Re: Engine wont turn now
Sounds like your clutch might not be engaging enough to make the motor turn over while under compression? If the piston is moving when the plug is out but when you put the plug in and release the clutch the motor doesn't turn (and the back tire doesn't lock up and skid) then your clutch is slipping. It SHOULD lock up and skid if the motor were to be seized or having too much compression....
If this is the case, your clutch being the problem, then you need to check the stuff others posted about the clutch. Make sure the lever is releasing the clutch all the way (Pull the cable off the clutch arm, does the wheel lock now?) If there is slack in the cable and turning the wheel still doesn't turn over the motor (or lock the tire) then you have an internal clutch problem. Grease/oil on the pads? Something coming loose in there? Tighten the flower nut?
If the clutch isn't greasy or something loose, then adjust the flower nut to make the clutch grab better. To do this, remove the screw on the flower nut, LOCK THE CLUTCH LEVER IN (which takes the pressure off the flower nut) then turn the flower nut in a little at a time until when you release the clutch it grabs firmly....