Jonsereds 49sp with too much compression? can't pull the starter unless plug is loose...

[Bruce Barney]

So I've got a 49SP that has never given me any problems before like this...It is almost impossible to pull the starter; I've poked around on the net for ideas and here's where I'm at: Engine rolls over great without the spark plug, Exhaust is fine, muffler OK, cylinder looks nice through exhaust port, I've removed the bar/chain/clutch so no load there, always been running non-ethanol fuel, starter cord is fine and not jamming into the pulley. I've looked through the plug hole and all looks OK on top of the piston. Here's the weird part - if I loosen the plug a turn or two, all is well and the engine starts and runs beautifully, if I tighten the plug, it's basically impossible to pull the starter cord. Any ideas? I've had the saw since new, cut lots and lots of firewood with it, and always started no problem in the past. Sat for a few months before this problem.