4.3L running rough after idoling or going slow

[scottkanderson]

Hi, I never post many questions on here, but I have learned tons in the last 4 yrs as a boat owner from reading. I tried looking for an answer to this, but did not know what to search for.

I have an 1986 OMC 800(yes they made stringers in 86) with a carberated 4.3 GMC with points. It starts right up and runs great, but if I go slow or idol for any amount of time longer than 5 min, when I go back to a faster speed the engine seems to be missing, or running like water in the fuel for 2-3 min and then starts running good again. It will have power then loose it for a sec, like the engine is surging. If I shut the engine off when I stop, or never go slow, this is not a problem.

I have not changed the plugs or points yet, but if it was one of these, wouldn't it run bad all the time? If you don't think it is one of these, what else should I be looking at?

Thanks,
Scott

 

[Bondo]

Re: 4.3L running rough after idoling or going slow

I would check the idle fuel/air mixture and make certain the choke remains fully open.

Ayuh,... I see a Carb issue....

 

[scottkanderson]

Re: 4.3L running rough after idoling or going slow

I don't know anything about carbs, how do i check idle fuel/air mixture? It is idling good now.

Thanks

 

[Maxumum]

Re: 4.3L running rough after idoling or going slow

My 4.3 was acting this way, it turned out to be leaking exhaust manifold allowing water into the cylinders when running slow. At speed the water would be blown out with the exhaust. If your boat is raw water cooled these manifolds rust out quickly especially in salt water.

 

[what the]

Re: 4.3L running rough after idoling or going slow

Is this a new problem or was it like this when you bought the boat?

To set the air/fuel mix there should be two screws located at the front base of the carb with springs on them. with the engine running at idel and warm with choke open slowly turn one screw in just untill the rpm drops then slowly turn it out just untill the rpm rises. repeat on the other screw this will set it close and will not load up at idel.

The exhaust manifold could be the problem I havent seen it but it makes since I would set the air/fuel then remove all sparkplugs and check for rust and then with all plugs removed connect flush hose and let water run for 15 min then bar the engine over and check to see if you get any water out of cylinders (engine needs to be warm)