Maybe the rings are wearing out.
Did you do a plug chop.
I sure hope the piston is not being slowly fried, as a result of a small air leak not picked up because of a lack of leak testing.
If your old Dad wont let you have a leak down tester, at least pressurise it and spray some soapy water around.
You can check the crank seals by immersing the gearbox vent tube in a container of water.
I implore you to do some kind of leak test before it bites you.