Backfire Issue - First Generation Pontiac Firebird (1967 - 1969) - Firebird Nation

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Backfire Issue


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#1 BlueDuck

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  • Engine: 455
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Posted 12 May 2020 - 09:32 AM

455 aluminum headed Firebird.  Fresh from the rebuild and have put around 50 miles on the motor.  After I got my carb adjusted I decided to hook up the distributor advance can to full manifold advance.  The car started better then it ever has, no cranking.  Got it warmed up, went to get out to adjust the curb idle speed and the carb backfired and died, no sooner than I opened the door.  Unhooked and plugged the vacuum line and the car would not idle.  Since then I have replaced the power valve.  I did observe that BOTH sight glasses had the fuel half way up after I adjusted them to the bottom, with the strong odor of gas.  Also, a day later I checked the carb and the fuel was at the bottom of sight glass.

 

What would cause this random backfire?  Is it possible both needle and seats had crud?, bad floats? Also, my proform 750 has blowout protection, would this still allow a blown out power valve?  I am stumped as to the backfire.  

 

Thank you,

  



#2 sea dog

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Posted 12 May 2020 - 11:07 AM

Backfires are usually caused by very lean mixtures, improper crank to cam timing, or ignition timing way off. There are other causes but these are the most common.

 

Have you checked your spark plugs for proper mixture. Is ignition timing correct? And since engine was just rebuilt, could crank to cam timing have slipped a tooth or 2?

 

Have you checked for vacuum leaks? They can cause very lean running if leaks are bad enough. And your carburetor, could it be malfunctioning giving a very lean mixture? Again spark plugs can tell the story.

 

And a vacuum gauge can be a good tool to diagnose running problems. If you don't know how to interpret vacuum readings, there are sites on web that can help you. I'm betting there are youtube videos on the subject also.



#3 BlueDuck

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  • Engine: 455
  • Transmission: 4 SPD

Posted 18 August 2020 - 10:02 AM

This issue ended up being a BAD MSD READY TO RUN distributor.  






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