i have about 300 miles on my new combo and it runs great other than seeming to be lean in the transition circuit and accelerator pump circuit.

Combo is ; 400 .030 over, Aussie Cleveland heads ~ 10:1 comp XE 262 cam, springs, lifters, retainers and timing set. Stock distributor vacuum advance disconnected and re-curved with lighter springs - 12* initial. Edelbrock performer intake, brand new Holley 4160 vacuum secondary. Hedman headers, 2-1/2" duals with flow master 40 copies.


Seems to have an off-idle lean stumble when accelerating gently. I've backed the idle mixture screws about 1/2 turn out which helped a tiny bit. Seems to have a lean stumble sometimes when going WOT. Changed the squirter to a 35 and then later the pump cam to blue and re-adjusted the linkage.

Thought the WOT acceleration was good, checking from a standing start. Then I went WOT on the interstate on-ramp and it stumbled. Let out a little and got back in it and it cleared up and went.

Idle is not overly rich, I'm thinking about backing IMS screws out more. Problem is worse when the motor isn't fully warmed up

 

Have you tried advancing that timing a bit more? The idle mixture settings won't fix a flat spot. I would also confirm that the floats are set high enough. Most flat spots are due to carb problems but some are from the distributor advance.

 

Float levels are good. Nervous about cranking up the timing more but I could check advance operation I suppose. The idle circuit feeds the transition slot, so richening the idle richens the transfer slot fueling

 

Why is your vacuum advance disconnected. For street driving you should use it. You might have to retune after you hook it up. I was recently having the same kind of trouble with off idle response. Tried adjusting the idle mixture a bit with no response then found a bad diaphragm in the advance canister. Replaced it and had to adjust timing due to slight part difference and retuned idle mixture problem went away.

 

Agreed, unless you are WOT all the time, use vacuum advance. This improves part throttle driveability. What distributor is it, and is it new along with the rest of the motor?

 

Vacuum advance is disconnected because I changed the distributor mechanical advance springs for lighter ones and am worried about pinging with the high compression. It's a stock distributor and not new.

No offense, but if I have a lean stumble I think adding more timing will make it worse, not better. Your bad diaphragm probably caused a vacuum leak; hence the stumble.

 

It sure sounds like a weak pump shot, but also sounds like you have a handle on that. Idle mixture won't/shouldn't have any effect.

Vacuum advance only comes into play at steady cruise/high engine vacuum vacuum. Always use vacuum advance though, unless at WOT all the time.

 

I don't think I've ever had an "new, out of the box" Holley perform perfectly. I mean, they are sized the same for a million different engines. If your having a "lean" stumble, and your positive it's not timing related, she needs more fuel. Increase the jet size? Your already are pumping more fuel out of the accelerator pump? Did the accelerator pump mods change anything?

 

This is the part where you fine tune with a vacuum gauge. Dial in the carb, dial in the dizzy...
Jefffafa over in the 67-72 kingdom is VERY knowledgeable on Holleys

 

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What's the news on your stumble?
I was curious, if you close(screw in) both air/fuel screws, does the engine die?

 

Haven't had a chance to look at this further guys. Playing with the accelerator pump certainly helped the WOT stumble and I haven't noticed the slow acceleration stumble for a while. Seems it only does it when I'm putting around

 

Don't worry about playing with the timing. I would advance the timing until it pings and then retard the timing until the ping goes away. Ping for a short time wont hurt anything. 4 to 5 degrees at a time should be safe.