The first thing to check is the piston and cylinder by removing the muffler and peering inside. This is especially important on cut off saws, they generally operate in filthy conditions and need constant air filter maintenance. If the piston is scored or sloppy in the bore, if the rings are loose in the grooves, no amount of tuning is going to help.

canguy,

I assume you would expect to see low compression readings with the scored cylinder, bad rings, etc? I was thinking that the pressure/vac test would not highlight this problem as it only measures leaking out of the crankcase where it should not (seals, sparkplugs, etc.).

I would say that you missed something that is blocking the low speed circuit in the carb. Just dipping a carb in carb cleaner does little. You need to clean passages with wire and/or air and carb spray to verify that everything is open. Be sure to remove the low and high speed needles and clean the passages they are in.

The compression (just checked) is 120 psi. I think nominal might be 137-139 psi (manual just says 9.5:1). One guy was telling me that he thought about 130 would be needed for a decent idle. Any thought on that?

If compression is not the issue, then I'll be pursuing the blocked low-speed port idea.

John , just my two cents I agree with both Can and Rday if the Piston is scuffed or the low speed idle circuit is even partially plugged your idle issues will prevail. The Compression you have indicated is in the ball park. Did you pull the saw over at least 5 times to ensure saccurate reading ? With the compression you are at I would lean towards the Idle Circuit myself or perhaps a weak Diaphram...since you have replaced everything else which would cause you saws condition...keep us posted Bro !

On account of obvious wear on the piston skirt (worn semi-circular area on cylinder below intake port), I installed a new cylinder-and-piston assembly, but that did not solve the problem.

Though the carburetor low-speed ports did not seem to be plugged (squirted carb cleaner through with no obstructions) and I had already done a carb soak and installed a carb rebuild kit, I had other advice from a mechanic that works on Stihl saws who said that despite all that, this was probably a carburetor problem. He sees it often with the Stihl cutoff saws. He advised that I simply replace the carb at this point.

I did, and now it idles beautifully!

It looks like this gasket is leaking between the gas tank halves.How do I replace the gasket? The saw sat for 3 years. Any recommendations?
Thanks,AThomas