It's probably an air leak (lean running). On that saw, there's several possibilities. No. 1 is a cracked fuel line, which is relatively easy to diagnose. Just inspect the line for any cracks and leaks.
I've seen a lot of those saws with loose or broken carburetor mounting manifolds. (The plastic block between the engine & carb) Grab the carb and wiggle it to check for any movement (there should be none).
Other possibilities are a blown cylinder gasket, leaking crankshaft seals, a internal carburetor leak, etc. These can be difficult to diagnose without doing a pressure test of the crankcase, which requires some special tools.
Before you do anything to it, I'd remove the muffler and inspect the condition of the piston and the cylinder wall for any damage. Most of the saws have unlined aluminum cylinders, and will score very easily. Depending on how long it's had a lean running condition, the engine could already be trashed.