M33 magneto matters
Rick generates the right info for a 1949 BSA
Brian from Leeds emailed to ask two questions: where locally he could get his 1949 BSA M33 magneto rewound, and how to time it up upon its return. Seems to me that FTW (01142 336269) might be his best bet, as they’re not too far away in Sheffield.
Setting the ignition timing is relatively straightforward, but it’s essential to use a magneto pinion puller when dealing with BSA singles. The end came for my dad’s BSA M20 when he tried to lever the pinion off the shaft and broke the back off the crankcase – it’s not as strong as it appears. Pullers only cost around £15.
Timing needs to be set with the ignition lever at full advance. That’s when the camplate under the points is pulled as far as it will go AGAINST the magneto’s direction of rotation. Next, check the points gap is correct (0.012in) then set the engine to the firing position, which my BSA service sheets say is 7/16in (0.4375in) before TDC on the compression stroke – coming up to the top with both valves closed – taking care not to go over compression and set it on the down stroke; it’s easily done. Now stick a cigarette paper or similar between the points and, pulling it gently, turn the magneto shaft in the running direction until you feel it release. Then fit the pinion without moving anything. A light tap onto its shaft will help it grip while you tighten the nut; once it’s all tight, go back and check that the points still break at the right point.
Once the timing is set correctly, it’s worth making up a tool to stop the piston at the right place in future (see page 88).