Since it doesnt "jam up" with the spark plug installed perhaps the decompression valve has either been replaced with a a spark plug or it could be jammed shut, The decompression balve is located ontop of the head, to find it you will need to remove the black cowling ontop of the motor. I dont have any pics on the computer to illustrate but if I get out the back in the next few days I will take a couple.
Thanks Joe, I have the cowling off but couldn't see a decompression valve, have another look today.
