Engine backfiring from slowing down from high revs - bad caps or joints?

[japlytic]

I remember a neighbours' late-80s Mazda B2200 going "bang" after slowing down from high revs, and I am curious about the possibility of bad solder joints and/or bad electrolytic capacitors in the ECU would cause that.

[kc8adu]

Re: Engine backfiring from slowing down from high revs - bad caps or joints?

i doubt it.but my neighbors geo tracker damaged its airbox and maf from backfiring when the caps leaked.it was a mess.after i fixed it up it stopped backfiring and would start every time.

[larrymoencurly]

Re: Engine backfiring from slowing down from high revs - bad caps or joints?

Isn't that often due to the air injection into the exhaust not being turned off during deceleration, such as due to a problem with its gulp valve, so when oxygen hits all that raw gas in the exhaust, they explode? That was a huge problem with early 1970s Mazda rotary engines, which used exhaust air injection to burn the gas directly, rather than to help the catalytic converter burn it.

[kc8adu]

Re: Engine backfiring from slowing down from high revs - bad caps or joints?

an exhaust leak can do it too.
letting the throttle snap shut suddenly lets a burst of over rich mixture through.a touch of air getting into the exhaust and bang!

[dood]

Re: Engine backfiring from slowing down from high revs - bad caps or joints?

I remember a neighbours' late-80s Mazda B2200 going "bang" after slowing down from high revs, and I am curious about the possibility of bad solder joints and/or bad electrolytic capacitors in the ECU would cause that.

Nope... mostly likely just a rich air/fuel mixture on deceleration, plus maybe a lazy exhaust valve allowing some extra combustion gasses into the exhaust.

[severach]

Re: Engine backfiring from slowing down from high revs - bad caps or joints?

Overly rich mixtures don't go bang. Most of the gas and oxygen is gone. All that's left is a smidge of gas, maybe a pop's worth but not enough for fireworks. Even with a nice stream of fresh air the mixture isn't mixed well enough to produce a bang.

It's lean mixtures that go bang. When the mixture gets lean enough and the tired engine runs poorly enough sometimes the cylinder misfires allowing a complete cylinders worth of unburnt fuel mixture out. When that slug of oxygen and gasoline find a hot place to cozy up, BANG! Carburetors couldn't regulate well during deceleration and often strayed lean leading to backfire. Even a poorly operating fuel injection regulates well enough to prevent backfire.

You can cheat on carbonated engines by flicking the key off when passing someone of note. When the key is returned to the operation position, WA-BANG, off goes several cylinders worth of fuel mixture, more than enough to overcome the flimsy muffler and impress passerby's. Fuel injection cuts the fuel off so this trick doesn't work.