The saw has been assembled now for a while, but it still doesn't run right. I did raise the exhaust port about 0.04" which correspondingly made it a little bigger, but that was it for work inside the cylinder. Because of the goofy setup on this saw for the impulse port, machining the 0.040" off the base meant that the impulse port became larger than the area it was "assigned" under the base gasket. I tried a first JB Weld repair, which didn't work - the saw had an air leak. So since I bought the saw to learn on, I bought a MightyVac and took it apart again, including changing the crank seals (which were still fine). I pulled the cylinder and filled the impulse port (again) completely with JB weld, and then modified it. I put the cylinder in a drill press, with the face of the impulse port perpendicular to the bit, and re-drilled the impulse grommet port 0.040" higher up the cylinder, figuring that the little tube might seat better in the grommet if it was higher up. I connected it to a new small hole in the bottom of the cylinder, where the old hole was.
After re-assembly, the saw would pass a vacuum test but would not hold pressure, only about 1.5 PSI before the impulse tube would bypass: The impulse tube extends from the carb mount plate into a little rubber grommet at the cylinder base, and the tube is tapered at the end -- by design it doesn't look like it would ever hold pressure so I didn't really worry about it. I didn't see how a little positive pressure leak there would do anything as bad as a vacuum leak. The vacuum held great though.
But it still didn't run right. So I rebuilt the carb, using an aftermarket Walbro kit (saw has a WT 170, I think). That immediately wouldn't work, could barely get the saw to start. So more reading, then I pulled the carb (again) and put the old pump diaphragm and gasket back in place. Saw ran then, but still pretty bad, and seemed to guzzle fuel. It ran okay up top, but if the saw was under load, it would bog and seems to run out of fuel. If I released pressure and let it rev up again, it would cut again for a few seconds and the repeat the mess. If I just pulled it out of the wood it would rev okay, but seemed like it really wanted to stall. If I stopped revving, that's what it would do. Nothing I did to the low screw seemed to have any significant impact.
More reading, and I think maybe I set the metering lever too high. So I open up the carb, and the lever is actually a bit too low, so I raised it up just a hair. Now the saw runs a little better, but has the same run-out-of-gas feeling.
So its back on the shop floor. I've ordered a cheap aftermarket carb to try that, but I'm still not convinced that it's a carb problem or the air leak is still there. The trick here is that the carb mounting plate - the part that attaches to the intake port - can't easily be made part of the pressure/vac test. I used an old one where I filled it with silicone (read that here somewhere) but that obviously can't be used on the running version of the saw. I'm wondering if I somehow damaged that goofy little grommet when I pulled the blocking plate off and replaced it with the (new) open one.
Does anyone have any ideas?