You've got a combustion leak..
It's either a leaking head gasket.. Or through the valves.. One way to check if it's down through the rings, is to squirt about 5 or 6 cc [teaspoon] through spark plug hole. Turn engine over a couple of times.. Then check compression.. If it rises alot it's leaking down through rings..
Check to see there is a tight valve clearance holding the valve open ??
Check clearance when lobe on the cylinder is facing up...
I sent the work experience boy off once for a tin of compressed air.. He came back with a Truck air tank. Later sent the same guy out for a long weight. He went across the road to the pub and won $700 on the pokies and took us all out for dinner. I later suggested to the boss that we should see if he could get a skyhook. My workmate got all upset as he knew someone that did get one (a sky hook). That person that got one was me and my boss (at the time) was not too impressed with the big bill from Sikorsky Helicopters.. I got him a hook from a Sky Crane helicopter...10K USD thank you at the time..
On subject you either have a burnt valve or rings gone.. Time to freshen the motor up