looking for some advise on trying to remove an off-idle stumble on a 600 cfm holley carb (80457). Carb is on a rebuilt 289 in a 77 comet.Car idles fine, but has a major stumble when I lightly tap the pedal. I have cleaned the dishcharge pasages with air, set the linkage cam (orange type) so it taps the accelerator pump right away and changed dishcharge nozzle sizes. Factory was a 31, dropped down to a 28 then up to a 33 and 35. not much change, bog is still there. Accelerator pump membrane looks fine. Should there be a steady stream of fuel even if the pedal is lightly pressed? this seems to be problem as the stream starts off at a minimum value before reaching a full stream. also when changing nozzels, I had notice some "spiting" of fuel. Not sure what this indicates, accelerator pump appears not to have presure at this point. Any help would be appreciated.
Try adjusting the accelerator pump so that it is preloaded a bit if that doesn't work try the other way so it has a little free play.Holley sugessts that it be set so that there is 15 thou clearance between the pump arm and the adjustment screw at full throttle.
I have found that ignition timing also contributes to an off idle stumble. With my street boss combo I am at 18 degrees static (stock boss is 16) my stuble has since left the building.
Yes double check timing as Thack says,,, I would also pull accelerator pump apart and check diaphragm along with the rubber check valve. Squirter flow should not spit but even flow, maybe dirt, when you have pump apart blow air from squirter back down and watch for dirt
You might look at the power valve too. I've had instances where with the wrong size power valve I've had an off idle stumble.
ive had the idle mixture set too rich and had an off stumble. it had been adjusted to what felt to be the smoothest idle. i later used a vac. gage and found that the vac went way up when i started to lean the idle mixture.
My 650 DP does it too. Timing made it better but couldn't eliminate it. I think it's the accelerator pump too. Doesn't do it when not under load.
I bumped up the timing to 12 BTDC. Stumble or fluuter still there. Ended up removing the float bowls and front metering block. Inspected everthing for dirt or clogged passages. Finally repaced the accelerator pump and rubber valve, problem Gone! If I were to guess the real problem I would say it was the rubber valve not sealing even though I thought it had looked good. I had purchased the carb about 6 years ago at an automotive flea market and just got around to installing it now. Carb looked new and was still in the box. The seller said he had wanted to go back to the original carb for his application. but i think he tried to tune it and gave up. The rear float adjustment had dropped down causing the carb to flood and have hard warm start problems. Also the accelerator pump appeared to have very litle slack. I adjusted it backto about 0.015. Does this have to be adjusted with each cam change? This could have damaged the accelerator plug. Thanks for the help offered
