Regarding your M20, you can check compression and/or clutch slip simply by standing on the kickstarter. It should hold your weight easily unless you're unusually heavy. You don't say whether or not the bike starts easily, which can be a good indicator of various things. Next, check that the valve lifter has some free play. You could be inadvertently holding the exhaust valve off its seat and losing power. Next, check the float in the carburettor. You could be starving the bike of fuel, although you'd probably have other indications of that (coughing, banging, etc). A weak mag could be your problem. The spark might be adequate under a low load, but might fail under a heavy load - but this is unlikely to be the cause. It could also be the exhaust valve (common fault). It might have a burnt seat, or some carbon grit holding the valve off its seat, or might need lapping in. Either way, get the head off (after you've stood on the kickstarter and tested that for compression). Check your points too. Worn points will put the ignition timing out. Maybe you could post some more details about general starting when hot and cold, tickover, performance, etc. Also, this site has some good information on M20s:
www.sump-publishing.co.uk. The comment about clutches on military bikes is correct. WM20s have single spring dry clutches behind an ally drum. If you overfill the primary case, you'll wet the clutch plates and the clutch will slip. But you should notice that on starting (you can easily clean off the plates after dismantling). But some WM20s have later 6 spring clutches fitted, which are designed to be wet (but still won't want over-wetting). I suggest you try a hill climb experiment with different plugs just to rule that out. I hope some of this helps.
