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Old 03-18-2005, 09:15 PM
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Off idle stumble

Rebuilt 355.........
Motor specs:
350 0.040 over, decked,align honed, Scat crank, 9.5-9.6 comp,
CompCams ,1.5 roller rockers, AFR 180 heads, KB Hyperutectic pistons(KB106040), Edelbrock RPM Air Gap ,Holley 770 Avenger
I have tried 3 carburetors, 4010 660, 4778 700 and 770 Avenger. Gap is .50, timing is 12 initial , 38 total.
With all 3 combos I sm experiencing off idle stumble/or stall. If I put my foot in it slowly, the rpm's rise fine, if I stomp the pedal it stalls. Do I have a timing problem, or what.

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Old 03-18-2005, 09:29 PM
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carb

I would have guessed carb but since it is a rebuild. It sounds like a valve setting or wiped out cam lobe.
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Old 03-18-2005, 09:44 PM
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Is the fuel getting to the carb. ok? It could be that something has gone wrong in the pick up or fuel pump and when the gas is stomped on it can't give what is needed and stalls. Does the engine stumble out like a putter then die or does it flat out shut right off?

Chris
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Old 03-18-2005, 09:53 PM
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How big is the cam? Who much does it weigh? What transmission, rear gears, and tire size? If an automatic what torque converter?
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Old 03-18-2005, 10:30 PM
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Brackateer,
I hope I don't have a wiped cam lobe...The idle wouldn't ever be right, would it?

strikingthematch,
7lbs pressure at the carb...when I stomp it it stutters and then takes off..

tresi,
Cam is 218/226@.50 462/480 lift, weight 2400 lbs, 700r4, 3:89 gears, 30 inch tires, 2400 stall..but it does the same, if the car is stationary.

Last edited by AngliaBob; 03-18-2005 at 11:22 PM.
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Old 03-19-2005, 07:12 AM
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You need to tune the carb in properly.
here is Holleys site with a free downloadable manual and troubleshooters guide. Help is also in the Knowledge Base. This is asked a few times a week.
http://www.holley.com/HiOctn/TechServ/TechInfo/FMCTech.html
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Old 03-19-2005, 07:58 AM
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Try this before you rip anything apart. Put the timing light in the drawer that you store it in. Loosen the dist a little and while you stand beside the car rev it and slowly move the dist. Set it for the least amount of stumble. Now tighten the dist and try it on the street. Does it improve or no difference. A timing light is a good reference but sometimes you got to go out in left field. I have done this for 35 years and it works for me.
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Old 03-19-2005, 08:56 PM
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ditto

I had the same problem but I had an edelbrock 750 carb. I simply adjusted the accelerator pump for maximum shot and it went away. Dont know much about holleys, but i would figure out how to adjust the pump and start there. I'll bet you just need to tune the carb properly.
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Old 03-19-2005, 10:17 PM
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Your .050" spark plug gap is a too big. Try .035". If you have an ignition box .045" max.

You need to make sure you have instant accelerator pump shot. AS soon as the butterflies open, there needs to be a strong shot. Bend the metal accelereator pump arm if you have to. Make sure the accelerator pump spring on the arm is stiff enough to start instant shot. Tighten it if it isn't.

If you do have enough shot, make sure you have the right powervalve. If that is right, increase your squirter size.

The Green pump cam gives the most initial shot. The pink cam has considerably less instant shot. The pink one almost always causes a bog on my engine.
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Old 03-21-2005, 04:31 PM
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Thanks to all for the input

I have read all the material at the Holley links and have talked with tech support.
They seem to agree that the accelerator pump nozzle needs to be increased from .025 to .031. I have ordered .031 and .035 and a cam kit. Should be here 3/23. I'll install the .031 first, if it fixes the problem I will start to fine tune with the cams.
Thanks again,
AngliaBob
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Old 03-25-2005, 09:05 PM
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Well, I put in the .031 and I still have a stumble. As a matter of fact, I experienced a backfire thru the card(almost curled my eyebrows and beard). . I looked in the carb during acceleration, and the shot looks good. It seems the main circuit is coming on late, or the shot isn't long enough). I will put the.035 in tomorrow and see what happens. I am running the orange cam now. I am not seeing a puff of black smoke, during the transition.
Thanks,
Bob
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Old 03-25-2005, 09:55 PM
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carb stumble

i have played with these holleys some. Try increasing the primary jets a little at a time. If the squirters dont fix it go up 2#s on the jets each time and take it out to see if that doesnt take care of it. The tech line will sometimes not agree but that damn air-gap leaves a hole in you initial shot. I went nuts and did the same thing you did and finally tried larger jets and it improved. Good luck
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Old 03-25-2005, 10:11 PM
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carb

If that carb is shooting flames. It is not a carb problem. Timing off or valve hanging open.
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Old 03-26-2005, 08:54 PM
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I bought a vacuum gauge and set idle mixture. Went up to .035 nozzles today. The stumble is almost gone. Thanks to Bracketeer, I checked my timing, and total timing was at 45 btdc @2700 rpm. Not correct, I am going to bring it to the professionals, I give up.
Thanks again for all the help,
Bob

PS: I was getting 22 on the vacuum gauge, should I be running a 10.5 PV?
It has a 6.5 PV in it now.
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Old 03-26-2005, 10:50 PM
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Have you tried turning the mixture screws out a litte and taking some of the timing out maybe to 36 total. Do you have a advance kit in it? Did you foul the plugs from the other carb? If you did raplace or sandblast them. Bad plugs will make it run like crud. Hope this helps some
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