Gomower,
If it is a mulching mower yes it should have 4 blades.
As to the over reeving did you check the ring gap when you replaced the piston and rings (ring gap should be between 10-17thow).
Check the top "O" ring (the large one) on the starter assy as it could be leaking causing the mix to be lean.
Check the carbie "O" rings as well (you have two) and make sure they are in good condition.