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Old 12-15-2011, 11:54 AM
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Carb stumble

Okay guys what is the checklist for identifying/correcting cause of a "tip-in" stumble? At speed under load, there is an obvious and irritating "hesitation", "stumble" when I go to accelerate . It gets past it and will accelerate fine, but trying to get this thing smooooth. Timing, accelerator pump???
It seems to run better with more inital timing ~20 and the vacuum advance disconnected. Better than when I set initial at 15 and use PORTED vacuum.
Proform says the distributor has 24 degrees mechanical...
Carb is a 770 Holley Avenger (vacuum secondaries) ; max vacuum of about 14 at idle with each mixture screw turn out 1.5 turns. Would changing the color spring out help? Holley tech paper says it is set up with the spring that gives best all around performance and economy. It se
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Old 12-15-2011, 03:23 PM
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What cam you have in that motor? Stock. Check Accelerator pump first, make sure you have 0 clearance between pump arm and plunger. You may also have throttle plates open too much creating a "tip in" issue. With radical cam you may have to drill throttle plates.
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Old 12-15-2011, 03:30 PM
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hello

If your DP bogs going WOT at low end, wait until you put a vacuum secondary on! The bog is a lean bog that gets covered up by the pump shot. Double pumpers are nice because they give you 2 pump shots. On a vacuum secondary you only get one. I have never had good luck covering up that lean spot using a vacuum secondary carb (on a higher performance engine). The solution is usually to delay the scondary opening (with a heavier spring in the vacuum pod) which means some loss of performance. i did not write this .
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Old 12-15-2011, 04:33 PM
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@ Richiehd; not a stock cam. Duration at .500 is 234/224 and gross lift is .569...nothing too radical. Has about 14" of vacuum at idle.

@Corybuice; I did not read it ...lol
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Old 12-15-2011, 04:47 PM
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Originally Posted by Hwyhogg
@ Richiehd; not a stock cam. Duration at .500 is 234/224 and gross lift is .569...nothing too radical. Has about 14" of vacuum at idle.

@Corybuice; I did not read it ...lol
Sounds like you need a different color acc. pump cam.
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Old 12-15-2011, 07:27 PM
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Pull that carb off and see how far open the throttle blades are. If you are past the transfer slot (rectangular slot) you need to drill a small (1/16) hole in each primary butter fly so you can close that down some.You should also make sure the thottle blades are very slightly open on the secondaries also. Try a bigger pump shot, if you have a 32 go up to a 34 squirter. MAKE SURE you have instant pump shot when throttle is opened, I have seen brand new carbs out of adjustment.
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Old 12-15-2011, 08:01 PM
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yes this is a brand new carb...only adjustments I've made are on idle mixture screws and idle. BTW, I'm sure the blades are slightly open as there is some vacuum on the ported port at idle; however, I have to have the idle screw set this way to maintain a decent idle with the initial timing under about 20 degrees. I'm hoping to get it dyno tuned and the fuel/air mixture checked soon, but wanted to try to save some of the dyno time. I'm new to carb tuning so I'll be going slow. Obviously would like to start with the easiest to hardest fixes. Do guys with edelbrocks have these problems out of the box?..lol Thanks for the direction guys.
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Old 12-15-2011, 09:59 PM
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What size engine you got?
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Old 12-16-2011, 09:22 PM
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Does this post make me look dumb??

Sqzbox it is a 460, (.030 over so a 466 now )


Okay, To all who took time to help me I apologize. It wasn't wasted, because I NEED to learn all the information you guys provided. When I started double-checking everything on the carb, throttle adjustment, mounting nuts, etc. I noticed the accelerator pump arm was rubbing on the aftermarket throttle bracket. I never noticed it because I never felt any binding or resistance and there was a notched out area on that part of the bracket, so I ASSUMED it wouldn't make contact. I will try to attach a picture. Sooo, I removed bracket and filed the area down until NO contact was being made. Took it back out on the road and that solved probably 95% of the "stumble". So there you have it, me in all my "let's make a problem where there is none" glory. So as for now, I gotta say the Avenger 770 has really been pretty dang close "right out of the box".
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