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Backfire on Overrun - Mixture or Timing? (archive)

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Posted by Hamish on March 07, 2000 at 13:01:04:

My 76 (32/36, pertronix, blue coil, no emissions eqpt) is increasingly starting to backfire on the overrun. My question is, is this a timing, or a mixture issue?

The engine conditions are one of high vacuum and low load. With the throttle plate closed, this would make the idle circuit the primary factor in how much gas reaches the engine. However the higher than normal vacuum will pull extra fuel through and increase the mixture richness, correct? The fact that it is backfiring indicates to me that all the fuel is not being burned and is making its way out the exhaust valves.

Though I have removed the smog pump and thermal reactor, my 76 still has its vacuum retard lines attached (whether they are actually functioning properly I cannot say). Is it possible that the higher vacuum is actually retarding too much and causing this problem? Would I be better off disconnecting the retard altogether? Or is it a carburetion issue after all?

Thanks for your help.


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