Is there any advantage. Sound or weight. looking for light waight and louder sound than stock muffler. OS61 side mounted.

Louder? I'd guess so.
Lighter? I'd believe so.
Advantages. Very doubtful.

Take the baffle out of the standard stock muffler and achieve similar results...
free of charge.

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Any advantages over what? Slimline's Pitts mufflers have a smaller muffler box (body) then Bisson and I would guess is the louder/lighter of the two. Are you talking about a Pitts muffler or what? Both are good mufflers. What your asking here is very vague!

One thing to be aware of... I have a Bisson muffler on a ST .75 and as of yet have not gotten it to run below half throttle. It seems there isn't enough back pressure to get proper fuel flow. I've been told, but have not yet done so, to plug (or restrict) one of the two exhaust stacks which should help.

I have a Bisson muffler on my TT46 Pro. I was advised that the Bisson had smaller outlets than the Slimline, and thus better back pressure without plugging any outlet. The engine runs well without any changes, so am happy with the Bisson.

A good muffler will not overcome poor plumbing, improper tank locations, or a bad carb design. The S.T. carb has been a pain for ages so don't look at the muffler as a cause. It's quite possible the opposite is true and you now have too much back pressure.

Bisson mufflers are manufactured with reduced openings to assist those that need the extra back pressure. Those that don't cut off the piched outlet and pich up a couple hundred rpm. I've been using Bisson mufflers for years and prefer them over the Slimlines. Lower price and longer lifespans that Slimlimes.

I like Bissons a little better myself. But Slimline mufflers are a little smaller and I have used them when the Bissons would not fit. I think both are good mufflers with a slight edge going to the Bisson for the sound. I have gotten great back pressure using either muffler.

FWIW: I am running a Bisson Pitts muffler on my SuperTigre G-75 and it purrs like a kitten. Note that the ST carb does need careful adjustment on the low-end needle. I had to spend some time (and get some help from a more-experienced engine guy) to get it just right on idle. No problems with back pressure or fuel flow.

As others have said, the Bisson mufflers are a bit larger than the Slimlines and probably a bit quieter. Go with what fits, I guess.

Hey Fledermaus, how far is your idle screw adjusted out from being fully closed? When I initialy set mine up, I put fuel line on the carb nipple and lightly blew air into it from a closed position on the idle screw and opened it just to the point where air would pass through the carb. I've since been adjusting more and more out with no luck.