Backfire when switching ignition off

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Barletta
Cucciolo - the Lil Pup
Posts: 8
Joined: Tue Sep 21, 2010 12:51 am
Location: Sydney Australia

Backfire when switching ignition off

Post by 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
860 GT / GTS
Posts: 272
Joined: Sun Jul 06, 2008 9:37 am

Post by mizike77 »

timing advanced too much?
79 Darmah
78 900 GTS
86 F1B
80 SSD
02 Harley FXD
72 H2 750 triple
08 KTM 300XC
there has yet to be a motorcycle line made that is as satisfying to ride, hear and look at as the ducati bevels
wdietz186
Cagiva Alazzura
Posts: 703
Joined: Sun Sep 10, 2006 5:40 pm

Post by 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
Cucciolo - the Lil Pup
Posts: 20
Joined: Wed Apr 07, 2010 3:46 pm
Location: Swansea, NSW, Australya mate

Post by 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 :lol:
I survived :-)
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DesmoDog
750 GT
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Location: Dexter, MI USA
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Post by DesmoDog »

My 750GT with Dyna ignition does this too. Kind of disconcerting but nothing's wrong.
MartinMille BANNED
BANNED
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Post by 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
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DesmoDog
750 GT
Posts: 156
Joined: Fri Mar 19, 2004 10:26 am
Location: Dexter, MI USA
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Post by DesmoDog »

Martin Mille / SSD wrote: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
Mariana
Posts: 35
Joined: Tue Jul 30, 2013 2:57 pm

Re: Backfire when switching ignition off

Post by 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.
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