I don't want to hear any more about that cheap 46cc saw having no business with a 20" bar. The 20BPX made a world of difference, and the saw pulled it just fine thank you. I ran it through some ash and then cut up some nasty crotches from the white oak I've been working on. Those were at least full bar with plenty of knots and it cut quite quickly with no bogging. I'm not claiming it's a race saw, but it's far better than just adequate.
But I did do some muffler work on it this morning. I took it off to look at the baffles, and I enlarged them and added a couple on the bottom (because I was leaving the screen in and wanted to have more access to the area on that side). I noticed that the original muffler had the same problem as the one I put on the 142, namely that the top edge of the inlet opening intrudes into the port by a good 0.090" or so at the top. I don't know if that was intentional, but I opened it up. I'm still running the screen and the one original outlet (opened up a bit).
The saw certainly sounds serious now, and it's perfectly happy pulling that chain.
One last thing if anyone has an early version like my Type 1, replace the muffler shield right away. The part that's formed around the outlet is weak and fatigue fails, allowing the exhaust to melt the brake. The replacement is different and stronger.