It depends on the hydro. Most homeowner mowers have non-serviceable hydros. Upper end homeowner and pro level ZTRs have serviceable hydros that need a fluid and filter change at certain intervals.
Did you actually measure the squish? The later 660s have notoriously tight squish from the factory. My 660 would not even turn over with the base gasket removed.
Make sure the small guide roller stays well lubed. For me the guide roller is is an important part of the process. You can muscle the slide enough to flex it and the result is inconsistent cutter length. If you stop filing just as the guide bar contacts the roller the cutter length will be super...
Hi Ben. The facts are that I'm a Stihl guy and I will be the first to tell you the the 550XP is the choice in the 50 class. Run one, it will make a believer out of you.
I can see poc marks from detonation on the land above the top groove. Crap gas is likely the cause, and the non-AT saws may have been tuned rich enough to pass. Looks to me that once again the problem lies behind the handle.
Pull the air filter and look inside the carb for excessive dust. Check that all the AV mounts are intact and that the handle and crankcase have no breaks or cracks at the AV areas. Make sure the spike mount on the case isn't broken.