Welcome to JUST ANSWER small engines, let me try to help you !!
Sounds like the valves are too tight and need adjusting.
The valve covers are on top of the block and held on with a single bolt in the center. The valve lash is .008 on intake and .013 in. on the exhaust.
As te engine warms, the clearance goes away, and the longer engine runs, the more they leak. After engine cools, it will just repeat.
under intake manifold?
ia access to adjust clearance under intake manifold?
Yes, it is not easly accessable, one will have the vent line in it.
The rectangular openings.
would compression be a lot higher when engine is cold?
It will help compression and power.
Give it a try and let me know how it works. Thank you.