Date: Tue, 10 Aug 1999 09:08:24 -0700
From: Bill de Garis 
Subject: Re: Kill Switch problem

Chris L Johnson wrote:
> > Chris,
> > grateful for advice/suggestions on following problem .......
> >
> > Bike is 97 Scorpa 250
> >
> > Kill switch works fine when bike is idling or throttle approx 1/2
> > way open or less
> >
> > At higher revs - particularly if throttle stuck open, the kill
> > switch doesn't work (which is of course precisely the time I need it
> > !)
> >
> > Regards,
> > David Bee
> > Hartlepool, UK.
> 
> This is actually normal.  Bikes will run on their own when stuck wide
> open.  I think it is igniting off the glowing plug tip.
> 
> The way to stop a bike stuck wide open is to shift up and clamp on the
> brakes, and/or put the choke on, and/or cover the muffler hole to choke
> off the engine.
> 
> Chris Johnson

Chris, 
You are right for some instances particularly in the old days when we used to
get a lot of carbon build up in the combustion chamber.

However the problem David is experiencing is quite likely different.
What may be happening is that the power (voltage times current) of the primary
circuit is high enough at high revs to drop sufficiant voltage in the leads of
the kill circuit to still generate sufficiant voltage to fire the plug.

This is most common when people use a ground (earth) return kill button. (A
single wire kill circuit that relies on the handlebar and frame of the bike to
complete the circuit).

David, use a two wire kill button with nice fat wires (lots of copper in
cross-section), and ensure that the ground side is clamped to the same point as
the ignition primary ground.

If that's already the case, check to see you don't have corrosion where the
wires connect to the ground. Check also that there is no place where the kill
wires have been pinched and perhaps partially severed internally.

Billy d
DG Instrumentation Ltd.
http://www.corp.direct.ca/ginseng/dginst/main.html