5.0l 305 running rich

Steve Diomin

Cadet
Joined
Jul 30, 2010
Messages
9
I have a 1998 5.0l merc carberator engine running rich. The engine has 250hrs. I have just put in new plugs as the old were black and sooty, put a new rotor and cap also. The boat runs great cold but as it warms up it seems to richen up. The electric choke is closed when it starts and slowly opens as it warms. I ran some carb cleaner threw the tank but that seemed to not help. Sometime while running 20mph you can feel the engine want to sputter out or choke. When puttering back to the dock it almost wants to stall as it is burning so rich. I beleive the cylinders are getting way to much fuel but i dont know how to combat this or whats making it do this. A guy thats knows what he is doing looked at it last year when bringing it out of storage and he did make the coment about how much fuel was shooting out the spark plug holes as we were turning the boat over without the plugs in to clean it out. Like i said it runs fine cold, one pump and it fires right up, 75% of the time when warm and starting up the engine it turns and turns like 10 times before it fires and you really get that flooded feel and smell. No one has ever tinkered with any of the engine and the carb seems to be lined up to the specs...


What the heck is going on?.
 

Alpheus

Lieutenant Commander
Joined
Aug 3, 2009
Messages
1,757
Re: 5.0l 305 running rich

What kind of carb???

Could be:

Heavy fuel soaked float or floats.

Float or floats out of adjustment

Bad needle and or seat.

to much fuel pressure.

If its not the fuel pressure your carb needs a rebuild...
 

Steve Diomin

Cadet
Joined
Jul 30, 2010
Messages
9
Re: 5.0l 305 running rich

its ia a 2bbl mercarb with thunderbolt V ignition.

I am going to try and test the fuel pressure today, if it is to high can it be adjusted or do you just replace the pump. I dont think it is high becuase i but a clear piece of tube going from the tank to the filter just so i could see the flow and it didnt at all seem excessive, i t didnt seem like alot at all but i will still do a pressure test.
 

Aloysius

Chief Petty Officer
Joined
Aug 21, 2010
Messages
484
Re: 5.0l 305 running rich

first, rebuild the carb..no big deal. Only 250 hrs. in 13 years, probably really gunked up from lack of use.
 

John_S

Rear Admiral
Joined
Jun 21, 2004
Messages
4,269
Re: 5.0l 305 running rich

its ia a 2bbl mercarb with thunderbolt V ignition.

I am going to try and test the fuel pressure today, if it is to high can it be adjusted or do you just replace the pump. I dont think it is high becuase i but a clear piece of tube going from the tank to the filter just so i could see the flow and it didnt at all seem excessive, i t didnt seem like alot at all but i will still do a pressure test.


A '98 may be an elec fuel pump, but if mechanical, you might be getting gas through the site tube line. It means the pump is bad and needs to be replaced.

No pressure adjustment on mech or elec fuel pumps.
 

Steve Diomin

Cadet
Joined
Jul 30, 2010
Messages
9
Re: 5.0l 305 running rich

its an electric fuel pump. Whats the chances of the fuel pump delivering too much fuel pressure...........is this possible?? or should i be concentrating my efforts on the carb?
 

John_S

Rear Admiral
Joined
Jun 21, 2004
Messages
4,269
Re: 5.0l 305 running rich

It definitly happens with mech pumps, and believe I have seen tech bulletin or other posts with check ball issues that can cause pressure problems on elec pumps. If you are going to shotgun, which is more likely? Probably the carb, but its not my dice. If you were going to measure the pressure, follow through. You will kick yourself if you don't and go through the carb, and it is still a pressure problem.
 

Alpheus

Lieutenant Commander
Joined
Aug 3, 2009
Messages
1,757
Re: 5.0l 305 running rich

its ia a 2bbl mercarb with thunderbolt V ignition.

I am going to try and test the fuel pressure today, if it is to high can it be adjusted or do you just replace the pump. I dont think it is high becuase i but a clear piece of tube going from the tank to the filter just so i could see the flow and it didnt at all seem excessive, i t didnt seem like alot at all but i will still do a pressure test.

How can you tell the difference between 4psi and 1000 psi just by looking at it in a clear tube???

Most likely its your carb that needs a rebuild, but before you go tearing it off, just perform a simple fuel pressure test to rule it out...

Good luck...
 
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