750 + 400 ci = too much (now with vid)
#564724
12/28/09 12:51 PM
12/28/09 12:51 PM
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Posts: 472 Hartselle AL
67MIke
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I have been plagued with this problem since I built the motor. The motor is a 400 stock bore, 256 comp cam, manifolds pretty much all stock. Well I have a bad stumble right at 950 t 1100 rpm's. I have adjusted timing up down etc. Changed the pump cams, bigger seems crate a backfire. no difference with a smaller cam. I reused this carb from 440, doesn't seem like I had any problems when I used them together. Thought it might be timing, but still cant get it tuned right. Any ideas - Oh yeah it screams after 1100 rpm's and runs great Thanks
Last edited by 67MIke; 12/28/09 07:56 PM.
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Re: 750 + 400 ci = too much
[Re: 67MIke]
#564725
12/28/09 01:02 PM
12/28/09 01:02 PM
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Anonymous
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Anonymous
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holley? check for a leaking powervalve. if thats ok check the idle mixture. the transition from idle circut to the high speed circut can be a pain if not properly tuned. holley's never stay in tune for long.
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Re: 750 + 400 ci = too much
[Re: 67MIke]
#564729
12/28/09 02:13 PM
12/28/09 02:13 PM
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Joined: Jan 2003
Posts: 5,444 Indiana
YO7_A66
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master
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""Well I have a bad stumble right at 950 to 1100 rpm's.""
You need to hook up a vacuum gage to read manifold vacuum while you are driving. Just add a long piece of hose from the carb into the cab so you can watch the gage while driving at the 950 to 1100rpms. Then you can rule in/out the power valve. Off idle rpms like 950-1100rpms would point to needing a larger squirter size to me. I would try the next size up on the primary squirter and then retest the pump cams.
1970 YO7 A66 [Canadian Export] F8 Challenger 340 (Currently in shop for stroker assy.)
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Re: 750 + 400 ci = too much
[Re: YO7_A66]
#564730
12/28/09 02:42 PM
12/28/09 02:42 PM
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Joined: Jun 2005
Posts: 4,041 michigan woods
imfixinmopars426
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first,ck the pump arm and adjust. many times holleys get put on the bench,or shelf,and the pump arm[s] get bent,and you get a mis-adjusted pump clearance. w/o proper adjustment,it will have a stumble off idle,w/o a "pump shot" to raise rpm's. make sure there is 0 clearance at idle,and NO bind at WOT. check manuals for full descriptions on adjustment. also ck mech advance,and is the timing at 35-38deg all in at 3k rpm or so? any vacuum leaks? lean cond will cause soggy low end also...good luck,mike
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Re: 750 + 400 ci = too much
[Re: Dodgem]
#564732
12/28/09 03:13 PM
12/28/09 03:13 PM
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67MIke
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dual feed vacc. secondary 4160 One thing that might matter, is I pulled apart an old 400 dizzy that had lighter springs as the 400 i have had a centrifugal advance ( i think) not sure on the total timing, just have a standard light, plan getting tape to check it, initial timing is around 12-14. I set it here, drove it, and backed timing down until I had a good combo. of driveability, starting and no pinging... Thanks - Mike
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Re: 750 + 400 ci = too much
[Re: 67MIke]
#564733
12/28/09 06:14 PM
12/28/09 06:14 PM
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Joined: Nov 2003
Posts: 36,041 Lincoln Nebraska
RapidRobert
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Circle Track
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what's your in gear idle speed? and in gear vacuum? What # power valve. yes as said add a vac gauge & note the vac when it hesitates. Is this from part throttle or stomping on it? ported or manifold vac adv? EDIT any possibility of a vac leak?
Last edited by RapidRobert; 12/28/09 06:15 PM.
live every 24 hour block of time like it's your last day on earth
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Re: 750 + 400 ci = too much now with vid
[Re: AndyF]
#564735
12/28/09 07:52 PM
12/28/09 07:52 PM
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67MIke
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Thanks for all the input, I hope I have a vid that might help out just ignore me being stuck and the loose belt I'll double check the carb when i get it out power ram
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Re: 750 + 400 ci = too much now with vid
[Re: 1fastrunner]
#564738
12/29/09 01:10 AM
12/29/09 01:10 AM
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Joined: Jan 2003
Posts: 27,421 Balt. Md
383man
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Alot of things can cause a stumble and most of the time it's a lean condition. But also make sure the idle is not to high that the transfer slots are uncovered in the carb. This can cause a problem as it will have you leaning out the idle screws since the transfer slots are already starting to pull fuel. Then when you step on the gas you dont get the extra fuel that is used in transition from idle to main metering system. Thats why alot of guys drill holes in the throttle plates to give it the extra air for the higher idle but still keep the throttle plates closed enough. We use a 750 DP on my sons .030 over 400 in his Dart. He tried and 830 anular carb and an 850 DP but it still ran its best with the 750 DP. Ron
Last edited by 383man; 12/29/09 01:12 AM.
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