[a1sailor]

I am new to forum and having a real problem figuring out how to fix a loud exhaust on a fresh water run 02 Yamaha 15 hp 2 stroke, model 15 MSHA. The noise got worse at low idle and some exhaust leaks out of the lower cowl on forward side and does not seem to come out of the upper exhaust opening on lower cowl as much as in the past. If I push the motor lower into the water the exhaust noise is less but still more than normal.

I pulled the power head and replaced the exhaust manifold gasket, I could not see any cracks in the exhaust manifold, there were some signs of discoloring on the painted manifold due to heat. The old gasket did not really appear to show signs of major exhaust leakage but possibly some, it was hard to tell, the bolts seemed tight but not over tight when removing, I really thought the gasket was the problem but a new one did not fix the noise. It was and still is also louder than normal at full power and has now lost some top end power after reassembly, at first I had the throttle cable out of adjustment but re set it and the butterfly on carb opens up fully, the other levers and controls attached to throttle seem to function ok.

Can anybody suggest where to look next ? Cracked block?

[WillyBWright]

I think you might have to pull the powerhead and see if the base gasket is blown.

[a1sailor]

I did replace the "upper casing gasket" at same time, p/n 63v45113-A1-00, this is the one between the power head and upper casing, no sign of it blown, it tore on dis-assembly. Did you mean to check another gasket?

There were two rubber gaskets that were not repaced, one right on the bottom of the exhaust manifold (I turned this one over on re-assembly) and the other called a plate seal damper, they both looked ok.

[a1sailor]

Does anyone out there have any further suggestions on this exhaust noise problem?
Thanks