BSA M20 - niggles or serious?

[CarlHolm]

Hi there,

I have recently become the proud owner of a 1943 BSA M20, and although the bike is in very good condition and starts first kick every time, there are some problems. Any tips that you can offer as to the cause and remedy of any of these would be greatly appreciated!

1)   The major one, is that it seems to be running very hot, and in fact yesterday I went on a short run (12km) and the motor was  smoking by the time I got home. The insulation tape on the plug lead had also melted, it was that hot! As I pulled into my driveway, she just gasped and the fire went out. It took a long time to cool down and stop smoking, and I haven't tried to kick it over since. I have a feeling this may be connected to the fact that I can't seem to set the igniton to full advance and the lever only comes about half-way back. I hope I haven't done serious damage to the motor! (Seized piston or warped cylinder or head or worse... is worse possible?)

How does one adjust the travel in the ignition advance lever? How does one knoe if it really is going to full advance?

2)   The gearbox likes to select false neutrals when I change up (downward movement of the shift lever.) It then finds the gears (sometimes) when the lever is moved upwards.  Is this simply a question of clutch adjustment, or is it likely to be the selector arm? (or something else?)

3)   The choke won't stay open and the lever keeps wandering forward, towards the closed position. How does one adjust this and then make sure it stays where it's supposed to?

4)   What grade of fuel should I be using, and what additives do people recommend?

[mini-me]

The absolute go to site for WD M20.

http://pub37.bravenet.com/forum/static/show.php?usernum=3155626639&frmid=16&cmd=show

[R]

The ignition timing is somewhat important, as you say - your engine life depends on it. !
If it doesn't get full advance with the lever all the way up, you need to correct that, pronto.

You need to have a study of its action on the points in the magdyno.
Normally, pulling the lever all the way up gives full advance.
And most motoring can be done like this.
You only need to let the lever back down part way for starting, and periods of idling.
It will get hot, really hot, if the timing isn't advanced enough at 'speed'.

The action on the choke needs the top nut tightening, to prevent it moving by itself.
If tightening doesn't work, you need to go into the assembly order of the bits, and make sure it does. Running with the choke partly on is not good, not good at all....
These are the biggies for now.

Have fun !

[CarlHolm]

If I understand it correctly from the manual, the lever pulled towards the rider advances the ignition, pushed away from the rider retards it. Is that correct?

[Rex]

If it's the original MO mag then no. It's pull to retard (assuming some mug hasn't put the lever on the right hand bar, of course)
If it's a later mag it's pull to advance, so you need to check what's going on, assuming you know what to do there.

[iansoady]

The easy way to check is to pop the cover off the contact breaker. Turn the engine over with the kickstart and note which way the contact breaker assembly rotates (you may need a helper to do this).

If the advance / retard wire pulls in the same direction as rotation it's slack wire advance; if it opposes it's tight wire.