Rebuilt B40 spitting through carb

[Dabbist]

My head just imploded. I guess that 15 degrees of advance, marked on the dizzy is at the points, which equals 30 degrees at the crankshaft.
Does anybody have a conversion from crankshaft degrees to piston position for the B40?

[JulianS]

That is correct.

Makers stopped using fully retarded figures to set timing because variations in manufacture and wear in the a/r unit caused inaccuracy at the more important fully advanced figure.

When using a degree disc to set timing it is crucial to accurately find TDC - using a piston stop the easiest way. Several degrees can be lost around TDC when using rodnthrough plug hole method.

[Dabbist]

Very true, but finding tdc isn't really my problem. Trying to determine a sensible timing figure is.
The very fact that there's a 10 deg difference between the 1962 and 1964 figures rings alarm bells for me.
If I take the '64 ss90 figure of 5 degrees as correct, then depending on whether the advance figure on the dizzy is at the points or the crankshaft I get either 20deg, which seems very retarded or 45 deg, which seems insanely advanced.
Add to that the fact that cdi produces a much faster and shorter spark than points (and an adjustment for modern fuel) and it all starts to look somewhat random.
With a dizzy, I'd just twist it to get the best running, but that's not really practical for moving the stator.

[Dabbist]

Oops, arithmetic failure...35 degrees.

[JulianS]

In 1963 BSA changed the distributor from 15 degree advance to 10 degree advance but the fully advanced setting remained the same.

Hence the up to 1963 retard setting of 15.5 and 5.5 1963 on.

[Dabbist]

Thanks Julian, I didn't know that.
So is that also implying that the advance figure on the distributor is at the distributor, hence the ten degree change at the crank shaft?

[JulianS]

Thats right.