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Backfire?

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Old 09-22-2007, 10:47 PM
  #1
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Join Date: Feb 2007
Location: Boardman,Ohio
My Boats: 1986 Imperial
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Default Backfire?

Any good engine guy may know this one. I have a 6 year old GM350 HO crate engine with a Holley 750 on it. I was on the Ohio river today running good all day. I increased rpm's to about 4900 which is just about WOT when I heard a thump and it practically shut down the engine. I thought I hit a log of some kind that was floating in the water, but found no evidence of that on the outdrive. I now think it was some kind of backfire. The engine did run fine after that noise.
The only thing I can think of is about a month ago I did replace the front needle valve,float and gaskets. Bowl levels were fine and mixture screws were adjusted. Is it possible the mixture is wrong? Would that cause a backfire at high rpm? This engine has been perfect every season and is still running good. I just want prevent another backfire if that's what it was.

Thanks
Tom
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Old 09-23-2007, 12:05 AM
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The mixture screws only count at idle and are not able to cause a high RPM misfire or backfire. Possible lean condition if the backfire was at full RPMs out of the carburetor.. Check fuel pump and separator. Dirt in the float bowl can plug-up a main jet orifice. Many times a carb backfire will mess-up the Holley power valve and allow way too much gas to leak into the intake manifold and the idle will be erratic.
Good Luck, hope this helps.
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