Topic: backfire question

[wsup55]

i've been experiencing a fast popping type backfiring under decel on my 77 750K.  Today i discovered imperfect seals on the rubber tubes into my airbox.  The rpms at idle will change slightly if i spray cleaner on the seal.  Questions: Can an imperfect seal here be the culprit behind my backfiring?  Are there any tips besides softening them in trying to get a good seal?  For softening, do you go about it in the same manner as the carb boot thread in tips & tricks?

Thanks

Darren


edit for improper terminology

[eurban]

Small air leaks between the airbox and the carbs won't typically cause lean running.  Leaks between the carbs and the engine will.  If you don't find air leaks to be your culprit, you will likely have to go through the carbs.  The idle mixture screws may be improperly set or more likely you may have crudded up idle circuits

[wsup55]

carbs were my first suspect.  I just had them off and everything is looking ok.  That's why i've started poking around trying to eliminate any other possibilities before i pull them out again.  The bike is running very strong in every other respect and the plugs have good color but when it's time for decel pop...pop..pop.....pop

The connections between the carb and engine look good.  Is there anything else that can cause it to go lean on decel or am i basically down to just carbs?

[BlackMax]

carbs were my first suspect.  I just had them off and everything is looking ok.  That's why i've started poking around trying to eliminate any other possibilities before i pull them out again.  The bike is running very strong in every other respect and the plugs have good color but when it's time for decel pop...pop..pop.....pop

The connections between the carb and engine look good.  Is there anything else that can cause it to go lean on decel or am i basically down to just carbs?
[/quote



I'd say "POP POP" while decelerating (very common condition in motorcycles) is usually caused by a lean idle mixture.  Fuel isn't burning in the chamber due to the lean setup (lean burns slooow), and is finishing in the exhaust..thus "pop".  I'd adjust the pilot, or if it's mild, ignore it.  All my bikes pop a little on decel.  

Usually a "pop" is lean.  A "booom" is rich.   Both are fuel combusting in the exhaust due to incomplete burning of fuel in the chamber.  At least that's my understanding.  

[Laminar]

I'd say "POP POP" while decelerating (very common condition in motorcycles) is usually caused by a lean idle mixture.  Fuel isn't burning in the chamber due to the lean setup (lean burns slooow), and is finishing in the exhaust..thus "pop".  I'd adjust the pilot, or if it's mild, ignore it.  All my bikes pop a little on decel.   

http://forums.sohc4.net/index.php?topic=53815.0

http://forums.sohc4.net/index.php?topic=4089.0

http://forums.sohc4.net/index.php?topic=3060.0

[1timduke]

Could there possibly be a leak in the exhaust, perhaps at the headers or an undiscovered hole somewhere down the pipe(s)?   I've had this same problem when I switched exhausts.

My VW bus has had some mild to bad exhaust leaks, causes a disturbance in the force leading to major backfiring on decel.   I just need to get new headers and muffler, aluminum foil gaskets only seem to make it a few days

-Tim