Yamaha 150 2 stroke won't idle with the cowling on

[mvollmer]

I have a 2004 150 2 stroke with about 400 hours on it. It starts easily but won't stay running if the cowling is on. If I take the cowling off it idles great. Runs great under load with the cowling on. Had my 19' Key West Sportsman gliding along at 40 MPH yesterday but when I slow down to dock, it just stops. I can put the cowling on and it'll start to die. If I lift it off, it'll come back alive. I've replaced spark plugs, emptied fuel water separator on the engine and replaced the one in the bilge. I removed and checked the thermostats, inspected the carbs, they appear clean. No excessive oil in the carbs or in or around the engine. Flame suppresser is clean. I adjusted the idle up, with the cowling off of course and it seemed to be idling fast. When I replace the cowling, it seemed to run a bit longer before stalling but it still stalls. Is there an air-mass sensor or automatic choke sensor that could be causing this. Any help would be appreciated.

[DennisG01]

Check the "air intake" in the cowling. If that's partially obstructed, it may be just enough to cause issues at low RPM where the engine isn't pulling air in as hard as at high RPM's.

[99yam40]

exhaust leak might cut down the available O2

[mvollmer]

Lemme check. Both seem plausible. How would I test for an exhaust leak? Is that common?

[mvollmer]

Problem solved. . . but not repaired. It appears that a former owner discovered this problem and applied some glue over a hole in the deteriorating exhaust manifold. See attachments. The first photo is taken from inside the engine enclosure and shows the glue. The second shows the same spot from the aft view facing forward with the engine trim removed. The third and fourth pics show the hole with the old glue removed.

Now I'm looking for some advice. Cosmetically, the engine looks to be in great shape. It was running great before the leak reoccurred. It's a 2004 with about 400 hours on it. I'm interested in any suggestions. Is it worth a proper repair (taken apart and exhaust manifold replaced)? I was thinking I could fashion a piece of stainless steel or other resilient material to match the shape of the area where this hole is. Clean the corroding area then slather some high temp rtv over the entire area and place the part mention above on top. Finally, maybe secure that plate with a screw or two. I'm just not sure whether a screw might make things worse. Any thoughts or suggestions or advice are welcome.

[mvollmer]

Thanks again for the suggestions. I had thought about the cowling and was planning on inspecting that next but exhaust leak hadn't occurred to me at all.

[mvollmer]

BTW, I posted a long response with photos but that post appears to be stuck in the moderator's inbox awaiting review.

[99yam40]

BTW, I posted a long response with photos but that post appears to be stuck in the moderator's inbox awaiting review.

did you find anything that would indicate a gasket or other problem?

[mvollmer]

I responded but my post is still held in the moderator’s inbox for review. I described what I found and added photos. Does anyone know how to kick the moderator and wake them up?

[mvollmer]

Problem solved. . . but not repaired. It appears that a former owner discovered this problem and applied some glue over a hole in the deteriorating exhaust manifold. See attachments. The first photo is taken from inside the engine enclosure and shows the glue. The second shows the same spot from the aft view facing forward with the engine trim removed. The third and fourth pics show the hole with the old glue removed.

[mvollmer]

Now I'm looking for some advice. Cosmetically, the engine looks to be in great shape. It was running great before the leak reoccurred. It's a 2004 with about 400 hours on it. I'm interested in any suggestions. Is it worth a proper repair (taken apart and exhaust manifold replaced)? I was thinking I could fashion a piece of stainless steel or other resilient material to match the shape of the area where this hole is. Clean the corroding area then slather some high temp rtv over the entire area and place the part mention above on top. Finally, maybe secure that plate with a screw or two. I'm just not sure whether a screw might make things worse. Any thoughts or suggestions or advice are welcome.

[mvollmer]

This is the view from the back of the motor with the cover removed