Alternative methods of ignition timing

[pdg]

I've just got hold of a "BSA Model A7 and A7 Star 500cc Twin Motorcycle Instuction Manual 1947" in which two alternative methods for ignition timing are described. The first is the normal "3/8" before TDC with the ATD fully advanced. The second is "@ TDC with the ATD completely unadvanced.

The second method is clearly much more straightforward from a practical perspective. Also, arguably and desirably, using one method allows a check on timing accuracy with the other method.

Has anyone tried the "@ TDC with the ATD completely unadvanced" method?

Thanks, Peter.

[Butch (cb)]

I guess that the issue is that the non-advanced setting is relatively unimportant, and so if there is imprecision between the two states then you will have it set precisely for the non-advanced setting but are at the mercy of variation for where fully advanced actually sits.

[Triton Thrasher]

You'd have to know the exact range of the advancing mechanism and even then I don't see the point.

Check and measure the thing you want to know, which is timing at full advance.

[pdg]

Many thanks.

I'll stick with setting it at 3/8" before TDC and the ignition fully advanced.

[Rgs-Bill]

You can double check your timing at 3/8 before TDC, by retarding the ignition all of the way, and bring it up to TDC, then see if the points is just releasing a fag paper, because that is where you cold start the magneto when you have a manual advance/retard  system.  If  the points are just releasing the fag paper at both settings, your bike is spot on..  Now the only other concern would be that the cam ring does not fire exactly at 180* and a 180*, that requires some machining work.  Brand new old stock cam rings have been known to be out by 5* or even more, right from the get go, which a lot of fellas fail to check,   (and then machine things to correct them).

[terryk]

Hi Peter I wondered years ago why there was two different setting for the early longstroke. I have later plunger A10s and A7s but I understood why they state the unadvanced setting when I purchased a 1947 A7 and checked the timing. My 1947 has a steel cover over the advance unit so you cant wedge them to advance it. The bike runs very well and when I checked the timing its exactly as the unadvanced setting states in the manual at TDC.