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
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
