4.3L with a lean sneeze at idle

[jrs_diesel]

A friend of mine recently bought a 2004 Bayliner with the 4.3L V6, it has Thunderbolt 5 ignition and a 2 BBL Mercarb. I've been helping him fix some things on it. One of the problems we have noticed is that it has a lean sneeze at low idle, mainly when it's in gear. We had to get the cap off of the mixture screw, and when we tried to adjust the idle mix, it had no effect. This boat had sat for at least a year, maybe more before my friend got it.

It starts easily, the electric choke works. Accelerates just fine and runs great anywhere above idle. It only seems to have the lean sneeze at idle in gear. I'm hesitant to pull the carb because it runs great other than low idle in gear. I suspect the idle mix circuit may be clogged. Is there anything we can do with the carb on the engine to get the idle mix screw to do what it is supposed to do?

 

[Scott Danforth]

Pull the carb, tear it apart and clean it.

 

[Bondo]

Ayuh,... Sounds like crud in the idle circuit of the carb,...

Pull it, 'n rebuild it,....

 

[alldodge]

It will continue to do this until the carb idle circuit is cleaned.

 

[Brian.Burress]

First off, I want to thank everyone for their suggestions.

I am the friend jrs_diesel was referring to. We pulled the carb this weekend to rebuild it and once it was removed we found one of the throttle plates was bent in towards the intake manifold. It looks as though someone jammed a screwdriver down the carb and forgot it was there and closed the throttle, bending one of the throttle plates. It is now straightened out and we rebuilt the carb, but upon reinstall of the carb a nut found it's way into the intake manifold towards cylinder #4. We pulled the intake manifold and the valley of the engine looked like someone poured a Wendy's frosty in there. Water and oil have definitely mixed at some point. We cleaned it out with carb cleaner and some shop towels and I will be replacing the engine oil tomorrow. I hope I don't have a cracked block or heads. A friend of mine thinks I might be alright just replacing the intake manifold gaskets and trying again. Can anyone give any suggestions of tests I can run to find out where the water is coming from?

 

[Bondo]

I hope I don't have a cracked block or heads. A friend of mine thinks I might be alright just replacing the intake manifold gaskets and trying again. Can anyone give any suggestions of tests I can run to find out where the water is coming from?

Ayuh,..... Welcome Aboard,.... There's the possibility that you found condensation,...

Put it back together, 'n get it runnin',....
When ya do, watch the oil Level,....
If the oil level rises as it runs, ya probably got a cracked block,....

To test for a cracked block, 'n to find the crack,...
After the intake is back on, drain the block, isolate the block's coolin' passages from the drive, 'n exhaust manifolds, 'n pressurize the block with air, 'bout 15 psi....
If the block is cracked, you hear the air escaping, follow the sound to the crack,....

If the oil level stays stable,... run it up to temp, to steam off any remainin' water in the oil,....