I used some valve lapping compound on the leaky decomp valve and cleaned it out with carb cleaner and lubed it a tad. I pressure tested it and it seems to be good now. I'll keep it for a spare and test it in a cylinder next time I pressure / vacuum test a saw with a decomp in it. I know the used one I put in this saw held pressure and vacuum during testing no problem. I would think these would be somewhat self healing since it gets hammered 9,000 times a minute while you're cutting wood. I have read that peak pressure in the combustion chamber of a saw can reach upwards of 2,500 psi. But I don't have a whole lot of experience with them as "good" or "bad".
I don’t really know either I know they work till they leak then they get replaced. It will be interesting to see weather these can be salvaged or not
Not sure about salvaging one but I took one apart to thin the shaft to bleed off more pressure and deepen the detent groove to make it harder to pop closed. One of my ported saws has 240 psi compression and the decomp would pop shut as soon as you started to pull. I "tuned" the decomp so it stays pushed in upon pulling and blows 135 psi. Easy to pull and still closes tight upon firing.
Know I understand I was thinking you where trying to salvage an old valve out of a saw you where restoring. Know it makes sense on what you where doing
Compression was 180 on my cheapo tester that you have to hold on the cylinder, hold the saw, and pull the thumper cord. This was with 2 tanks of mix through the saw so far and I'm sure the new caber will seat in a little better with some more run time. I'm just happy it runs better than before and feels strong.
Got out today in between mowing to play a little. 12" Poplar blow down. https://youtube.com/shorts/Cy3hKtlY4OY https://youtube.com/shorts/NAMPe3F5
I've run this saw hard and cut several LARGE trees with it using every bit of the 32" bar it wears. Usually 40:1 red armor. I was noodling a blown down 22" sugar maple today and it was running great then the saw went HARD STOP... Stuff rattling around in the muffler. I opened it up and the piston had let go by the exhaust port. I was assuming it hung a ring as part of the ring had snapped out and was in the muffler. Oddly enough, I can't really see any damage to the cylinder at all even around the exhaust port where I thought there would be severe damage from a hung ring. I did use the lightweight HyWay piston when I built it and lightened it even more so it's possible it just came apart on it's own. The circlips were in place so they didn't come out. I'm not sure I want to go back to a HyWay piston although this saw has performed well for 2 years of firewood use. It hasn't been used daily or even weekly, but when it is in use it's usually for 2 to 6 tanks of mix for a hard days work. Parts in muffler, piston chunks and part of the caber ring. top of piston. Was I running too lean??? Spark plug electrode gap was hammered closed from debris in the chamber. But no other damage in the ports or cylinder was found. Even the chamfering still looked good. Front of piston GONE... spark plug. Saw was running great, but I got lazy and did not check the tune on it today. It had run fine last time I had used it so I never even thought to check the tune... I need to form better habits... Plug doesn't look lean to me, but I'm just a hack, not an expert. I had noodled probably a half dozen rounds at least before this happened.
Wow Monte. There are not signs of piston crown pitting, that I can see, that would indicate a long term lean condition and the plug actually looks like it was quite rich. It looks black and wet, not the cardboard box tan or a bit darker you look for. My guess is you either took to much material off the piston, like you stated, or you got a piston that was at the low end of chicom quality. Out of curiosity, for unrelated reasons, what does the exhaust port look like?
I bet that made a glorious sound. Lol The sound we wished to hear with all my oil testing but never were rewarded with.
Yikes. I see you thinned the top of the piston in a couple spots on the exhaust side to match the port, but it doesn't look like you took so much off as to compromise things. I'm also curious about the ring locating pin too. I can definitely imagine that maybe there wasn't enough of a press fit on that from the factory. After enough heat cycles and heavy loads that could've backed out. Voids in the casting is another possibility, or maybe the aluminum alloy composition was never quite right either. Whatever the root cause, it's a real bummer. Hopefully you can flush out the cases and be off cutting again with just some top end work.
Just came in from using other saws today. It hit 80 degrees here and kinda muggy so working outside is brutal today. I just saw everyone's posts. The sound wasn't glorious at all... WOT and it just STOPPED. No noise other than a couple tink, tinks from the chunks in the muffler. I held the saw up and shook it and sure enough there was metal bouncing around in the muffler... I just put it on the tractor and continued with the 357 to finish up the evening. The ring end pin looks to be snapped off and the ring was rotated around the piston till it got to the exhaust port and hung. In this pic, the ring has been pulled out of the groove, but is still on the piston, just up out of the way so you can see the pin has either snapped off or worked itself in, but I don't see how it could go in... The end walked around to the exhaust port. When I took the saw apart this end was still in the groove but the other end of the ring was snapped off and inside the muffler. The exhaust port has some carbon and is a little damp but no damage that I can see. bottom of ex port Most likely I will go with a new meteor piston / caber ring for the replacement. I won't guarantee that the spinny thing won't touch the new piston though... I'll flush out the crankcase and rotate it a bunch to ensure it is clean. I'll probably not mess with the cylinder as I liked the way this saw ran. I may have to double check the intake numbers cuz this broken piston did have the skirt trimmed for the intake timing.
