Matchless G3 350 with a Weird Problem

[Michael Crawshaw]

My 1961 G3 stopped running one night after I switched the headlight on and the fuse blew burning the plastic fuse case.. Subsequently she flat refuses to run. The following have been replaced, battery, main harness, coil, ignition switch, light switch, rectifier, headlamp bulb and of course fuse and fuse holder. The harness was installed by a very excellent competent guy, subsequently I've checked it and it all looks perfect.

Turning the ignition on does nothing, no spark etc. If I turn the lights on before the ignition they all function fine, side light, main beam, dip and rear plus brake. As soon as I turn the ignition on the lights go out and the bike wont generate a spark again. If I isolate the points the same thing.

Any ideas (no matter how bonkers because I've run out of road on this) welcome.

Mike

[SPRIDDLER]

My 1961 G3 stopped running one night after I switched the headlight on and the fuse blew burning the plastic fuse case.. Subsequently she flat refuses to run.

I've never had an alternator and coil ignition bike so it may be a daft idea but my initial thought is that the ignition coil primary winding may be shorting out. (Possibly as a result of a voltage surge when the alternator was called upon to increase its output to cope with the headlight demand.............. )

I can think of several reasons why the above is unlikely but it'll only take five minutes to check the coil with a meter.

[Groily]

Does your machine have an ammeter? I see that '60-66 bikes seem not to from Neill's book in the Archives, but some alternator ones did as there's a wiring digram for that too, on pp 120 et seq. To me, it's the one essential drop-dead necessary instrument . . .

If it does have one, it may tell you if anything is happening when you turn the ign switch on. It should show the drain when the lights are applied.
But if there isn't one, you need to be careful when playing as you don't want a melt-down. Maybe insert one in the battery live wire??

If Spriddler's thought isn't daft, you've got a serious dead short when you turn the ign on, enough to snuff everything else. Maybe you also have that if you turn the ign on without the lights? Only an instrument will tell, or another blowing fuse of course if there's one in place.

Your problem sounds perhaps to be an error in the power supply between switches and / or ammeter. On the face of it the light switch is operating correctly. But it may be that the power to the ignition switch has been taken off the wrong terminal on the light switch (if it's supposed to come from there?) and perhaps to the wrong terminal on the ignition switch, resulting in either 'no power' or a dead short at the ignition switch.

I think a good session with a meter is needed to check all the main elements afresh. Rectifier to switches (ammeter if there is one) and battery, and all wiring between them. Might all look good, but something fundamental is wrong. Not easy to decide where to start, but I'd put an ammeter in, at least temporarily, I'd check meticulously that the switch terminals are wired according to whatever diagram you're using for the bike, and I'd check for voltage at the coil low tension feed when the ign sw is on, for starters.

Nothing worse than these sorts of hassles - but the only way to approach them is 'one thing at a time' and very methodically. It's probably no big deal in terms of fixing it when you find the problem, but the consequences of messing about without being able to 'see' what current is flowing where and when, and where there are volts, can be pretty horrible.

[EddieDay]

One other thought. The problem started when you switched the headlight on. That possibly suggests a link between the headlight wiring and the ignition wiring. As you have replaced both the ignition switch and the light switch (I am assuming 2 separate switches on your model) is there something amiss inside the headlamp shell? The wiring diagram for 1960 - 61 singles shows three wires running between the two switches. The new loom may not have included replacements for those. It would be worth testing the continuity of those 3 wires.

[Guzzibiker]

A very useful tool when playing with dodgy wiring is a high wattage bulb. I use a headlight bulb, same voltage as the battery, with both filaments wired up. I fit this in place of the fuse. If I've got a dead short it lights up bright. I can then go round wiggling and diconnecting things until it goes out. It saves any fire risk and saves blowing a shed load of fuses.

[SPRIDDLER]

A very useful tool when playing with dodgy wiring is a high wattage bulb. I use a headlight bulb, same voltage as the battery, with both filaments wired up. I fit this in place of the fuse.

Quite so. I've made up the same kit (but with a 12v lamp with linked filaments) which I also use for testing the output from 6v dynamos.......no risk of the 12v lamp blowing when checking 6v dynamos when the unregulated voltage soars as the revs are increased.

Dynamo test rig (640x480).jpg

[Ralph]

I had this on a modern bike, the head light bulb had a intermittent dead short to earth, worth a check.

[g80csp11]

check the new ignition switch are the terminal in the same orientation, is it a Lucas 88SA LU34427B type ?
Ive see some of these that look similar but are not wired the same ( look at links under the switch )