What causes a backfire?

[Anonymous]

A basic question, I know, but what causes a backfire?

Thanks

[Jeff Tate]

Without going into too much detail, it' because for some reason(s) raw fuel has gotten past the exhaust valve and ignited creating a pop up to blowing the muffler off... Good fun!

[Anonymous]

It happens to me when I eat too many beans 8)
GY

[Anonymous]

OK, a bit more specific

Gerry, sounds like you'd like the soup I've just had - chickpea, lentil and kidney bean! For a non-vegetarian that's a pretty punishing regime.

OK, my bike didn't used to backfire. Now it does. What's happened in between then and now is that I dismounted and then re-mounted the carbs and I changed the spark plugs.

Oh, yes, I forgot to mention - most of the time the bike doesn't start at all now so I guess you could say the backfire is an occasional problem.

Cheers

[argonsagas]

You did not say what bike you have....little bits of info like that are helpful....


If you have a 4 cylinder GS...and I am guessing you do, then if you did nothing but remove and re-install the carbs, and made no other adjustments except the spark plug change, you need to check your plug wires. They are probably crossed.

One coil fires cylinders 1 and 4 the other coil fires cylinders 2 and 3

[Anonymous]

I agree with Ron and I also think you need to check a couple of things:

Is the connection between the carbs and the intakes airtight? No cracks in the intake manifolds?

Did you check the gap of the plugs? Should be 0.7 mm.

Is the ignition timing correct? If not, this would explain the bad starting...

[Hap Call]

You did not say what bike you have....little bits of info like that are helpful....


If you have a 4 cylinder GS...and I am guessing you do, then if you did nothing but remove and re-install the carbs, and made no other adjustments except the spark plug change, you need to check your plug wires. They are probably crossed.

One coil fires cylinders 1 and 4 the other coil fires cylinders 2 and 3

That would also explain why it is not running.

You will also get popping when you have a leak around the exhaust gaskets on the header pipes. This allows cool fresh air into the pipe, mixing with the unburned gases (very seldom is all the gas burned in the combustion cycle, thus allowing some un-burnt gases to escape into the exhaust) that are at a sufficiently high temperature to complete the combustion requirements (fuel, oxygen, and ignition source).

Hap