watch the squirter while you open the throttle. You should see a stream of fuel immediately into both venturies. Adjust the pump arm if you aren't getting that shot just as the throttle opens. In my case I had to increase the jet size to fix a bog but if the carb was rebuilt with all the same settings it's probably just an adjustment somewhere. Check to make sure all the nuts (carb to manifold) are still tight since you just installed it recently.

 

[Tom Mobley]

the jets aren't generally involved in off idle stumbles. or are you talking those off-idle stumbles the involve WOT?

as always and forever, these carb problems usually end up being solved in the ignition, likely the initial and total advance. do you know what you have here?

 

[barrrf]

My off idle stumble was cured by installing the accelerator cam on correctly. I installed a yellow cam which has one hole labeled with a #2. I, however, installed it in the number one spot on the arm. So even though the pump arm was resting on the cam, it took too much throttle travel to get it to work properly.

Additionally, I adjusted the accelerator adjustment screw with the choke engaged. So the throttle was back just a ways from hot rest. I didnt realize I had moved the throttle at all so I didnt know I had screwed that up.

Just a couple things from my experience. This was the first carb I had EVER attempted to tune.

 

[made2cut]

the jets aren't generally involved in off idle stumbles. or are you talking those off-idle stumbles the involve WOT?

Tom is correct (as usual ). My bog showed up more during light throttle opening at low speed cruise. Using a huge squirter helped the problem but jumping 6 jet sizes was the fix.

 

[barrrf]

Tom is correct (as usual ). My bog showed up more during light throttle opening at low speed cruise. Using a huge squirter helped the problem but jumping 6 jet sizes was the fix.

I think the stumbles that do involve jetting are ones that you can continue indefinitely with light throttle - the period of time when the accelerator pump stops pumping and before the secondaries are open - or lean cruising.

My initial jetting was 68/76. I found that if I held the throttle 1/4 of the way it would hesitate/pop/stumble indefinitely. Had nothing to do with idle mixture, accel pump, ect. Im at 76/82 now - I feel it might be a tiny bit rich but wont know without Wideband.

So if its a brief hesitation, prolly accel pump adjustment. If you do it indefinitely, like when trying to climb a hill without downshifting, most likely youre too lean and need some jetting (assuming pressure, idle mixture, accel pump, ect is in tune).

 

[Adam G]

Thanks everyone for the suggestions. Appears to be an accelerator pump adjustment required. It only happens when coming off idle slowly to pull away from a standing stop. No problems whatsoever with WOT or aggressive acceleration. The only problem incurred then is keeping the rear tires from going up in smoke

 

[tjstarduster]

Check your power valve. I'm guessing you need a 4.5 with a high performance engine.