Hi Geno, welcome to Outdoorking.
Since the saw can run at high speed but not at low speed, and it surges as you lower the speed, it sounds as if the mixture is lean at low speed. Because the compression is good, there are two most-likely causes of this. One is that the carburetor is producing lean mixture at low speed, and the other is that there is an air leak into the intake system. Because the intake vacuum is high at low speed and low at high speed, an air leak tends to have its greatest effect at idle or light throttle.
I suggest you begin by confirming the lean mixture diagnosis. To do this, partly close the choke as you drop off from high speed, and see if there is some choke setting at each speed that gets it to run well. (If it doesn't have a choke, remove the air filter and partly obstruct the intake opening with a piece of plate. Be careful doing this though, in case it spits - wear safety glasses, and keep yourself out of line with the carburetor's air intake.)
The results of this test will tell us whether it is a lean mixture problem. Once we know that, we can move forward to curing it. Please post pictures of the saw, and of the carburetor, as well as the carburetor brand and type number.
