Wants to stall from idle when gassed then ok

[JPL]

Hi all, real quick, 440 w/ 750 Holley vac secondaries/electric choke. Starts up fine, runs fine, revs nice while in park, but when standing still in gear & then given gas, it wants to stall, then it catches itself and is ok after its rolling........

[John_Kunkel]

Off-idle stumble is usually caused by a lean mixture, either the idle is set too lean or the accelerator pump isn't functioning correctly.

With the engine off and the choke open, look down the primary bore as you open the throttle...there should be a healthy stream of fuel from both pump nozzles.

[JPL]

Yup, I looked and there is a healthy stream of fuel from both pump nozzles, just wondering if they are too big or too small............

[John_Kunkel]

In some cases where the butterflies are open too wide at idle, the transition ports are uncovered at idle and this will also cause a stumple. Try enriching the idle mixture and closing the butterflies a little...IOW, use the mixture screws to set the idle instead of the lever stop screw.

[JPL]

In some cases where the butterflies are open too wide at idle, the transition ports are uncovered at idle and this will also cause a stumple. Try enriching the idle mixture and closing the butterflies a little...IOW, use the mixture screws to set the idle instead of the lever stop screw.

Should I try this first or change the nozzel? It's running pretty good as far as mixture, a little rich if anything.....

[JPL]

Going to change the accelerator pump nozzle from a .028 to a .037, I think I should have done that when I changed the accelerator pump from a 30cc to a 50cc, fingers crossed......

[JPL]

Whelp, changed the nozzle to 37 and still have the same problem, and yes when I look into the carb I can see the gas squirting through the nozzle............

[Paul G]

Sometimes the simplest thing can cause this type of bog just off idle. Check your accelerator pump adjustment.

Quote from Holley website

"The first adjustment to check is the clearance between the pump operating lever and the pump diaphragm cover's arm, at wide open throttle. This clearance should be around .015". The purpose for this clearance is to assure that the pump diaphragm is never stretched to its maximum limit at wide open throttle. This will cause premature pump failure. Once this clearance has been set take a good look at the pump linkage and work the throttle. Make sure that the accelerator pump arm is being activated the moment that the throttle begins to move. This will assure that pump response will be instantaneous to the movement of the throttle. These adjustments can be made by turning the accelerator pump adjusting screw that is located on the accelerator pump arm together with the pump override spring and lock nut."

There is lots of Holley tuning help on Holley's website

http://www.holley.com/data/TechService/Technical/Carburetor%20Tech%20Info.pdf

http://www.holley.com/TechService/Library.asp

[Chryco Psycho]

Is there any play in the linkage to the pump , it has an adjuster bolt on the arm , there should be no Play at all .
The cam on the throttle shaft could be worn or a better cam profile could be used to got more immediate squirt

[JPL]

Yeah, that stuff is all tight and pretty new, anyway I went from a 23 accelerator pump nozzle to a 37 with no difference, you would think that if the thing was starving you would see some sort of improvement with such a big jump.....

[Paul G]

Did you rebuild the carb or do something else prior to having this stumble problem occur.

[JPL]

Did you rebuild the carb or do something else prior to having this stumble problem occur.

Nope. This happens right off idle, lightly off idle, if I press the pedal about 1/4 way its fine, a quick take off, slight stumble but alright, but lightly from idle it will die, & I looked down the cab and it definitely squirts from the accelerator nozzle & upped the nozzle from a 23 to a 37, no difference, maybe I should rest the accelerator pump arm against the bottom of the diaphragm instead of having a slight clearance........

[gtx6970]

ck it for a vacuum leak

[Chryco Psycho]

so you are saying you have slight clearance between the pump arm & Diaphragm . there should be NO clearance , extend the bolt in the pump arm until all clearance / play is removed , the diaphragm has to move with any movement of the throttle shaft

[FLG]

so you are saying you have slight clearance between the pump arm & Diaphragm . there should be NO clearance , extend the bolt in the pump arm until all clearance / play is removed , the diaphragm has to move with any movement of the throttle shaft

Yep, needs to be touching but not too much as Paul wrote earlier...you should be able to just get a .015" feeler gauge in between the arm and the diaphragm without much trouble.

[Paul G]

so you are saying you have slight clearance between the pump arm & Diaphragm . there should be NO clearance , extend the bolt in the pump arm until all clearance / play is removed , the diaphragm has to move with any movement of the throttle shaft

Yep, needs to be touching but not too much as Paul wrote earlier...you should be able to just get a .015" feeler gauge in between the arm and the diaphragm without much trouble.

That is correct. That .015 is at wide open throttle I believe. They just want a little clearance at wide open throttle to make sure the diaphragm is not bottoming out. Should be no clearance at idle.

[Musicman]

They just want a little clearance at wide open throttle to make sure the diaphragm is not bottoming out. Should be no clearance at idle.

This is the correct statement