??? about lean running motor.

3 replies to this topic
  • huffa

Posted April 13, 2002 - 12:04 AM


I just posted a few topics down about back firing and understand it is a lean condition which is what I was always told. Till I was told though I always thought it was a rich condition (I'm sure there's others out there that think that too) being the case with the backfiring and the flames I figured this is un-burned gas which would indicate a rich condition. So that's what I want to have a good understanding of - Why a lean condtion creates a backfire?

  • Boit

Posted April 13, 2002 - 12:42 AM


Eric Gorr has a site that discusses this penomenom. It has a ton of information about many motorcycle questions.


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  • Taffy

Posted April 13, 2002 - 02:19 AM


there are unburnt gases in any lean or rich engine. the difference is that the lean engine runs at 1300 degrees instead of 800 degrees. this means the metal is so hot it will keep the gas heat high and burn the unburnt fuel where it finds it.


  • Mark_Cantrell

Posted April 13, 2002 - 05:10 AM



When I have had fouling problems, I get big sharp or soft backfires with or without flames out the exhaust while starting. This is because a correct mixture of air and fuel are being exhausted directly into pipe. On the rare times when it does fire, it ignites the fuel in the pipe, backfire. We used to intentionally backfire cars and trucks (because we were young rednecks and it was rural TN) by driving at speed, cutting the key, letting gas build up in the exhaust, and cutting it back on.

All of that to say that a bad ignition or other source of misfiring causes this type of backfiring also.

Maybe Taffy can shed some light on this, but if you have a too rich mixture and some unburned or partially burned fuel gets into the exhaust where will it get the air to re-ignite it. It was way hotter (maybe more than 1000 degrees hotter) in the cylinder right after ignition than in the pipe. If it wouldn't burn in the same mixture there, why would it burn in the pipe? It didn't fire because of wet plug?


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