Engine sparking when I turn off the key! BOOM!

[Jumpinjimmy]

HI folks!  I'm a new member and am writing to you from France.  I've got a problem with my R65 (see subject) and am really stumped.  Switching off the key creates a spark on both spark plugs--the bang from the exhaust is impressive.  At first, I thought it was the ignition module (under the gas tank), so I have changed that, with no effect.  I actually removed the electronic ignition from the front cover of the bike and it STILL sparks.  I want to try and go through a logical procedure for finding the problem rather than changing parts and hoping for the best. 
CAN ANYONE HELP OR GIVE ANY SUGGESTIONS??  First time I've ever run across this problem in 40 years of biking.  PS. It seems to have occurred after a long ride.

[georgesgiralt]

Hello JumpinJimmy,
The spark when you turn the key off is normal behavior. It is due to the way the ignition circuit is made.
As per the explosion, that means your cylinders are filled with fuel when this spark arrives. This means you have a carburetor related problem, not an ignition one.
I'll bet your carburetors will benefit from a clean and complete revision. How old is the bike and when was this last done ? How many km ago ?
Have a nice day.

[tunnelrider]

Hello and welcome to the asylum also JumpinJimmy, 40 years riding respect!  I first thought of perhaps a hot spot on a piston, from carbon build up or a sharp edge?
Also as George suggests maybe carb related and perhaps your throttle isn't fully closing, check that your throttle cables are slack before turning the engine off after a long ride? 

[nhmaf]

Does the engine run fine without bogging or sputtering otherwise?  As others indicate, it is more likely a carb issue.. but there are ignition problems that can result in this sort of thing, like a severely maladjusted timing or an intermittent ignition module or coil..If ignition drops out on one side and the cylinder pumps a number of gulps of fuel into the exhaust system without burning it, then the last spark from the collapsing coil field when turning off the key ignites it.  Running severely rich or excessively high fuel bowl level or also suspect

[mrclubike]

Does it happen often.
If not it may not be a problem
I am sure a perfectly normal bike may do this occasionally
I know mine has done it but it wasn't very load.
 
How  high is your idle

Do you shut he fuel off when parked

Are you leaving it on the side stand
Like most Airhead owners I generally  park on the center stand

Is the engine running when you shut down and then you get a back fire
If not why would you need to turn the key off if the motor isn't running 
 
If you ever have a no spark issue and it still sparks when shutting off the key that points to a bad bean can
Good to know

[Justin B.]

It happens occasionally.  If the coil happens to be energized and the cylinder has some fuel vapor in it.  When you turn off the key the juice to the coil goes away and the magnetic field collapses generating voltage in the high voltage windings which produces a spark.

[Jumpinjimmy]

Thanks for the input everyone!  It happens every single time I turn the key off.  I hooked up a couple of spark plugs and laid them on the cylinders heads so I could see what was happening and it sparks every time I turn off the ignition.  Has anyone ever checked to see if their bike does this??

[Jumpinjimmy]

I have to admit that I have a problem thinking this is a normal condition and that the issue lies with the carbs.
Think about this:
both plugs spark at the same time when I turn off the ignition switch---even with the complete electronic ignition unit removed from the bike (but module and coils in place).  Personally, I've wrenched on a lot of bikes over the years, about a dozen of them being my own bmws and I've never encountered an ignition system sparking when you turn the key off.  Maybe I've missed something along the line, but I'm pretty sure I would have caught this at some point if it was normal.  As for the carbs...they were recently taken off and cleaned, so I'm not inclined to think that's the problem.

[Barry]

That both plugs spark when you turn of the ignition is not uncommon even perhaps with the ignition trigger removed. If the ignition module is still in place then current can be flowing through the coils and it will be interrupted when the ignition is turned off. Being a wasted spark system both plugs will always spark together.   

That you get a big bang from the exhaust every time is not normal and points to an excess of fuel in the cylinders for some reason.  Even though the carbs were recently cleaned I'd investigate the float valves and levels to make sure the carbs aren't flooding.   Any other clues such as poor MPG and signs of richness on the plugs.

[georgesgiralt]

When I was a young lad, in the street I lived in, where a garage specializing in tuning all makes of cars. The owner an old guy told me many many times that "when you think it is ignition then check the carbs. If you think it's the carbs, check the ignition". It has proved true anytime since.
As per the spark occurring at power off, the coils are driven by dwell, so permanently energized when the ignition is on. (this is why the ignition cuts off after a few seconds if you do not crank the engine) so when you turn the key off, the primary voltage drops to zero, and, provoke a change of flux in the coil, giving a spark in the secondary.
You'll find the same behavior on every (quite) recent ignition system on petrol cars.
If you do not notice it it is because there is no left over petrol in the cylinder to light up and explode.
So we are back at the carb(s) issue....

[Tony Smith]

Thanks for the input everyone!  It happens every single time I turn the key off.  I hooked up a couple of spark plugs and laid them on the cylinders heads so I could see what was happening and it sparks every time I turn off the ignition.  Has anyone ever checked to see if their bike does this??

All of the post-1980 airheads with hall effect ignition behave as you describe.

In fact, doing the test you did is the primary way used to determine if the ICU is 'alive" or not by flicking the kill switch off/on you should get a spark every time, if you don't, you have a problem.

So the good news is that your ICU is in good health.

[mrclubike]

Just to Clarify
You may be missing something in the translation

Yes I have checked this on my R65 and it does it
 

It is absolutely normal for the spark plugs to spark when you shut the ignition off
It depends where the engine stops 

 Yes it will do it every time if you disconnect the bean can

[Runninn1]

Turn off with kill switch and then turn key off...same result?

[Tony Smith]

Turn off with kill switch and then turn key off...same result?

No!

You will get a spark every time you interrupt power to the ignition circuit.

If the ignition is on and you flick the kill switch - you will get a spark.

If the ignition is on and you turn it off - you will get a spark.

BUT, if the ignition is on and you flick the kill switch off and leave t off (and get a spark) you will not get a further spark if you then turn the ignition switch off cause there is no power to the ICU/coils). similarly if you turn the ignition switch off (and get a spark), you don't get more sparks no matter how many times you flick the kill switch.


As others have said, in possibly less blunt terms. if your bike is running OK, getting good mileage and is nicely responsive. then build a bridge and get over the occasional detonation that you get when you turn the ignition off.

On the other hand, if your bike is not running well, and is not getting good mileage - then rebuild your carbs.

You do not have an ignition problem!

[davidpdx]

Just to add a little preventive maintenance to the discussion. Be sure that both of your Carbs are connected well to the intake and air filter. When your bike backfires through the carbs and one falls off in a parking lot it is embarrassing. How would I know you might ask?