Kill Switch Ignition Trouble

A word of pre-caution for those who may not be familiar with some of the Lucas bits and wiring that leads to an ignition problem and often many hours of trial and tribulation, i.e. swearing and tearing of hair followed by much drink. On the later 750 and 850 Mk2's the handlebar Lucas switch blocks have a kill button. Depending on which model, year, US or UK version it may be on the left or right side. On the weekend I replaced the handlebars and thought I'd look inside the switch blocks as they are 37 years old and I haven't done anything to them in 7 years, and rust never sleeps. A quick clean with WD40 and gentle wipe out seemed fine, none of the soldered connection brittle or wiring sheaths punctured. Bars back on, bike wouldn't start. OK, seems the last thing played with is most likely to be the culprit and a quick check of the spark plugs laid out on the head showed no spark at all. Pulled the switch block off and nothing obviously wrong, so it was time to trace the wiring.

First thing was to see if the Boyer was getting power. This is the amp box white lead connecting to the blue/white off the harness. Nothing. Attaching a wire from the battery NEG to the Boyer amp box white showed sparks jumping at the plugs, so that verifies its not the Boyer.

To simplify the circuit for those not comfortable with the wiring diagram, the power runs on the white leads from the igntion switch up to the junction blocks under the front frame tubes and then splits off into the handlebar pigtails. One of these white pigtail goes up to the switch block. Inside the switch block is a metal terminal plate that has a yellow white cable soldered on. The kill switch has a spring loaded metal strip that cuts power to the yellow white connection when the button is pressed. If that metal strip is corroded or grubby it can break the power circuit to the yellow white cable, that in turn runs through the pigtail back to the junction blocks undert he front frame tubes and connects to the blue white that feeds power to the white Boyer amp box cable.

The trick here is to ensure the kill switch is not sticking or corroded. Removing the guts is a simple but fiddly job. I'm no fan of kill switches so I cleaned up and soldered a small blob between the white power and yellow white contact plates to ensure I got a good 12V supply to feed the Boyer.

I hope this helps explain some of the unreliabilty issues that occur, its really just old electrics that need attention.

Mick

I did exactly the same thing to my 850 long ago. The kill switch would become intermittant while riding and the bike would stop. Very frustrating. I tried every trick I had for cleaning and adjusting the contacts all to no avail.

I would not recommend removing the kill switch from the circuit. If you don't trust the stock button, consider adding a separate kill switch. If you ever have a throttle slide stick open (It happens too frequently with Amals) it can be quite exciting trying to find that key under your knee while the bike continues to accelerate in traffic! DAMHIK.

Agree with Ron - A kill switch is a good thing. The oem Lucas switches can be opened up completely if you are so inclined for a thorough cleaning. Some Deoxit and some dielectric grease in there and it will be fine for the next 30 years! Or, as noted, you can add a separate switch or go with the CNW switch gear that they use with the change to the Brembo Mastercylinder. ( I REALLY recommend the Brembo MC change for disc braked bikes - the sleeved oem MC is a big improvement over stock but the Brembo and stock caliper stopping power is limited only by the stickiness of the front tire)