Chainsaw Repair

Yes, one could melt the piston on a brand new saw just by adjusting the carb incorrectly.  You can sit it on a bench, and slowly turn the high screw in and get it really screaming, say 16,000 to 18,000 rpms for 40 or so seconds.

You need to "richen" the mixture to slow them back down to 13,000 or so, depending on the model.

Also, since the lube is in the fuel, the leaner the mix, the less lube there is for the engine, which makes it overheat
exponentially.  So if one is using his saw, and it starts acting squirrelly and dying, he needs to shut it off and fix the problem.

WOW! Fish said it all and then some!

Those that want to set up their saws by "sound"....usually end up with scoring of the piston and cylinder. Buy a good reliable tachometer, then tune! I have seen many a good saw go to ruin by those that relied on "set to sound" method of tuning. In fact, the shop where I retired from, all new Stihl saws came with a nice little orange handled screw driver...we actually had several hundred of them all bunched up like a bouquet of flowers in a big vase....in the shop area! All for the simple reason we didn't want the "new owners" tuning their new saws! When we sold a new saw, we serviced it before the new owner got it into his hands. Hardly ever a time we had to actually "tune" a NEW Stihl product! I can't say that about the others we sold or serviced!
We also had a saying in our shop with "some" warranty claims that came in....warranty doesn't cover stupid!

Dave

Well, I put in the new piston and rings, it started normally, I ran for about 15 seconds, felt like it had good compression, then I turned it off for the night, planning to put on a bar and chain today. 

Now it's back down to 90 PSI.

Does this indicate I might need a whole new cylinder/piston assembly?

Compression psi only tells you that an engine will run, not how well.  It also takes a little longer than 15-20 minutes of run time for your new rings to seat onto the cylinder wall.  Run it through a couple more heat cycles and then test it again.  Keep the carb rich for the first few tanks of fuel and try a different gauge.  If the cylinder has alot of time on it, it may also need to be replaced.  To give you an idea how long it may take for those rings to seat, some engines take 6-10 tanks of fuel before everything seats.