AJS & Matchless Owners Club

Have a Model 20 1956, just recond engine, the mag was recond some 4 years ago, assembled everything as per the manual, would not start, then discovered that it would run on offside cylinder with the plug lead to the nearside disconnected, when I put a spare plug into the nearside lead and earthed the plug body a lovely fat spark, when I took off the lead and put it back onto the plug fitted to nearside the engine stopped as if I had thrown a switch, any answers please
Regards
Len

Dear Len,

I had a similar sounding problem on a BSA A7 a few years back. It turned out that the mag pickup brushes (supplied by a BSA specialist!) were as soft as a 2B pencil. They had smeared a film of graphite round the pickup slipring, so that when the mag generated a spark, it was going onto both plugs at once. I suspect the reason why you got a lovely spark on the plug out in the open is that it's much easier for a spark to jump the gap at atmospheric pressure than it is when the plug is in the cylinder and the air between the points is compressed, so all the spark jumped across that plug, with nothing across the plug that was under compression. A quick check on the pickup brushes is to take one out and try to see the surface of the slip ring. It should be nicely burnished, and clean, with no carbon smeared over the gaps between the two metal surfaces. If it's got a black streak of carbon right round it, effectively both sides of the slip ring are shorted together - it doesn't take much carbon for 10-15,000 volts to arc across! If it's difficult to see the slip ring, try wiping the pickup brush across a piece of paper. It should leave as little mark on the paper as possible - more like a 6H pencil than a 2B. Hope this helps.

Kes Enright

black.five@cwcom.net