1986 searay 5.7 mercruiser running rough

the newfoundler

Recruit
Joined
Sep 3, 2009
Messages
5
Hi guys, I am new here to the forum and I am looking for a little input with my problem. I got an 86 searay with a 5.7, alpha 1. The fuel pump ruptured last fall and dumped gas into the base. I did not run it long after the pump failed as it was smoking and bogging down so I shut off the engine right away. Replaced the fuel pump and changed the oil and filter. Seemed to be running a little rough but I didn't run it long after that time. it was the end of the season and i was pulling the boat in for the year soon so I never thought about it until now. I put the boat in today and it is still running rough. Starts right up, revs up, sounds good while cold and at the dock. I did notice that she is smoking a bit. I noticed that there are 2 hoses, each one go into the top oil pan on each side of the engine and connect to the flame arrester. After the boat warms up, I notice a lot of fumes coming out of these PVC hoses. If I remove the hose from the oil pan, the boat will rev a little and seems to smooth out. When I put the hose back into the oil pan, it dies down and runs rough and smokes.It runs especially rough under load. I checked the compression last fall, all around 135-140. NO sign of water in oil or top pan, PVC hose etc.. But the oil still smells a little like gas. Has anyone ran into this problem before? Or can help me sort it out? Does it sound like just the oil, or something more serious like rings, head gasket, etc...
Thanks
 

RCAnderson

Seaman
Joined
Oct 5, 2011
Messages
68
Re: 1986 searay 5.7 mercruiser running rough

I think the 'top pan' you are referring to is your valve cover ? Then that would be the PCV valve\hose that is connected to the flame arrestor. And it sounds like they are clogged. Is the carb flooding when idleing ? Do you have a mechanical fuel pump or an electric one ?
 

the newfoundler

Recruit
Joined
Sep 3, 2009
Messages
5
Re: 1986 searay 5.7 mercruiser running rough

Yes, you are right. Valve covers. It has a Holly mechanical fuel pump. There are no PVC valves connected to the hoses. Just open fittings that fit in the valve covers and connect to the hoses going to the flame arrester. a lot of blue or black fumes coming out of the valve covers.
 

the newfoundler

Recruit
Joined
Sep 3, 2009
Messages
5
Re: 1986 searay 5.7 mercruiser running rough

It does not seem to be flooding at idle, but runs rough, like an ignition miss, until I take the hoses off the valve covers. Then it seems to clear itself.
 

jze1212

Petty Officer 2nd Class
Joined
Feb 11, 2006
Messages
156
Re: 1986 searay 5.7 mercruiser running rough

I'd rule out any ignition problems if the symptoms go away when you pull the PCV.. If it is running nice when it is cold then begins to run poorly it may be possible that you did not get all of the gasoline out of the pan, and as a result when it gets warm it is causing excess crank case vapors , resulting in a rough idle.. After you rev a few times it will clear and then happen again after at low RPM ( rinse and repeat.. )
 
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