4.3 L Mercruiser running rough when warm

[Chuthmak]

I have a feeling this is a common question BUT here we go.

Boat is a 1991 Sea Ray 170 with 4.3L Mercruiser V6, 2BBL carb

Boat starts instantly when cold. Will run 95-100% perfect for 10-15 minutes full speed then it starts to miss and bog and run rough.

What has been done so far in order:

New in line fuel pump with filter (me)
New coil (mechanic)
New distributor module (mechanic)
Tested on water with mechanic pouring gas directly into carb while running bad. No change.
Mechanic tested different Thunderbolt IV but it was for V8 so don't think that was helpful.
Filter in mechanical fuel pump is clean
New distributor cap (me)
New rotor (me)
New spark plugs MR43T 0.035 (me)
Filter at carb was packed with granulated something, maybe varnish? Cleaned it (me)
Tested it twice, same problems
Filter at carb was once again dirty with same stuff but not too much. Cleaned it (me)
Sprayed Carb cleaner into carb while underway, sometimes it was helpuful, sometimes not.
Drained tank replace with fresh gas
Sprayed Carb cleaner into carb while underway, no change

Mostly it won't go past 1800-2000 rpm

Thinking next steps are replace TBolt IV or rebuild carb.

When it runs on start up it runs great.

Thoughts anyone?

Charles

 

[Bondo]

Filter at carb was packed with granulated something, maybe varnish? Cleaned it (me)
Filter at carb was once again dirty with same stuff but not too much. Cleaned it (me)
or rebuild carb.

Ayuh,...... Welcome Aboard,...... Ya got a fuel delivery issue, why would ya replace the ignition,..??

 

[Scott Danforth]

Spraying carb cleaner down the throat of a carb is like paying a stripper, however not seeing the show....... Just a waste of money

To clean the carb, you have to pull it apart and properly clean it

 

[Chuthmak]

Bondo: mechanic did his thing. It was his call on the electric. I have since not trusted his judgement.

danforth: I hear ya.