My buddy's saw won't run under throttle so I am going through the motions as a favor for him. This one has a good bit of carbon buildup but a view from the muffler looks fine. The top of the piston when viewed from the plug hole has a noticeable bump or divot right where the plug electrode would line up. Is this common for a worn piston? He has used this thing for a good 10+ years. I don't think I will be able to take a clear picture without pulling the jug which I would prefer to avoid. More of a curious question as we are mostly looking for a fuel related/ air leak based on age & symptoms but I like to learn.
Hmmm, ten years old, you may want to consider pulling the jug and putting a new ring and a carb kit in the old girl. Is what you are seeing a convex or concave spot in the piston?
Yes! Carb kit is a guarantee. Unfortunately my compression tester is rather inaccurate, and perhaps if it were my saw I would dig deeper. I will see what he wants to do. The divot goes into piston, not a buildup but looks like more of a removal of material.
Sometimes carbon can build up evenly across the piston crown but a small indentation in the built up carbon will sometimes exist directly under the plug. This is because it is hotter there and the carbon doesn't build up as much, or (much worse) the carbon has built up to the point that the piston crown plus built up carbon is contacting the plug.
Never saw any products that do that. I remove the jug, coat the top of the piston with WD-40 (seems to soften the carbon build up) then gently scrape the carbon off with a single edge razor blade, sometimes use a scotch grite also. Remember to put a rag under the piston so that the carbon does not go into the case.
That picture I posted is a hole starting in the piston. Material has been removed and there is a depression. Echo backpack blower. Backfires through the carb and stalls right after it starts. Found ignition locked in full advance. New coil, back in service, although I pointed out that the hole "could" let go without warning.
Stihl sells some. I don't have the PN handy but we used to decarbonize high hour 4-mix engines all the time without disassembly.
I have a Husqvarna 357 with a dimple in the piston. I finally figured out that it was some "thought he was" saw mechanic stuck a hard piston stop in the cylinder to get the clutch off or something? Other than the dimple in the piston, the saw looks brand new.
This particular Echo piston definately did not come with a dimple, you can easily see the craters from pre-ignition. This isnt the first time I have seen this on an Echo product, and that first lesson taught me to look at the built in timing advance on their coils
Running BelRay H1R @ 32:1........... has removed some carbon from my stuff, and I imagine that with more use, it'll clean up even more
