O.k... I get to see quite a few big saws with burned out pistons, and they all look just like the two (three) shown... The guy usually tells me "it was working great and then it stopped". Also see the same on Blowers where they run at their max HP while slowly disintegrating the piston over a 30 minute run time.. Got another in today as a matter of fact.. It ran great today..., just wouldn't restart. Compression 100lbs... piston melted on exhaust side.
It's almost always gas related (Oil ratio, old gas makes the saw run hotter, crappy gas or low octane gas does the same), or mixture. The gas part you can fix easily - mid grade or above, no alcohol etc etc. use a decent oil, and mix at 50, 40, 32 or whatever, I don't care.
Mixture is another issue. One part is under your control. If you tune your carb to the max, run the saw hard in hot weather, don't clean your cylinder fins or flywheel fins, you can expect more problems than if you tune it it conservatively and run it nice... like NOT goosing the last drop out of the tank..
Another mixture related problem you can't control - an air leak in boot, seal, gasket will take your saw out in a heat-beat if you tune to over come a developing problem. I always check carb settings - if the L screw is 1.5 plus turns out, there is a real good chance something is leaking. Fix it! And listen to your saw.. if it starts to idle fast, or run on after a long cut, you likely have a problem in the works.
Split fuel hoses are often intermittent and may not show a problem unless at full flow... then they suck air. Same with impulse hoses.
Water in the gas is a real problem. It can be "clean" water (recent rain etc) or old water in a metal tank. Both indicate a problem with gas care so I usually suspect bad gas. Old regular from god knows what farmer's pump, or Arco (10% alcohol out here) bought last season and left in the can/saw. I recently has an 026 that I rebuild the carb three time before the owner fessed up to water in the gas can - Only a tiny amount but had rust particles that got into the fuel filter, through the fuel filter, into the carb and every where else. Remember me telling a previous poster to chuck the filter out?
Water in the carb WILL rust the metering arm screw. The rust then gets into the bore and the outlet valves. PITA to get out. Whatever you do, put a new screw on the metering rod!
So... I HATE putting in a new piston if I can't see a cause. I quiz the customer to death on his gas an usage. I drive my boss crazy by "wasting time" finding problems, but you'd be amazed what I find...So far, none have ever come back... yet...
Not only pressure/vac test the saw, but pressure/vac test the tank though the fuel hose. A bad tank vent will lean your saw out just as easily as you doing it, at the carb and it's real hard to detect .
If your carb is NOT close to standard settings, find out why... Many failures are because of blocked air filters and a carbs set real lean to compensate. Then one day, the filter gets cleaned or replaced... Hmmm... that saw ran just great after the filter was changed, for a short while...
Anyhow, what is the actual question that is to be answered?
