140 merc blown head gasket

[Gettinther]

OK so here goes, last year I replaced the head gasket on a fairly new engine figured the cause was water in #3 cylinder because it never got winterized soon enough the fall before.
Replaced the head gasket, had the head resurfaced and checked for cracks, checked the block for cracks and at that point looked good .
Ran the boat on muffs for half an hour then put it in the lake.
15 minutes in the lake and blew the gasket again.
Took it back and removed the head blown gasket in 3 cylinders ​this time so I figure has to be water inside but oil is clean, no sign of a crack in any cylinder or the head.
So I start checking the manifold, riser and elbow.
I pressure tested the manifold riser and elbow at 40 psi with soapy water and no sign of any cracks.
Next I think it has to be the flaps in the exhaust but they look like new.
Has anyone got any Idea what I might do or be missing.

 

[Alumarine]

Is the the head mating surface on the block straight?

 

[Gettinther]

about as close to perfect as ya could want, but this time gasket damage is at #2,3,4

 

[Bondo]

I pressure tested the manifold riser and elbow at 40 psi with soapy water and no sign of any cracks.

Ayuh,.... The riser/ elbow has dump ports, how'd ya get it to hold air,..??

Was there signs of water in those 3 cylinders,..??

 

[Gettinther]

Thanks Bondo

The elbow only has one dump port and had to shall I say fashion a plug and clamp it in,
As far as water in the cylinders I actually never saw any but signs of fresh rust spots in the manifold but just spots no great accumulation.
A couple more questions or ideas I was wondering.
When the manifold is cold is it possible a crack could close to the point of no leak like on muffs or off the engine?
Would acetone be of any help to find a leak? ​
Only reason I am so interested in fixing this is because it belongs to an old friend so I become quite inventive.