Noise when turning off ignition key

[MisterCinders]

This may be related to a flooding carb issue I am dealing with in another thread.

With all the starting with diagnosis, I had to put my battery on the charger for a bit. No problem, she bounced back fine.

Other than trickle charging the battery a bit, I have done nothing unusual to the electrical system. Lights and starter work fine.

Here is the strange bit. After pulling the carbs off and on several times and starting the bike on the recharged battery, I noticed that the bike occasionally makes a strange noise when I turn the key to the off position. It doesn't happen every time, but it has happened with or without the engine running first. In other words, I have encountered it when shutting down from just lights no engine, and when shutting down an idling engine with the key (instead of the kill switch).

The noise comes from the left side of the bike near the starter motor area. It sounds a bit like the starter engine is spinning for a second after turning the key. One time the noise included a single popping sound, more tonal than backfire-like, but not the horn.

It doesn't happen every time I turn the key from on to off. But NOTHING should happen when I turn the key from ON to OFF, so what the hell would trigger the starter motor or something else when you key the bike off? My carb struggles have included fuel flooding, and the carbs sit above the starter motor. Could the fuel spillage somehow be causing a weird short that trips when you close the electrical system??

[mike_of_bbg]

When you turn the key to off (assuming the ignition switch is in run position), it will pull the power from the coils and, if they've been grounded, they'll fire one time. That's the only thing I can really think of.

[MisterCinders]

When you turn the key to off (assuming the ignition switch is in run position), it will pull the power from the coils and, if they've been grounded, they'll fire one time. That's the only thing I can really think of.

Yes, this happens when turning key from RUN to OFF.

By "grounded" do you mean a short in the coils? Should I be checking connections there?

Coils firing fits the one time it "popped" - as that sounded similar to a backfire, but the spinning noise is like the whirring of the starter motor after a failed start. Would the coils trigger something like that?

[Killer2600]

When you turn the key to off (assuming the ignition switch is in run position), it will pull the power from the coils and, if they've been grounded, they'll fire one time. That's the only thing I can really think of.

I'm gonna go with this theory.

[mike_of_bbg]

The way coils work is like this:

First, they are energized. They get a constant +12V when the bike's ignition is on. The ignition's job is to ground the coils, creating a current in the coil primary circuit. That current builds a magnetic field around the primary where the current is flowing, but also around the intertwined but much larger secondary. This does not cause spark.

When it comes time for the coil to fire, the ignition system pulls the ground, and the magnetic field collapses very quickly. That magnetic field collapse is what induces the current in the secondary. It's when the coils are turned OFF, so to speak, that spark occurs. On a points ignition, the coils fire when the points OPEN.

Coils are basically step-up transformers. Rather than sinusoidal transfer, it's a repetitive pattern of slow build and sudden collapse. The induction in the secondary is the derivative of the surrounding magnetic field. The sudden and rapid collapse of the surrounding field induces a large voltage in the secondary.

When you turn your ignition off, if the coil is grounded (has a magnetic field) - which at least one should be - the magnetic field will collapse and cause the coil secondary to fire. This is perfectly normal, happens all of the time, and nobody ever notices (except you'll occasionally see a thread where someone was checking their spark and notices it after leaving the plug laying against the head turning the bike off - they then freak out and wonder if their ignition is malfunctioning). But perhaps because you're leaking fuel into what should otherwise be dry cylinders you're actually igniting something when this happens. And of course that firing will not be timed to anything in particular. So it can happen in any particular (static) position of the motor: exhaust valve open, intake valve open, etc.

[MisterCinders]

Thanks, Mike. That detail helps a lot.

[chuck hahn]

+4 on the coils firing..that pop may be intermittent from time to time. And its a hint that just maybe the petcock may be allowing fuel to run into the vacuum line and thus into cylinder 3. If it continues regularly you may wish to do some checking around.

[Kaiser Sosei]

If you are talking about the bike in your sig with the Dyna ignition, that would be were I would start. My father had a Dyna ignition in his CB750 and it would do that fairly often. I can't remember if he had the "S" version or not. I'm not sure if it's a feature or a bug, but his did it for years. Might want to contact Dyna about it.