Small Engines (Lawn Mowers, etc.): Husqvarna 435 chainsaw problem, piston damage, crankcase

Question
I bought a Husqvarna 435e chainsaw 3 weeks ago. Within the 1st hour using the fuel from the shop it started to break down on full power when hot. Then it continued to break down on full power revving up and down by itself. Took it back to the shop 3 times where they said it was just tuning. On the 4th time they found the piston and cylinder were scored. These parts were replaced under warranty (reluctantly). Yesterday I got the saw back and today started it still with the shop fuel and when hot after cutting for about 10 minutes it does the same thing. After this filled it up with a fresh batch of fuel with Husqvarna oil but the problem still exists. ( I have a video of it if that would help)

Answer
There are a number of things it could be.  My first suspicion would be the carburetor or the intake manifold.  It might be the crank seals as well.

It sounds like the engine is leaning out, this would cause the cylinder scoring as the oil would be insufficient.  This is supported by the surging.  There are a number of crankcase and carburetor/intake pressure and vacuum tests that could be done to pinpoint the problem.  I don't know about your area, but around here, many shops don't have the tools to do these sort of tests and don't have an interest in doing them anyway.  Chainsaw problems can be extremely difficult to pinpoint so a lot of shops will simply replace parts until the problem disappears.  

The cylinder and piston damage is unlikely to be the cause of your problem.  I tend to think it's the result of the real problem, which is the engine is too lean.  Without pressure and vacuum testing tools, I couldn't tell you just what the cause is, but if you can find a shop that does this sort of thing, they should be able to find it.  Thanks, PK.