The new Sparkplug was only run for a few minutes.

No evidence of a head-gasket problem, coolant looks fine, top of piston has no hole in it nor does it look to have been "steamed cleaned" from a blown head-gasket.

compression tester working fine, tested at WOT & engine running.
Has good spark as indicated by an inline spark tester.

Did not test cyl. # 1 fuel injector for flow.

I know the timing belt replacement won't fix this or any other problem, just was telling myself that it's not worth opening up the engine to fix a valve problem.

Are these engines known for burning valves as they have self-adjusting tappets.

Sticky valve stem from crusty oil build up, worth doing an engine flush ??
Fill the engine up with kerosene and see what happens ??

Oil is relativity clean , car used mainly for short trips to work.

I used to work on a lot of Volvo's back in the day, never saw/heard of problem like this.