Backfire when switching ignition off

[Barletta]

Hello All,
Anyone ever experienced backfire thru the exhaust when switching the ignition off (engine not running). Also in place off a fuse box the bike (900GTS bosch ignition) is fitted with circuit breakers. At times when switching the ignition off you hear one of the circuit breakers tripping.
Bike starts and runs perfect.
Any advice would be very much appreciated before it does my head in.

[mizike77]

timing advanced too much?

[wdietz186]

The bikes will spark when the ign. is turned on and probably when turned off too[never checked].The backfire is probably residual fuel in the pipe or in the cyl. with a valve open,is the bike running rich?
On the circuit breakers,do any of them actually trip or is it just a click?
If it has been rewired it might be a relay for lights or something opening.
Without pics. it is hard to tell
If it runs well and everything works it's probably best to let it be.

[DaleMclean]

Just a ditto on the above. My old girl will pop sometimes if you turn the ignition on, don't start it and turn it off again.
It happens when all the stars align and one cylinder has a valve open and unburnt fuel in the cylider, each time you turn off the ignition it can also cause a spark , collapsing magnetic field in the coil if it happens to be actually at a point in the rotation of the engine where the coil is carrying current (ready to be switched OFF by the ignition timing to cause a spark)

Ah the joys of older machinery

[DesmoDog]

My 750GT with Dyna ignition does this too. Kind of disconcerting but nothing's wrong.

[MartinMille BANNED]

If I turn my SSD off at the kill switch she shuts down fine no backfire nothing , turn her off at the key and its a hole new ball game . Its like as if the igniter boxes get a spike and hence they discharge causing a pre-ignition

Should add the Millie does it as well

Marty

[DesmoDog]

If I turn my SSD off at the kill switch she shuts down fine no backfire nothing , turn her off at the key and its a hole new ball game . Its like as if the igniter boxes get a spike and hence they discharge causing a pre-ignition

Should add the Millie does it as well

Marty

Just a guess, but the kill switch might ground the coils out so the charge goes to ground instead of the plugs when the circuit collapses. The ignition switch cuts power to the coil so the circuit still collapses but makes the plugs fire (like they're supposed to).

[thessler3]

This is normal and expected if you think about it. On the old points ignitions the coil primaries get charged to 12V when the points are closed. When the points open, the primary current is broken and the magnetic field collapses, which generates a high voltage in the coil secondary and subsequent spark.

If one or both coil primaries happen to be charged when you turn the key off, current is broken to the primary and you get a spark, regardless of where the cylinders are. If a valve is open you may get a backfire or fart.

Note that both spark plugs seem to always fire on the Dyna setup whenever you turn the key off. No harm, no foul.