4.3 Mercruiser Car Engine has a stutter when starting

[jdelisle]

Mercuiser 4.3 engine The engine turns for s second while starting then a quick hesitation then spins and starts up. . I've replaced the the relay and starter and problem still exists.
Is it an electrical issue, timing, Any help appreciated.

 

[Bt Doctur]

a slight hydro-locking issue? oil clean? age of manifolds and elbows?

 

[jdelisle]

Oil changed in fall is clean and no traces of water. Manifolds and elbows original to 04 boat and run in salt water and flush after each use. However, I have to change or clean thermostat during the season due to scale debris. I'll keep a close eye on oil.Maybe I should pull plugs and try to start to see if anything spits out cylinders?