holley stumble

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terry

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hi folks,
i'm having a problem with a stumble starting at about 1000 rpm's to 2500 rpm
it will idle great and have good acceleration untill i put a load on it. if i accelerate quickly the carb responds well and seems to be fine
the carb is a holley 750 with vacumm sec. front jets are 70 and rear are 78.
choke milled off. i can adjust the mixture screws and get 12 inches of vacum and i can kill the motor by turning them in.
the squirter seems to work once i have moved the throttle maybe 1/8 inch.
hope i have given enough info for your input
thanks
terry
 
adjust the pump shot tighter 1st ,then report.

Photo 386.jpg
 
thanks i'll try it tomorrow after work. do you feel that if i have any throttle movement with no gas coming out of the squiter then thats an issue?
thanks
terry
 
first thing to do is check and make sure the screw that holds the pump cam on is in the #1 hole. to adjust the accelerator pump you will need to hold the throttle wide open with the motor off. take needle nose plairs and move the pump body lever down until it stops, be gentle with it. now take a .016" feeler gage and see how tight or loose it slides in between the pump lever and the cam rocker arm. adjust it by turning the bolt and nut to get the correct clearance.
 
thanks 1 wild and bOb
i will try both of your inputs tomorow. at least we all agree that it seems to be the pump shot volume.
terry
 
fishy68 said:
Jets are too small in the primary causing it to run lean off idle.
Click to expand...

yeah could be, but you gotta start with what's obviously wrong.


and btw he says if he stomps it, it go's and when he eases it, it stumbles.

If the jetting was too lean it would fall on it's face and jerk a bit when stomped on at any speed. It would have to be 5+ sizes too small to those results.
 
I missed the part about the accelerator pump not working until you push it 1/8". That's a big problem. Is that a fairly new carb? I ask because both me and a buddy had new Holley's that had problems. The coating came off them and plugged passages. My buddies actually ran ok but had the stumble. It was because the coating came off from inside the accelerator pump area (area where it screws to the front bowl) where the one way rubber accelerator pump valve sealed. When the coating came off it wouldn't seal good anymore. we both called Holley and they sent us brand new carbs free cause of the defects.
 
1wild&crazyguy said:
yeah could be, but you gotta start with what's obviously wrong.


and btw he says if he stomps it, it go's and when he eases it, it stumbles.

If the jetting was too lean it would fall on it's face and jerk a bit when stomped on at any speed. It would have to be 5+ sizes too small to those results.
Click to expand...

Read my repost I was writing as you were writing this. I overlooked the problem he mentioned with the acc. pump not working until he moved it 1/8". My bad
 
fish68

yes it is a newly rebuilt carb by a fellow who has a company in co. called stahle performance. he does great work and prices are good. however this time i am having this problem. but its ok i'll take the learning curve.
bOb- the screw that holds ther pump cam is in the middle or second hole. should i move it to the first hole? looking at the pump cam that would be the hole farestest to the left?
thanks
terry
 
ok folks
i have taken the screw out of the center hole on the pump cam and put it in the first hole ( to the left of center hole) and have adjusted the the pump with the trottle wide open to .015. now it appears to be better but still a slight stumble. should i continue adjusting the pump maybe a half turn of the nut at a time? could it be that maybe my adjustment might need to be closer to .010?
thanks for your help
terrt
 
Terry I doubt adjusting it more will help any. You only need some space so the pump diaphragm don't bottom out. Do you still have to move the throttle 1/8" before it starts squirting? If so there's still a problem there. I've ran into a couple of them where the pump arm didn't contact the pump cam right away and I had to do some delicate bending on it to make it contact the arm so the pump shot started right away. Look real closely at the pump arm and make sure it contacts the pump cam at idle. If it does and squirts right away try a bigger squirter. BTW: What size squirter does it have in it now?
 
what is the feeler gauge clearance between the throttle plate and bore, at idle. Is only .04 of idle slot exposed underneath plate at idle?
 
usually a 28 shooter with a red pump cam on #1 setting is good for this type of app, not to be too general.

But you should either go to the next size bigger shooter or try the #2 setting again, see you should only change 1 thing at a time so that you can better pin point the problem.

so with the tighter pump adjustment the #2 setting which gives more initial shot from that same shooter might be the fix, if not then back to #1 setting and the next size up shooter.
 
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