I finally decided to adjust the valves in my 1986 300 Bayou today because it was beginning to start really hard and I had ruled out just about everything else. It was definitely long over due because I bad to turn the set screw out several turns before I got any clearance at all. I adjusted the intake to .127 mm and the exhaust to .152 mm since I read they should be adjusted to .1-.15 mm for the intake and .15-.20 mm for the exhaust. The bike starts up great now but it rattles. I assume I may have one of the valves too loose but is there anything else I might have messed up, possibly when I turned the motor over backward when I was lining up the timing marks? I'll try adjusting the valves again in the morning when it's cooled down again and hopefully that will fix it.
I'm surprised it had any power at all with the intake valve so tight.

 

Turning the motor over ccw does not do anything. I would check the spark plug, see if has proper gap and is in good condition. Who knows? By the way you mention it used to run, maybe the plug is out of service.

When they say timing marks is based to have the engine at tdc. When I did mine I placed the crank onthe T mark, removed the spark plug, carefully inserted an old automobile dipstick into the hole and visually checked piston was at tdc.

 

It's actually starting and running great now, but the rattling is new. Hopefully it goes away when I readjust the valves in the morning.

 

I must have adjusted them on the wrong stroke. All is well now. I can't believe I made that mistake as many times as I've adjusted the valves on my old air cooled VWs.

 

I had the same thing happened to me.
By mistake I set the TDC on the exhaust stroke (incorrect) instead of the compression stroke (correct).