1985 Mercury 75 - Fine on land, dying at idle in water

[CharlieB]

The symptoms you describe are classic lean idle mixtures. Not removing the idle mixture scres and cleaning those passages left accumulated crud restricting fuel flow at idle. The additional back pressure of the water column in the exhaust once launched increases the motors demand for idle fuel.

Idle mixtures will need to be perfected once on the water running IN Forward gear slowly adjust each carb to achieve the Best Idle Speed. Because it is a 2 stroke and no accelerator pump it needs even MORE fuel for crisp acceleration otherwise it will hesitate, cough or bog. Almost another 1/8 turn fatter than Best Idle Speed.

 

[quicktach]

Make sure your idle adjustment isn't set too low; you may be flooding to try to get it started. Check that your advance isn't binding or sticking.

 

[EchoNovember]

How do I check the advance and the timings? Getting into TDC/BTDC/ATDC/?TDC is more than I have ever done with an engine, and I clueless about it.

For the mixtures, I was already thinking of taking the carbs back off and removing the idle jet and screws.

Do I have this right? Also, what is the small hole to side of the carb throat?

 

[EchoNovember]

Alright, here's what I've done now:

Replaced spark plugs
Partial de-carb of engine (thought I did full de-carb, then realized later on I needed to use more Seafoam, and for longer)
Checked I have spark, and I do
Checked compression, 150 psi on all 4 cylinders, give or take 1 psi
Replaced old carb gaskets
Replaced water pump as partially melted housing was discovered
Rebuilt fuel pump. Initially this was a preventative "why not?", but the diaphragm turned out to be stretched out.
Changed lower unit gear oil and greased zerks
Replaced fuel line from tank to motor
Replaced damaged prop with a good condition used one - Not going to affect the engine at idle, but I needed to do it anyways

The only things left I can think of doing are:
Check fuel tank strainer - will do this tomorrow
Replace internal fuel hoses - more work than I care to think about doing right now, due the amount of stuff in the way I would need to remove...
Adjust carbs while in water when transom is finished.

Anything else? I was already planning to do a lot of work on the motor over the winter, now I'm thinking I've done almost everything I was planning on doing, except tearing it apart completely to clean it and replace worn parts. Not sure I even need to consider that now...