M33 mag­neto mat­ters

Rick gen­er­ates the right info for a 1949 BSA

Classic Bike (UK) - - News -

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).

Mag­neto pin­ion puller is es­sen­tial for BSA sin­gles – don’t try to lever it off

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