1985 OMC I/O 2.5L Stringer 400 Rough Idle with engine cover on

[Seaswirled]

1985 OMC I/O 2.5L Stringer Rough Idle when engine cover is closed.
Lots of work over the last year, had her out 6 times last year no issues. This year, first time out died, wouldn't idle to get to the ramp. Since, I have done carb kit, points, rotor, cap, dwell, timing, plug wires, all addressed and set per factory service manual, Runs great in driveway with mouse ears on. but on the water will not idle with engine cover on. Sounds like chitty chitty bang bang without the bang. Pull the cover back, smoothe's right out, and idles. put the cover back on, rough idle, barely stays running. If I leave the cover partially open runs great all day. I am very mechanical, used to refurb old motorcycles. first time with marine engines. At my age falling off my boat doesn't hurt as bad. Any help would be appreciated.

 

[Maclin]

Welcome Aboard!

Be careful, that sounds like a very closed compartment with no ventilation and hi carbon monoxide content. Can you hear any exhaust puffs when it is running?

 

[Seaswirled]

Thanks for your reply.
Nothing noticeable, we did have to replace the exhaust manifold due to being left with water in it over the winter, froze and cracked the manifold, (former owner), during our rescue of this old boat I upgraded the bilge blower from 3in to 4in. Occasionally we get a little exhaust smell mainly when idling, I have the white pvc piping both in and out high and low from the vents on both sides to the bilge. Is there a way to do a test for exhaust leakage at the manifold?

 

[kenny nunez]

Squirt automatic transmission fluid down the carburetor and crack the throttle to keep it from stopping. Is the choke tube in the manifold?

 

[Scott Danforth]

Thinking exhauat leak at the manifold/head joint

 

[Seaswirled]

Thanks again gentlemen for your replies, I will be checking for leaks in the exhaust system this weekend. I am a working dog and only get time to do stuff on the weekend. rest assured we won't be taking her out again until I correct the issue. So a couple questions about the transmission fluid in the carb: just regular type F fluid? Should I put it in a spray bottle to somewhat atomize it and spray it into the carb, or just pour slowly? What will happen? will it smoke where there is a leak, or am I looking for fluid seepage at the gasket? The choke heat pipe is well into the manifold, no seal there just a snug fit. The engine compartment is very sealed except for the louver vents on the sides, and some access holes for wiring and control cables.
That does make me wonder if I did something wrong, when I got the boat there was only one air pipe installed from the blower to the exhaust vent on the port side. I added another on that same side from the above bilge area to the vent, and added two on the starboard side to the vent one high and one low. when I got the boat there were none on the starboard side.

 

[southkogs]

I'm not sure I'm following your setup on the blower, but what you should be going for is a directed flow OUT. My vent hose runs from a low point in the bilge (gas fumes are heavier than air) to the vent exhaust on the starboard side (blower is on this line). I have one vent in (port side) and it's pretty much wide open - no hose. Basically I want to flood the compartment with undirected air.

My old boat was similar, with the exception that it had one directed vent out (with blower) and one wide open vent out on the other side (no blower, no hose) and two wide open vents in (no hose - one on each side).

Maybe that helps a little?

 

[Scott Danforth]

Thanks again gentlemen for your replies, I will be checking for leaks in the exhaust system this weekend. I am a working dog and only get time to do stuff on the weekend. rest assured we won't be taking her out again until I correct the issue. So a couple questions about the transmission fluid in the carb: just regular type F fluid? Should I put it in a spray bottle to somewhat atomize it and spray it into the carb, or just pour slowly? What will happen? will it smoke where there is a leak, or am I looking for fluid seepage at the gasket? The choke heat pipe is well into the manifold, no seal there just a snug fit. The engine compartment is very sealed except for the louver vents on the sides, and some access holes for wiring and control cables.
That does make me wonder if I did something wrong, when I got the boat there was only one air pipe installed from the blower to the exhaust vent on the port side. I added another on that same side from the above bilge area to the vent, and added two on the starboard side to the vent one high and one low. when I got the boat there were none on the starboard side.

you put blowers on both sides? if so, that is the problem. a blower can move about 30-50 cfm thru a hose. the motor needs about 400 cfm

only need 1 blower mounted low to evacuate fumes. this is to run for a few minutes until the motor is fired up as the motor will draw more air than a blower can move.

 

[Seaswirled]

Sorry to confuse ya all, I only have one blower, the hose goes from just below deck at the bilge compartment to the blower and out of the blower to the vent on the port side (exhaust). what I added was the other three hoses only. The boat did not have the other three hoses when I got it, and I'm thinking I may have restricted the air flow by adding the other hoses. I will be checking for exhaust leaks in the exhaust system though, to make sure that is not part of the problem. We are new boat owners and want to be safe above all else. I will update my findings. Thank you all for your responses.

 

[kenny nunez]

Run the engine to warm it up to normal temperature. Disconnect the throttle cable so that you control the carburetor, squirt any kind of transmission fluid in and keep the throttle open enough so that the engine does not stop, you want to really make a lot of smoke to find the leak. You may get popular with the neighbors by killing the mosquitos.

 

[Seaswirled]

Got it, thanks so much for the info. Maybe if I can make enough smoke I can get rid of the unpopular neighbor. LOL.

 

[Seaswirled]

Update: No exhaust leaks of any kind any where.
So I removed the other three air hoses that were not on the boat when I got it, so now the only air hose is from the bilge below the engine to the blower, and from the blower to the vent on the port side. The other holes in the vents are wide open. Ran fine in the driveway, no rough idle whether the engine cover was on or not, unlike when the boat is in the water.
The only difference I can think of is when in the lake, there would be some back pressure from the exhaust being under the waterline. so if I take off the prop, and fill up a big tub or barrel with the drive fully down and under water, can I run the engine, to see if there is any difference with the drive submerged?

 

[southkogs]

Won't work well that way. The Stringer doesn't really "draw" water up. You might have a hose fitting on it to cool, but you still won't get a real "lake pressure." I'd tow it to the ramp, pick a time when you won't be in the way and dunk it on the trailer and start it up.

 

[Seaswirled]

Ok got it. Fortunately the ramp is only 30 min from my house. We will be taking her to the lake, and report back.
Thanks for the reply.