Comet - Coil Ignition


Forum User
VOC Member
I have recently obtained a 51 Comet, which the pervious owner had started to convert to coil ignition, I was told that all the parts required were correct. However when I connected the battery (12 V neg earth) there was smell of burning, followed by a poping sound. When I checked the coil casing was bubbled.

Can anyone tell me does it matter what coil is fitted? other than being 12 V, and could anyone provide a wiring diagram? The bike is fitted with a Toyota 12V alternator

Also how do I check if the bike is pos or neg earthed? when I got the bike it didn't have a battery. I connected a battery as pos earth, the lights did not work, at this point the coil had not been fitted. When I connected a battery as neg earth the lights work fine.

Many thanks



New Forum User
Non-VOC Member
Sounds like your coil is grounding out. The lights shouldn't care what ground the bike is - they're DC, but the alternator rectifier board does care if it's positive or negative ground. In your case it is probably negative ground. Check for continuity through your coil. It should be massively resistant to current. If it's showing throughput, it's toast. Cheers,

Pete Appleton

VOC Hon. Editor
Staff member
VOC Member
VOC Forum Administrator
VOC Forum Moderator
Coil resistance


Ignition coils shouldn't be 'massively high resistance'. I am assuming that you are using points triggering on you bike. In that case, you will need a coil with a primary resistance (across the two small terminals) of about 4 or more Ohms.

If, as I suspect, you have a coil designed for an electronic ignition system then it will have a resistance of less than 2 Ohms.

If you fit an electronic ignition coil to a points system then when you turn the ignition on with the points closed the coil will overheat and the points will burn and melt.

Try Paul Goff for a points compatible coil.

Hope this helps.