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Old 04-19-2009, 05:58 PM
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What causes an engine backfire?

Just wondering what causes an engine to make that loud "pop" sound and backfire through the intake??

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Old 04-19-2009, 06:03 PM
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bad plug wire/crossed plug wire /timing can be off /fouled spark plug/carb adjustment/sometimes when a lobe is worn on the cam it will pop back through the carb also.could have a broken spark plug.valves to tight. sure theres more all i can think of right now. cole
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Old 04-19-2009, 06:10 PM
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hi there

very loose rocker arm nuts do this also sometimes i have found over the years a few times.

sometimes the threads wear out on the nuts and when they back off then the engine pops all over the place real bad just had this issue on a 79 camaro 305 engine very recently.

it was so bad the owner wanted to pull it and put in a 350

so we did and i put brand new rocker nuts on it and now it runs strong smooth perfect so im sticking it in a 78 truck of mine soon

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Old 04-19-2009, 07:29 PM
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It depends on when it happens,under aceleration on a warm engine is lean fuel mixture.
Try enrichening it a little bit with the engine at normal temp and see if it still does it.
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Old 04-19-2009, 07:39 PM
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It could be valves adjusted too tight, lean fuel mix, sloppy cam chain or improperly installed cam chain, or timing advancing too far. In the case of the last one, the engine won't run in that case, just backfire through the intake when you crank it.
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Old 04-20-2009, 08:22 AM
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If you're using a Holley carb make sure you check the power valve. One back fire and it's history, then you'll have continual backfiring problems. Good luck.

ZAPPER
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Old 04-20-2009, 07:36 PM
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my 67 short wheel base truck does that also. it shoots fire out the exhaust. my exhaust turns down under the cab and its diggin holes in the ground.
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Old 04-20-2009, 08:24 PM
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It is a Holley 670 street avenger carb, basically adjusted as it was out of the box, I haven't turned many screws except for the choke adjustment. It has shot flames out of the carb before I know this for a fact because my face was looking down the carb and I lost my eyebrows!! So you are saying that my power valve is shot because of one backfire?? Could somebody please explain what immediate adjustments should generally be done to a holley 670 vac secondaries right away and how to go about doing these adjustments? I am not very good at carbs to be honoust with you I am not even sure how to enrichen my A/F mixture on this carb. I guess I will need to replace my power valve by the sounds of it because it has backfired through the carb on more than one occasion. I also get a TERRIBLE off idle stumble when I first give it gas...... it sounds like its gonna die then just picks up. Man I really need a manual or something on adjusting this carb because I am so certain it is really not adjusted well for this engine.

Anyway all help is good help,
thanks,
keith
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Old 04-20-2009, 08:38 PM
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Quote:
Originally Posted by ZAPPER68
If you're using a Holley carb make sure you check the power valve. One back fire and it's history, then you'll have continual backfiring problems. Good luck.

ZAPPER

Since the early 90's most holleys have a provision for back fire prevention. This is accomplished with a check valve (ball) in the air channel to the PV.
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Old 04-20-2009, 10:28 PM
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Quote:
Originally Posted by sexypizzaman
I also get a TERRIBLE off idle stumble when I first give it gas...... it sounds like its gonna die then just picks up. Man I really need a manual or something on adjusting this carb because I am so certain it is really not adjusted well for this engine.
Sounds like maybe the accelerator pump is not working correctly. Your carb is a Holley but I had the same problem with an Edelbrock on a 327 that would backfire and stumble off a dead stop. Come to find out the accelerator pump shaft was completely gone. I replaced it with a Demon and solved the problem.
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Old 04-20-2009, 10:53 PM
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Could be timing. Mine was doing this because I was advanced to much. I never would have thought timing because I did not touch it.. but it moved on its own. Spark before fuel.. reset timing and problem solved. Check timing.
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Old 05-08-2009, 10:38 PM
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dont mean to bump this back up...but...dashpot? had that same problem with my z28 for the longest time. put a dashpot on and it went away
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