Weird ignition problems

Posted this in BritBike, also:

My '74 Commando, restored about 7 or 8 years ago and with about 9000 miles since, has been a paradigm of reliability, with one kickstart firing and handles beautifully. About a month ago, it died suddenly and had to be towed home. The main fuse (20A) had blown. Replacement fuses instantaneously blew, also. I checked all my wiring and all looked well, though I separated all wires and taped up anything that looked as if it might have contributed. Well, this time, the fuse didn't blow - until I kicked the engine over. Several fuses later, it appeared as if the TriSpark unit worked (flashing LED with kickstart) but no spark from grounded plugs. It looked like the coil was faulty. A quick call to Matt at CNW and, without batting an eye, he sent me a new coil! What truly great service, as I'd bought the TriSpark unit and coil off him at least 4 years previously!

The bike ran for a couple of blocks, but started sputtering and died again. More thorough review of wiring (yep, fuse gone again) revealed that the wires to my rear break switched had fallen behind the exhaust and the insulation hd burned off and bear wires were up against the (grounded) exhaust pipe. You may well think AHA! and that fixing that would solve my problems, right? HAH! That would be too simple.

Checked the new and old coils - both no good. Tried another TriSpark - no good, either. So, today, Richrd and I reinstalled a Boyer Mk 111 system - complete with old Boyer coil. Got it to start and run, pretty easily, but now it won't advance when revved! Any ideas?

Paul

If the pickup wires are reversed it will not not advance. Jim

phsDommie said:

Checked the new and old coils - both no good. Tried another TriSpark - no good, either. So, today, Richrd and I reinstalled a Boyer Mk 111 system - complete with old Boyer coil. Got it to start and run, pretty easily, but now it won't advance when revved! Any ideas?

Paul

Click to expand...

Thanks, Jim. We'd already considered that and switched the wires - no difference!
Paul.

Ok you have probably tried all of these... a different battery... disconnected the big blue cap, the zener or some other earthing fault... with boyer the pick up might be suspect...

Many thanks. The battery is fairly new - accepts full charge and has 12.7 volts at rest and falls to 12.02 with headlights and brake lights on, so doubt an open cell. Not sure what you mean by "big blue cap"??
Paul

the large blue 2 m c capacitor normally in the battery box .. disconnect the earth thats the red wire and if you can substitute the rectifier try that as well. a good indicator there might be something wrong in the charging circuit is if you take the fuse out and replace it , with everything switched off there will be a slight spark when its replaced... these faults can be a nightmare to track down.. good luck with it.

Make sure the Zener is not in full conduction mode. That is closed circuit to ground. When mine went south the fuses just blow when installed as you have a direct short to ground. Also, a Boyer will produce spark when run off the capacitor. The bike will get you home but it is not too happy. Mine restarted on that occasion but I was coasting down hill and releasing the clutch to turn the engine over.