Excessive gas in cyclinders

[jackmanjls]

I have a 2006 F90 that doesn't runs rough.

I had a problem about a month ago where the engine would stall when going to idle. After I trailored the boat and brought to the house I couldn't repeat the problem. It ran smoothly in the driveway.
I put the beast back in the water, ran great for about 30 min at WOT. Going out was fine but coming back I had the idle problem.

Here's what I know:

1. if I can get the boat into gear and run WOT is not problem it's the lower rpm's were I have the issue.

2. I currently have the boat on the trailer and had the wife start the F90, in the driveway. I was in the back and noticed that It smelled of heavy exhaust, you can see it in the air.

3. I also noticed that the exhaust that comes out of the prop had heavy black residue.

4. All 4 plugs have a heavy black carbon, similar to a "too much gas" problem.

I'm looking for ideas on what to check to resolve this issue.

[99yam40]

fuel pressures would be the 1st thing I would look at.

but hooking up to a computer just like automotive may lead you in the proper direction.

I would think that the motor in question is capable to give a read out, but not sure on that old

[rodbolt17]

check the fuel pump diaphragm also.I had an F90 with the oil so contaminated it would actually idle with the HP pump disconnected.