I decided to do a muffler mod on my MS290 just to see if it really makes a differenece. I followed the design of a guy who posted his work on another site since it was about the easiest thing to do, didn't bypass the internal muffler baffles, and didn't require cutting the muffler apart. Here's a link:
Just taking the cap off the muffler was quite revealing. The exhaust had to pass through two little slots about 1/8 x 1/4" long. It's surprising the saw cut as well as it did. It didn't take long to cut the opening larger, as shown in the pictures on the link. Actually, it took a lot longer to clean out all the filings, etc. afterwards. I also was surprised there was no gasket between the muffler and cylinder. Anyway, I put it back together and went out to give it a try. I immediately noticed that the saw was running leaner, so it definitely was pumping more air. I opened up the Hi speed jet until it started to 4-cycle and put it into a 12" ash log. Definitely cuts better now with much better pulling power when it's fully loaded. Now I need to replace the chain they put on this saw, RMC3 I think, with something more agressive and it should be a good firewood saw.