Jools,
Our BSA’s seemed to be best at lower advance numbers. I do agree that 36-38 Deg seems to be universal back in the 60s, but many owners today set them lower.
BSA Book values were:
Plunger GF – 11/32” = 0.3438” = 33.7 Deg BTDC (1950 Handbook)
Iron Head S/A GF – 13/32” = 0.4063” = 36.8 Deg BTDC ( 1971 Handbook Reprint )
Super Rocket – 13/32” = 0.4063” = 36.8 Deg BTDC ( 1971 Handbook Reprint )
The Lucas auto advance for A7 & A10 has an advance range of 11-13 Deg for 26 Deg ignition range, so obviously specified by BSA to Lucas. So at 36Deg, the start retarded setting is as high as 14.8 Deg, almost guaranteed to kickback.
The Lucas Triumph Twin ATD’s have advance range of 15-18 Deg, for 36 Deg ignition range.
Orabanda dyno-tested his bikes – worth reading his whole contribution
https://www.a7a10.net/forum/index.php?topic=3747.msg25975#msg25975and he arrived at the following for best performance:-
Plunger GF: - 30 - 30.5 degrees
Iron head swinging arm GF (2 off): - 30 - 30.5 degrees
Super Rocket: - 32 degrees. (Note 4.8 Deg lower than BSA standard)
These are all very much lower than BSA specified.
Modern factors which have changed the settings are:
• Top Dead Centre can be set much more accurately these days, removing a wide range of angles we used to do with stick down the plug hole - +/- several degrees here.
• Also need to keep in mind that my 31 Deg is set by timing light, whereas a fag paper uses numbers about 2 Deg more retarded, as you pull the paper out when the points have already started to open. (That is something I would like to quantify when I get the magneto back in the bike)
• Wide opinion that modern fuels with lower Octane RON Number require 2 or more degrees retardation compared to 1960s fuels at 100 RON. Determined by trials.
I did try to set mine at 30 Deg, but after tightening the bolt, it measured 31 Deg, so I left it alone, and very pleased with that am I. RDFella also seems happy with 31 Deg.
Stephen McFarlane requires a maximum of 33 Deg at full advance and no more than 2 Deg at start retarded setting to avoid kickback on the electric start. Thus why I am widening the range of my ATD.
Col
