Set the light at 24 degrees advance on the display. Yes, this this is BTDC.
Then, rev the engine until the pulley or flywheel TDC mark stops advancing (moving)- approx 4000 rpm. If the mark then aligns with the TDC mark, it is at 24 deg BTDC. If not, move dizzy round until it is.
It is normal for it to move about a bit (spark scatter) if using a chain and no tensioner. Even more if you have points.
I run a timing belt kit and there is practically no scatter (with Pulsar dizzy).
If running hot, it's probably a bit retarded.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
