Nothing...

Is there some point like carburator flame arrestor where i can measure right angle. And what is minium angle?

Here is a good old thread on this:
Teach me about Exhaust riser angle Please.

ok, and drain the block and use air only

Next thing is do a compression test

I check those before starting engine about 160-170 psi all 8.
Used air only with block drained.

you can use the flame arrester,,but you can just eyeball the tilt.think merc wants at least 13°drop.

I measure front & rear engine mount places in my boat.
I have to lower my engine rear mount in my test bench almost 2". Then it is same level than boat.
I hope that fix issue

Just jack the front of the pallet up and put some blocks or 2x4's under it

Change motor oils and rise idle little + angle motor right.
run it about 10minut idle and have cream + oil level is upper again...
I test compression again with warm engine, all 8 are 160psi.
Pressurize engine cooling system and everythink is fine.
Take off risers and there is water laying bottom off manifolds.

Pull some plugs and crank the motor over a few times with rags hanging in front of plug holes. See if rag gets anything on it besides gas