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Old 02-14-2005, 07:35 PM
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Backfire from only one side at idle

I have a ZZ502 crate motor with only about 150 miles on it. It has a Holley 850 and Hooker Headers with a Flowmaster exhaust. The headers have three normal primary tubes and one that is a two piece that wraps around the frame.

The motor starts fine and runs great with only two small problems. First, the engine backfires occassionally (once every minute or two) when idling. The backfire is only out of the right side of the exhaust and seems to happen more often once the engine is warm. This also happens sometimes while driving, but only at low RPM with the trottle shut. Second, the engine sometimes (maybe 1 out of 5 times it's driven) diesels for a few seconds after the ignition is turned off.

The spark plugs and exhaust gaskets are new, and I have verified that the header bolts are all tight. I have checked the timing and it is at 8 degrees at idle.

I am thinking that these symptoms might be due to a small exhaust leak and/or carbon deposits on the valves, but I am looking for some other options before I start taking things apart.

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Old 02-14-2005, 07:42 PM
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It sounds like it just needs to be tuned up. Did you set your mixture using a vacuum guage?

It also could be a rocker arm that is adjusted to tight.
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Old 02-14-2005, 07:49 PM
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I did not have a vacuum gauge to set the mixture. I put the screws at 2-turns open as stated in the Holley manual and then tried leaning it out a little from there. Leaning the mixture didn't seem to help.

I wouldn't think that it would be a rocker arm because the engine is brand new and adjusted at the factory. If I can't find any other problems first, then I'll take a look at that.
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Old 02-14-2005, 07:52 PM
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check your timing, vaccume leaks, air fuel mixture and check to see if the valves are adjusted right. too tight or too loose. also, with it dieseling after the ignotion is off that is a timing issue... put a timing light on it and check that. that will probably take care of yout problem and it should run fine after that is set. check the timing first.
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Old 02-14-2005, 07:54 PM
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Go buy a vacuum guage and use it to tune. The by ear method isn't accurate enough. See if that gets rid of the problem.

If the mixture screws aren't doing anything your floats are to high or your throttle postion is wrong at idle.
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Old 02-14-2005, 07:57 PM
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check the timing... that'll take care of your problem
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Old 02-14-2005, 08:03 PM
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Dieseling can also be caused by too high an idle speed
but you should recheck you valve lash, and Vacuum tune the Holley
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Old 02-14-2005, 08:08 PM
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Thanks for all of your suggestions.

I have checked the timing and it is at 8-degrees, right where the manual says it should be.

I have also noticed that the idle speed is not always consistent, it seems like sometimes the throttle doesn't always shut completely when you let off of the pedal and you have to tap the pedal to get it to idle down. Could this have anything to do with it, and if so, what should I try to fix that?
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Old 02-14-2005, 08:10 PM
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throttle linkage

check the throttle linkage to see if its sticking and chect to see if the throttle plates on the carb are closing all the way... they might be sticking open.
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