Hello everyone,

I've been visiting the forum for a few months now and I'm really impressed!

Now I have a BSA WM20 from 1944.
These were restored a long time ago and have been standing for a few years.
The engine no longer starts, but ran until the end.
I have already done the following work:
- Carburettor (276) cleaned
- Ignition set
- Oil changed
- filled with new petrol

There is a good spark and the spark plug is also wet.
Piston rings and cylinders also look good.
It still doesn't start.
In fact, the compression seems a bit low to me.
For comparison, I have an M21 from 1950 and it requires significantly more physical effort to start ;)

Today I'm going to grind in the valves because you can see some rust at this point and I could imagine that the missing compression is lost there.

I also noticed that it is blowing something out of the crankcase (main side).
Is that normal?

Best regards
Magnus

The crankcase breather on the wartime engines is screwed into the crank case just below the cylinder on the left hand side of the bike...The breather valve assembly is the same as the one screwed into the timing cover of your later M21...If the bike has been left standing it's worth stripping the breather to ensure the 'flap valve' is not stuck and moves freely...Ian

email (option): ian@wright52.plus.com

I unscrewed the vent. No change.

But what I noticed now when I closed the inlet by hand is that the engine both sucks and blows out of the inlet.
Is this normal with any camshaft setting?
Or is the inlet valve not closing properly?

The valve timing on an M20/21 is known as overlap. (Which is why it's important to adjust the tappets according to the BSA manual). The inlet valve starts to open before the exhaust valve has fully shut. The idea being that the exiting exhaust gas helps draw in the new fuel air charge.

However I'm not sure that it should be blowing back through the inlet. Ron

email (option): ronpier@talk21.com

Corroded or sticky inlet valve stem. Always store your bike with the valves shut.
If you have a multi-cylinder bike, that's just greedy!

Mark

email (option): pes.sales@btconnect.com

Don't trust anyone who says, "It ran until the end."
This was really the end at the end... can someone do something with the designation in the piston (82 and 35)?
The cylinder treads still look good. I'll measure it and then get a new piston and have the cylinder honed.

I'd say it's definitely nipped up at least once. Ron

email (option): ronpier@talk21.com