Gasket oil....

[mrcaptainbob]

The plan was to do a vacuum check on this rebuilt MS390. Since it runs great on the bench at idle and at wot, but bogs down to stall when in the wood, there must be something else going on. I pulled the moded muffler off and found much oil on both sides of the muffler gasket. Well, I usually don't lube gaskets when I install them, so this was a bit of a surprise. I'm also wondering if I  over moded the muffler, so I ordered new stuff and should be here in a few days.
What purpose does back pressure have on the running of a two stroke?
Would

 over drilling the muffler have caused a no power condition?

[Al_Smith]

The transfer follows the exhaust .When the exiting exhaust pressure reaches the incoming transfer pressure the transfer stops .If you have enough carb it can continue a little more as the exhaust pressure decreases .If you put too large or too many holes in muffler  the transfer blows the incoming transfer fuel right out the exhaust port .Some usually is good,too much it goes the other way and can cause a lean burn type of thing .As a rule you really don't want more of a hole or holes area any larger than about 85% of the exhaust port area . 

[mrcaptainbob]

Thank you, Al. That helps me understand this stuff a bit better.

[Al_Smith]

I don't think you're loosing it out the exhaust port .Just sounds to me you just aren't getting enough fuel in the mixture .Could be the high speed jet,if it has an adjustable jet is too lean . I really don't know much about consumer grade Stihls which a 290/390 is .I could be fixed or adjustable jet .
It should sound a little rich at wide open throttle then into the cut clean up and sound better .You don't try for more RPMs
by leaning it out,doesn't get enough fuel .
Pull the plug,it should be a light tan color.Black is too rich,whitish is too lean .
If it is the carb and if my parts list is correct it should be a Walbro HD 5 which shows an adjustable jet .If it needs a rebuild,which they do from time to time
it could be a situation where the internal soft parts have just became stiff and don't allow a large enough amount of fuel to get into a chamber .Some people have good intentions and "clean " them out which might appear to be a fix but seldom is .They need a rebuild which is cheap and easy to do .Walbro has a web site with the directions and trouble shooting .