1992 Yamaha 40hp 2 stroke

Hello everyone. Seems a new issue has came up on my Yamaha 40 MSHQ. It ran fine going out to fish but on the way back in it after about 10 minutes at wot it started sputtering and lose power but idled fine. After sitting a couple of minutes I tried wot again. It lasted about a minute then sputtered and lost power again. I ran it at about 1/4 throttle and made it in. I removed the cowling and there was maybe a couple of tablespoons of water in the pan. There may have been more and it drained out. Tale tail hose is good. Someone suggested to me the seal under the bottom cowling could be bad causing water and exhaust to get in the cowling and causing poor combustion. I was thinking head gasket. Any thoughts or ideas are greatly appreciated before I pull the crankcase to change the seal. Thanks.

Water in fuel as the cause of poor combustion, yes. Water getting into the lower cowling as the cause of poor combustion, no. If water intrusion is the issue you'll find the water at the bottom of the fuel/water separator (poor out into glass jar/cup). Likely the issue is fuel system related, but I'm leaning towards fuel delivery vs. water in fuel.

When you take the boat out again and experience this issue, try squeezing the primer as the engine sputters, see how the engine responds. Have you pulled/cleaned carbs and jets lately?

Water was in the bottom of the cowling. Carburetors where completely rebuilt along with everything in the fuel system replaced. The only thing not replaced was the fuel pump. The fuel is fresh none ethanol with Startron marine treatment. I’m thinking the water was fouling out something in the wiring. I ran it on muffs the other day at idle but couldn’t find a leak anywhere. Plugs looked good as well. It may only leak at wot but that’s just me and guess work. Just after rebuilding the carburetors I was adjusting the idle on the lake . The boat started drifting toward a swimming area so I put it in reverse a little and a little water got into the cowling then. Then I ran across the lake at wot and it did the same sputtering and lost power. I pulled the cowling and wipe out the cowling with a paper towel and fished for awhile letting it dry out good. It ran fine the rest of the day.

Update: I checked bolts today and found several on the exhaust cover barely snug. I torched them all back to spec. I hope this fixed the water coming in the cowling. I think the water is fouling out some wiring or something after I thought about when it happened before. Now for a good weather day to lake test.