bsa b31 plunger MO1L magneto

[750rideruk]

Can anyone please clarify the position for magneto advance for ignition timing. My magneto has the HT lead at rear left when looking at the points but it is marked MO1L. Is advanced timing tight wire or slack? Thanks for your input if you have the ''Knowledge''

[Dean Southall]

You should be able to work it out for yourself.
Turn the engine over by hand and note the direction the points rotate. Then operate the adv/ret lever and if the points then open later when the lever pulls the cable then you have slack cable advance. If they open sooner you have tight cable advance.

[Ian C]

Just to add to Deans explanation, please see attached.. HTH

[Calum]

The explanation with reference to control levers can sometimes be confusing - my girlfriend's NH Ariel has taught wire advance (as does my B31) but her control lever is forwards to tension the cable (I assume the control lever is a right hand one repurposed!)

If it's standard your B31 should be taught wire advance. Looking at the points cover, the control cable should enter the magneto on the right as you look at it. If so, then your control cable needs to be taught (magneto fully advanced) and the points just opening when the engine is at 7/16" before TDC. If you are using a tool that screws into the plug hole to measure the distance, do remember that it is at an angle and not parallel to the piston stroke (as a result you'll be looking for a figure nearer to 15mm rather than 11mm, hope this makes sense)