bf100 extension case seal? Exhaust Smel? What to use..

Thanks in advance.

Took apart an old honda bf100 cdi type to track down an exhaust smell I was getting in the boat. I expected to find a bad exhaust pipe gasket either where the extension case connects or at the engine block. Both appeared ok. So back together it goes. The case came off much easier than on the other motor I wrenched on and I wonder if it was not sealed properly and hence the smell. Which brings me to my question. The manual calls for a liquid sealer applied to the mating surface when installing the extension case. Makes sense. So what to use? Silicon? Liquid rubber?... Both 3m and permatex make a couple products.

And of course any suggestions tracking down the exhaust smell.. My old motor went tu but never had such a smell before so I was thinking exhaust was leaking before it made it down to the prop. Otherwise the motor runs ok. Looks like the gaskets around the exhaust plate are ok. The engine had no troubles failing to run as others have indicated caused by exhaust gasses leaking into the cowling.

thanks in advance for your thoughtful answers..

Pretty much anything that cures to stay flexible would probably work. I have used silicon gasket maker on that flange. I would NOT use 3m weatherstripping adhesive (elephant snot). It cures hard and brittle over time.

But I believe they were trying to exclude water from entering there not sealing in exhaust gasses. But who knows at this late date?

The engine may or may not run well with the exhaust pipe to engine gasket failure. I think it depends on how much air can enter the cowl. While I haven't done a million of them, I've found a couple of these that seemed to run great with that gasket completely blown out and hanging there.

Is this outboard in an engine well? If so and it's gagging you out then that engine to exhaust pipe gasket would be my first suspect. That or a hole in the pipe itself.

Something else I thought of might be too much back pressure. Small outboard exhausts don't have much chance to get plugged up but improper mounting could increase back pressure and fill the extension with gasses. If you run a long or x-long shaft deep on a low transom it might set this up. Just a guess though.

Good luck.