i have a st g-61 and was wondering if i closed off the end of the exaust stack enough to create back pressure and tap a hole in it and screw in a pressure fitting and use that instead of the big heavy muffler? thanks for the help

I haven't tried this but I'd say that if you closed it off enough to materially reduce the sound level (and you'd have to greatly reduce the sound level to make it acceptable) you'd absolutely destroy the engine's ability to scavenge its cylinder properly and you'd end up with a gutless, foul mannered, untunable evil pig as a result.

What you're talking about sounds similar to what's called a tongue muffler. I couldn't find a decent picture to attach so here's a link to one. http://www.brodak.com/shop_productde...ProductID=6687

However, these are only used on CL engines which are never run at full power so any possible power loss isn't a problem. They're still a bit on the noisy side though compared to a proper muffler, just much lighter.

Such a system also is called a "dam muffler"
The volume of the pipe should just allow the exhaust gasses to escape, and use the header pipe gas flow inertia to suck in the fresch charge. (mousse can mufflers are a bit like that, whereas tuned pipes use pressure waves)
Ideally, the pressure build-up stops the outgoing gas rush just in time to keep the fresh charge inside the cylinder. Available power may be very cloose to a tuned pipe if set up well. and the setup has appreciable noise damping. An extra added muffler chamber with smallish stinger further reduces the noise to very low levels.