what causes backfiring?

[Anonymous]

I occassionally get a backfire. I thought it was from too much fuel in the mixtures but it did it once after the adjustment

what s the cuase of this? general causes? timing? vacuum?

Chaz

 

[Anonymous]

Backfiring thru carb is a lean condition or timing issue. Lean mixture or vacuum leak may cause it. Ignition timing could be off too. Valve timing could be off to an extent leaving a valve open during the spark/comustion.

This differs from After-firing which normally is a rich condition with excess fuel dumped into the exhaust pipes and re-igniting. Similar to what you see road race cars decelerating in turns and fire shooting out the pipes.
Djogu

 

[Anonymous]

so I had number two...and now I have a slight number one. Hmm, more fiddling to come

 

[Anonymous]

A lot depends on the situation during the backfire. Backfire under deceleration is fairly normal--just not heard unless you have a straight exhaust or a leaky one. Under deceleration often there is not enough fuel and air in a cylinder to support combustion. This will collect in the exhaust and finally ignite, usually it will get blown out before this happens or the sound will be absorbed by the muffler. Take the muffler off a rideing lawn mower and ride down a hill under decel-- terrible racket. Another possible cause is a bad spot in the ign. wireing. Old trick is ride down the road under decel and turn off the key, leave it in gear-- pump the gas a couple times then turn the key back on--however don't do this much if you are fond of your muffler as it may spit it's guts out in the street. With straight exhaust you can time this to a fraction of a second -used to have a lot of fun doing this with a toy gun in hand(lived in rural America, don't do this in a large city).

 

[pedal2themetal45]

AOld trick is ride down the road under decel and turn off the key, leave it in gear-- pump the gas a couple times then turn the key back on--however don't do this much if you are fond of your muffler as it may spit it's guts out in the street. With straight exhaust you can time this to a fraction of a second -used to have a lot of fun doing this with a toy gun in hand(lived in rural America, don't do this in a large city).

The only thing this well do is cause you to have to buy a new muffler. I've see mufflers split wide open from doing this.. AS well as it can't be good for the motor, gaskets, rings and such with that kind of back prusher (from the explosion) :roll:
I would look into lean mix, vacuum leak, timming, valve adjustment.
Tim 8)