Hitchcocks Forum

Chaps

I want to use a momentary on switch as a kill-switch (batteryless '96 Bullet on points and condenser).



Am I right in thinking that, if I were to effectively wire to two LT terminals on the coil together with the switch in between, this would work without frying anything?

Johnny connecting wires across the coil does nothing, the coil still has power running through it but you are going to put 12 volts directly into the points. I have thought about this over the years never really come up with a good answer. I did one an Inter a few years ago by fitting a spring loaded switch I found somewhere that when you pressed it in it disconnected power to the coil but you always have to remember to then turn the ignition off because if the points are closed you can cause yourself problems. Why not just use the decomp to stop it and then turn the ignition switch off

Hi Norm



As I mentioned, I run batteryless, so the points being closed isn't a problem.



Effectively, what I'm trying to do is do away with the ignition key and was thinking that, by pressing the switch, you'd create a path to earth that was easier than chasing around all those windings - a deliberate 'short' if you like.



As you mentioned, using the decomp will stop it (so will simply stalling it) but I was after a more elegant solution, and I thought I'd read that using the decomp was a bad thing.

Well how are you powering the coil, the positive wire to the coil must be coming from the powerbox so that is the wire that needs to be cut. Not sure why using the decomp to stop it is a problem

Hi Norm



That would be the easy way, but the switch that I want to use is teeny-tiny (fits almost flush into a handlebar) so is unlikely to enjoy sustained current of any magnitude and - just to complete ruin things - is a push for momentary on type.



With the decomp solution, apart from lacking a certain elegance, I don't have a handlebar control for it (super clean handlebars, you see) so have to reach into a hot engine to operate it.



Not the end of the world, I know, but am elegant solution would be nice.

You would have to use a relay so that when you earthed it through the little switch the relay would open and cut the power to the coil. Most relays close when power is added but you need one that is closed all the time and only opens when the earth is activated

Count Johnny: If you short-circuit the coil by closing a switch connected across the LT terminals, you will indeed kill the spark as the current will follow the path of least (effectively zero) resistance through the switch, rather than the coil.



However, the switch will have to carry the full output of the alternator, which is a big ask and may well burn it out.



A better approach would be to short out the points (in effect, so that they don't open). That, also, will kill the spark, but this time the current through the switch will be limited by the resistance of the coil. It might still be enough to damage a really tiny microswitch, but it's a much more feasible approach.



You will appreciate that one side of the points is connected to earth, so you only need run one wire to the switch (from the live side of the points, or the terminal on the coil that goes to the points), and the other side of the switch can go to a local earth.

Simplicity and elegance oft times compliment each other whereas complication for the sake of complication is neither
My 2 drachma

Hi Norm



For the reasons that Beezabryan describes, a relay is not an option.





Hi Thack

That's interesting and accords with my first idea of shorting the points by switching the positive coil terminal directly to ground.



When considering that idea, I thought I would be exposing the switch to a higher current - but you have suitably educated me.



The coil lives above the engine, at the forward end of the main-tube, so I can make that work very neatly by mounting the switch in the centre of the handlebar and running a nice short wire to the coil.

Lex parsimoniae. Decompressor every time. Especially on E/S models.