Steve,
In laymens terms the cylinder in that particular engine is larger daimeter at the bottom
than the top. when the piston travels up it gets tighter. in some of these engines the piston
can actually become caught in the pinch (the tightest part of the cylinder) heating the engine
with a heat gun will cause the cylinder to expand and allow the piston to unstick. Like opjose
said when the motor runs for a while the tightness will decrease. I have had RC buggy engines
that were pinched so tight that after getting stuck I would have to remove the head and push
the piston back down with a wooden dowel covered with rubber tubing.