What would cause a bike to backfire when shut off?

[bradshaw106]

Like the title says, what would cause a bike to backfire when it's shut off?

 

[Splexin]

Unburned fuel in the exhaust. Not an expert but I believe it is a sign that you're running too rich.

 

[bradshaw106]

That's what I was thinking.

 

[jay760]

could be an air leak on the exhaust,

 

[1974jh5]

Another vote for run rich with some hot carbon in the exhaust or a valve open a smidge. Haven't seen an XS do this, but on the old Ford Duraspark ignition systems that was a sign the ignition module was on its way out. You could turn the ignition on, then off, and it would backfire once.

 

[bradshaw106]

I wanted to revive this thread, my buddies bike is doing this and want to see if anyone else has any thoughts?

 

[Splexin]

This might have nothing to do with your buddies bike, but I have a story....

After a hard high speed highway ride, the nut came loose on the rod going through my PAMCO assembly (washer+magnet+circuit board), causing the magnet to fall off when the bike was parked (tilted). At the time I didn't know and when I tried to start it, of course it wouldn't. Switch the ignition "off" and BOOM.....backfire louder than gunshot at point blank. So loud someone called the cops! For whatever reason no matter how tight I turn that nut, every few months it will come loose again. I think because the spin of the rod is not opposite of the way it loosens, eventually it unwinds itself loose. I know that's not supposed to happen and I'm sure it never happens to anyone else but I live with it

So when I try to start and it backfires this is how I know my PAMCO nut is loose

 

[XSLeo]

A lot of electronic ignitions will do this. My Harley does, my 75 with a Pamco does and the 81 with a TCI does.
When the engine is in the right position when you turn the key on the coil gets powered up ready to fire. If you then turn the key off the coil fires.
Heard as a kinda whoof sound. You hear it more with a modified air intake. The stock air intake is desaigned to quiet the rush of air coming into the carb, kinda like a muffler does on the exhaust. Removing the stock box lets you hear the intake morte.
You won't hear this everutime because the engine won't stop in the right place everytime.
Leo

 

[TwoManyXS1Bs]

It's rare, like leo sez: gotta be in the right place. Old points-type ignitions can do this from 'inductive kickback', ign key acts like points...

 

[jayel]

This might have nothing to do with your buddies bike, but I have a story....

After a hard high speed highway ride, the nut came loose on the rod going through my PAMCO assembly (washer+magnet+circuit board), causing the magnet to fall off when the bike was parked (tilted). At the time I didn't know and when I tried to start it, of course it wouldn't. Switch the ignition "off" and BOOM.....backfire louder than gunshot at point blank. So loud someone called the cops! For whatever reason no matter how tight I turn that nut, every few months it will come loose again. I think because the spin of the rod is not opposite of the way it loosens, eventually it unwinds itself loose. I know that's not supposed to happen and I'm sure it never happens to anyone else but I live with it

So when I try to start and it backfires this is how I know my PAMCO nut is loose

get a new nut, use blue loc-tite