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How many teeth on an auto advance/magneto sprocket?

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Hi,

I’m going to convert my Domi 99 ignition from magneto powered to electronic. I’m going for one of those ‘baked bean can’ jobs that houses the electronic ignition stator and rotor (replaces the mag).

The company from whom I’m thinking of purchasing the ignition kit requires me to send my auto advance unit with mag sprocket so that they can replace it with a fixed advance version. However, I want to keep my auto advance unit in case I ever want to put the bike back to points ignition. I therefore set about looking for a ‘spare’ auto advance unit on eBay. I found two and put low bids on both thinking that I might win one of them. Well, Sod’s law took a hand in things and I won both of them. Both were advertised as being for ‘Norton Dominator’, but one has 19 teeth on the mag sprocket and the other one has 18 teeth.

I think the one with 19 teeth is the correct one for my Domi 99 as the one in the engine when I bought the bike had 19 teeth. However, that’s not a guarantee of correctness as the engine is a real disaster zone. (Is 19 teeth correct for a 1960 Domi 99?). So, what is the 18-tooth sprocket/auto advance from - Domi 88 perhaps? It looks identical to the unit with 19 teeth (except for the lack of one tooth). I haven’t a clue what bike it’s from but hopefully someone out there can solve this mystery.

Thanks

Regards

Tony

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Before making a decision, on your choice of electronic ignition, consider "Thorspark", from Sussex Motorcycles.  The original mag is retained, and uses the existing auto advance.  The trigger unit replaces the points (Although you have already done so, it is not even necessary to remove the mag).  Several other members have fitted this system, and, I have heard of no complaints.

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Hello,

 

the sprocket at the Magneto must have the same number of teeth as the other sprocket on the chain. If not, the engine don't work.

Yours

Helmut

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The Advance / Retard Unit for all big Norton twins has 18 teeth. So Helmut is totally correct. The intermediate pinion carries a double sprocket both having 18 teeth but using different chain widths.

John's suggestion is a quick, easy and cheap way of changing over to Electronic Ignition but by keeping the Advance / Retard Unit you are retaining an item that is well known for being a weak link in the whole system. These Units are prone to jamming either fully open or closed or advancing the ignition too quickly. They also have a limited range of advancement which generally tops out before 4000 rpm. Whereas most of the electronic units carry on advancing the ignition at higher revs, thereby increasing the engine power available.

My advice would be to chose a system that changes all the old parts for a more reliable electric/electronic set-up using a plain drive sprocket. The only other point here is to make sure the plain sprocket matches the drive shaft of the trigger / sensor unit.  You can get both parallel or tapered shaft versions.

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When I fitted mine (RITA), I simply removed the springs and lock wired it on full advance.  If it has no springs, even without the wire it should fly to full advance by centrifugal force as soon as you kick the engine over, and I ran it without wire for a while.  No problems over about 15k miles.

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Tony. I'll second John Shorter on the Thorspark

I fitted one to my 1960 99 two years ago and have not had a problem.

The job is simple enough. The worst job was getting the original cam ring out, but the whole installation job only took around 4 hours, followed by another couple of hours setting up the timing and retuning the carb. The beast fires second or third kick every time even when hot, and runs really smoothly and there is more response on acceleration.

Sussex Motorcycles only keep a small stock but will build the appropriate unit when you order it if they don't have one in stock so delivery might be a bit slow. Apart from that a good piece of kit and best of all simple.

I think I have some photos of the unit on my bike and some wiring diagrams. Send me an email if you want anymore info

John Sunderland

 

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