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