You put old oil in it??
Mixed accurately to 25:1 ratio???
With fresh fuel???
No ethanol??
It could be time for a rebuild.
By power stroke i assume you mean power torque??
Pull the spark plug lead off and tip the mower on its side, spin the blade plate by hand and see how it feels. Try wobbling the blade plate. Any slop in the bearings??
Pull the cowling and fuel tank off the top, undo the 3 bolts and remove the pull starter. Theres an o-ring under it that seals it, check its condition, it is essential that it is in good order and doing its job.
Then turn the mower over again you will be able to see and feel more.
Put the piston up the top of its stroke and use a torch to check in the bore.
Pop the clip off the top of the muffler and take it off, look in the exhaust port to check the bore and the edge of the piston.
That will give you an idea of condtion.
