Paul H said:
Hi All. Any thoughts on this one?
When less that 20-ish miles of running... Turn off the key and the engine stops as expected, i.e. regardless of whether in gear of not.
More miles than that, (i.e. fully hot) engine will only switch off when in neutral. If trying to key-off whilst in gear (with clutch in) then it just keep on going!
SF 750, DMC2.
I think I have a 12v leak path working it's way to the DMC? Maybe grounded out (or reduced V) by the grounding effect of the neutral switch?
Cheers, Hag.
As for Haggis' problem, it's a bit of a puzzle.
The engine shutting down when cold implies that the ignition system is powering down as it should when the key switch is turned off. Why else would the engine stop? So we can assume the ignition switch is working.
Running on when hot after the ignition is switched off is not unheard of. Tends to indicate carbon deposits in the combustion chamber (from running rich or burning oil) that can get very hot and act like a glow plug and fire the mixture even if there's no spark at the plug.
How the neutral light switch is involved is a bit of a mystery. I'm not on the same page as Tippie and Kermit. It's not unusual to earth the starter relay via the neutral switch to prevent starting in gear. But if the ignition was earthed through the neutral switch, the engine would cut out as soon as you put it in gear, not the other way around (cutting out in neutral).
So for the ignition to only switch off in neutral has me somewhat baffled. The only way (in theory) I can see that happening is if the 12V power supply to the ignition module is taken off the wire that goes to the neutral light.
When the engine is in gear, the DMC would be powered via the neutral light globe. Remember that the neutral light has 12V supplied to it all the time, so that 12V carries across to the -ve side of the lamp when there's no connection to earth at the neutral switch. So the whole neutral light system is at 12V all the way to the switch when in gear. When you select neutral, that power supply is earthed and the 12V drops to zero on the earth side of the lamp. So the ignition would lose its power and shut down. I don't know what current the DMC needs to fire up its little electronic brain. If it's only a few milliamps then I suppose it's possible that a trickle of power through that little neutral globe is enough. But why would anyone wire it that way? It would mean the bike can't start when in neutral, and surely Hag would have said something about that!
But even in the above case, the coils would need a separate supply because there's no way they could pull enough juice through the neutral light globe. In that case, switching the key switch off when in gear should still kill the engine because it'll cut power to the coils. So they'd need to be powered directly from the battery, which would flatten the battery when the bike is parked - another thing I'm sure Hag would have noticed.
Now you see what I mean by it having me baffled.
The only conclusion I can come to is that it's not an electrical fault at all and the neutral light is a red herring. The ignition is working as it should (as I observed at the beginning of this diatribe).
So, if it's running on due to pre-ignition from glowing carbon embers, then the ignition system has nothing to do with it because it's already switched off as it should be. So it must be a mechanical phenomenon. Perhaps something to do with the drag on the clutch providing as smoothing effect to the engine when it's in gear as opposed to it being in neutral. Or maybe something related to the additional rotating mass of the gearbox main shaft when in neutral. OK, I'm grasping at straws. ???
To answer Tippie's question about the 3W globe. A 3W drain wouldn't cause enough voltage drop in the rest of the electrical system to affect the ignition.
bit like making a pillow out of a plastic bottle 
