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Old 10-23-2011, 03:25 PM
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Back Fire 350 V8

Hi Folks

I am new to the forum but not wrenching. I have a friend with a "72" chevy truck with a built 350. I dont know the actual build but what I can see is this. Holley 750DB w/ hiperf electric pump on a Edellbrock dual plain manifold, headers, camel hump heads, Mallory HEI and a roller cam of unknown specs.
The engine runs great and has no problems of the line under full or partial throttle. The problem is it backfires when it gets to higher RPM. If you are cruising on the freeway at 60 and punch it it does fine for a few seconds than starts to backfire. The timing is about 8 BTDC and a total of 38 BTDC the plugs look good at idle and full power. I have checked all the basics and cant find any obvious problems. He brought the truck to me in frustration as three other shops could not fix the problem. The only real build problem I see is that the carb CFM seams a little big to me. Any ideas?

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Old 10-23-2011, 06:15 PM
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Lean Mixter maybe

Carb may be running to lean
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Old 10-23-2011, 06:30 PM
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Popping thru the exhaust typically means its running rich or incorrect ignition timing. If the popping only occurs at top end i would look to the secondary side main jets or the valve springs are floating.

Verify ignition timing is correct as well as fuel supply and float level. Nail down the basics first.
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Old 10-23-2011, 07:03 PM
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It is actually popping back through the carb. Timing is correct but I did notice removing vacuum advance slightly improved the problem. I have a feeling you are correct that jets on secondary maybe to small as carb is to large for engine as far as I can tell by standard build specs. I will do another high speed run and shut it down to check the plugs for correct mixture.I have done this once and they looked ok as well as the tail pipe soot is a nice dark tan brown.
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Old 10-23-2011, 08:12 PM
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If timing is correct and mixture looks good from reading plugs than i would suspect there to be.some faulty ignition components. Could be spark scatter inside the cap from carbon trails.
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