So this happened.
View of the top of the piston:
It's a Dolmar 420c. I was present when it occurred. It was in a long cut, started to lose power, then died. After the usual stuff of checking fuel etc. I slowly pulled it over and noticed it had much less compression than it should have. Removed the front plate off the exhaust and saw that little divot in the top corner of the piston there. I took it home and tore into it expecting to see carnage. But - there wasn't any. Just that little divot in the piston and big signs of detonation or impacts on the piston itself. At first I thought it may have been due to piston bits in the engine, but there wasn't any sign of impact on the combustion chamber. The jug was in great condition. No signs of damage, even at the lips on the exhaust port it looked good. Sides of the piston looked great. In fact, when I removed the front of the muffler and looked at it, all I saw was the piston face at first, and I thought - that looks good. Then I slowly turned her over and saw the missing chunk on the crown and thought 'That's not so good'. Spark plug was perfect with great color.
There is plenty of carbon build up, though. And, this thing had no real work done on the muffler. These saws are high in compression with tiny squish bands straight fro the factory.
So here's what I think happened:
After 5 years of happy firewood duty, a chunk of carbon maybe got stuck in the combustion chamber on top of the crown, breaking off a piece of the crown but not making a mark on the carbon build up already in the chamber. The piece(s) finds it's way out quickly enough not to do any real damage, but due to that very sharp divot, loads of detonation began to happen. Eventually between the divot and the piston damage, she doesn't have enough compression to run once the engine is fully warmed up or running hot. Either that, or carbon build up on the crown itself caused high compression and detonation, leading to the crown damage. Whatcha think?
I didn't see any signs of extended detonation(usually discolouration) under the piston crown, though.
I put a spare piston from another Dolmar 420 that died long ago, a new caber ring, ported the cylinder properly, removed the spark screen and added another outlet, added more holes in the top cover to allow better breathing(but not crazy or anything like I did on the 241). She is pretty strong now, and sounds fantastic.
View of the top of the piston:
It's a Dolmar 420c. I was present when it occurred. It was in a long cut, started to lose power, then died. After the usual stuff of checking fuel etc. I slowly pulled it over and noticed it had much less compression than it should have. Removed the front plate off the exhaust and saw that little divot in the top corner of the piston there. I took it home and tore into it expecting to see carnage. But - there wasn't any. Just that little divot in the piston and big signs of detonation or impacts on the piston itself. At first I thought it may have been due to piston bits in the engine, but there wasn't any sign of impact on the combustion chamber. The jug was in great condition. No signs of damage, even at the lips on the exhaust port it looked good. Sides of the piston looked great. In fact, when I removed the front of the muffler and looked at it, all I saw was the piston face at first, and I thought - that looks good. Then I slowly turned her over and saw the missing chunk on the crown and thought 'That's not so good'. Spark plug was perfect with great color.
There is plenty of carbon build up, though. And, this thing had no real work done on the muffler. These saws are high in compression with tiny squish bands straight fro the factory.
So here's what I think happened:
After 5 years of happy firewood duty, a chunk of carbon maybe got stuck in the combustion chamber on top of the crown, breaking off a piece of the crown but not making a mark on the carbon build up already in the chamber. The piece(s) finds it's way out quickly enough not to do any real damage, but due to that very sharp divot, loads of detonation began to happen. Eventually between the divot and the piston damage, she doesn't have enough compression to run once the engine is fully warmed up or running hot. Either that, or carbon build up on the crown itself caused high compression and detonation, leading to the crown damage. Whatcha think?
I didn't see any signs of extended detonation(usually discolouration) under the piston crown, though.
I put a spare piston from another Dolmar 420 that died long ago, a new caber ring, ported the cylinder properly, removed the spark screen and added another outlet, added more holes in the top cover to allow better breathing(but not crazy or anything like I did on the 241). She is pretty strong now, and sounds fantastic.