550xp Mark 1 squish band cavitaiton

[whitesnake]

Yikes. This reminds me, I need to refill my jug of 100ll.

 

[Ketchup]

I always assumed air could be drawn in through the decomp or spark plug on the downstroke. I think the explosion could draw oxygen from a lower pressure source during expansion. Then once the exhaust opens the change in pressure could be compensated by leaks.

But I have also had many leaky decomps with no negative effect. I guess I see it both ways.

I just replaced the top end on a 362. The only leak I could find was the decomp. Original cylinder was ruined.

 

[drf256]

To low of an octane likely from stale fuel.

Serious question. How does a decomp in a running 2T suck air?
What part of the cycle is there a vacuum? whenever the ring/rings are above the exhaust port going up or down it's a compression stroke or combustion stroke both positive pressure.
When the exhaust port opens exhaust gasses escape under pressure as this process is going on the transfers are filling the cylinder with a fresh charge under pressure.
I'm lost help me out what am I missing when is there a vacuum to pull air in through the decomp?

Thank you.

I’ve been trying to figure out how a leaking decomp valve can cause any engine damage at all (exc when AM stems fall in) for years now. On the upstroke, there is compression pushing gas out. On the downstroke there is cylinder pressure pushing gas out. How in the hell can gas (I mean the gaseous state of substance ie fuel/air charge) get into the cylinder via a leaking decomp valve?

To the OP, that’s detonation. Did you advance the ignition timing? Odd that octane would do that, as you retained a gasket and have at least 20 squish. Plus, 2T engines really are running static compression ratio’s around 7:1, much lower than the need for more octane than pump fuel give you.

I betcha all those little hot spots are making a bad situation worse. They will further detonation for certain.

 

[Motorka]

If you look at pictures the saw is really dirty. And inside cylinder are also marks of high temperature.

 

[cus_deluxe]

To low of an octane likely from stale fuel.

Serious question. How does a decomp in a running 2T suck air?
What part of the cycle is there a vacuum? whenever the ring/rings are above the exhaust port going up or down it's a compression stroke or combustion stroke both positive pressure.
When the exhaust port opens exhaust gasses escape under pressure as this process is going on the transfers are filling the cylinder with a fresh charge under pressure.
I'm lost help me out what am I missing when is there a vacuum to pull air in through the decomp?

I don’t know the exact answer, but I can tell you for sure that I had a leaky decomp in my 181 (I installed a decomp that entered from top into squish band) and with a brand new piston after just a few mins of run time there was a noticeable black circle of carbon directly under that decomp, which leads me to believe that it was creating a hotspot, which means it had to had been pulling at least a little air in somehow?

 

[rogue60]

Did some more looking around the net for a definitive answer didnt really find one went down a bit of a rabbit hole have a headache now lol
Going out on a limb but I've kind of came up with an answer and that's No a leaking decomp will not let/suck in fresh air in a 2 stroke.

Keeping in mind naturally aspirated engines 2T and 4T don't suck air in on intake, on the intake stroke the piston lowers the pressure in the cylinder(4T) or crankcase(2T) to that less then atmospheric pressure air is pushed in. Supercharged and turbo engines suck air in.

Ok here's why a leaky decmp can't let air into the engine you need an absences of pressure or less than atmospheric pressure and it can only happen when the exhaust port is open as compression and combustion (rigs above exhaust port) are both a positive pressure way above atmospheric pressure.
If say at this time in it's cycle with exhaust port open and there was a true negative pressure above the piston some how below that of atmospheric pressure (its about 14 psi at sea level) fresh air would be pushed in through the decomp and the exhaust probably bringing exhaust gas back with it to fill this negative pressure in the cylinder equalizing it to the outside air pressure.
I can't see how or this ever happening in a running chainsaw.
That's the best I can come up with I may be way off lol

 

[Wonkydonkey]

As others have said, to much timing adv = heat and thus detonation . Or as I had a 200t with the cylinder fins clogged with sawdust & oil, then the heat built up and detonation started.

 

[tickbitintn]

Coming from the motorcycle world I always enjoyed Kevin Cameron's articles (in Cycleworld and anywhere else I could find them.)
He is an incredible wealth of knowledge that can explain in simple terms, yet in such a detailed way that it is easy to understand for the larger crowd and yet still enlighten the most knowledgeable on the subject matter.

I have run into p/c's looking much like this in many saws. In my cases I have found a large amount of neglect (very dirty. crusted saws), and poor fuel quality.

I have not had much time for saws or the site lately but I'm glad I had a bit to get on the laptop and read this thread.
Good discussion and show of knowledge here.

Doing a storm clean up one time I noticed cut speed seemed a bit slow. I figured I had dulled the chain on some sort of debris but the chips still looked ok and it wasn't blowing dust. I kept cutting for a bit before taking a break. Catching my breath, having a drink and sitting for a minute I realized some leaves and a stick had stuffed in and was holding the decomp open a bit.
I can hold the decomp in on a couple saws I have while cutting.... the cut speed slows down a bit but that's about the only effect I noticed! For full disclosure I only did it for a few seconds!! YMMV!!

 

[davidwyby]

This was with a loose/leaking spark plug thread insert.

 

[NPKenny]

This was with a loose/leaking spark plug thread insert.

This is remarkably similar to how this saw was acting once warmed up.

@davidwyby is pretty close geographically to where I cut.

 

[NPKenny]

To the OP, that’s detonation. Did you advance the ignition timing?

No ignition timing advance. I went very mild with this saw to get a feel for autotune.

I do cut in the heat which has given me trouble on multiple occasions.

 

[davidwyby]

Same climate. I don’t cut much over 80 f. 90 only test cuts n stuff. Gas goes bad really fast in the heat.

 

[Mastermind]

I don’t buy the “decomp caused a scored cylinder “ theory

I never have.

Never plugged a decomp either.

 

[Stump Shot]

I never have.

Never plugged a decomp either.

I did up three saws for a Western Forest Firefighter and he was fairly adamant that all three have the decomp plugged, as he had a saw ruined fighting a fire from the decomp leaking.(at least that was what the repairman had told him)
The only decomp failure I ever had was one that I had inadvertently chose the wrong bit to enlarge and made the exit hole too big. This made it hard to start sometimes for the user. Took some cypherin' to come to that conclusion. Once replaced with the correct hole oversize the condition ceased and never returned.

 

[Mastermind]

I did up three saws for a Western Forest Firefighter and he was fairly adamant that all three have the decomp plugged, as he had a saw ruined fighting a fire from the decomp leaking.(at least that was what the repairman had told him)
The only decomp failure I ever had was one that I had inadvertently chose the wrong bit to enlarge and made the exit hole too big. This made it hard to start sometimes for the user. Took some cypherin' to come to that conclusion. Once replaced with the correct hole oversize the condition ceased and never returned.

I shouldn't have said "never". I'm sure I have when I built a saw and couldn't find one in my box o junk. LOL

 

[Stump Shot]

I shouldn't have said "never". I'm sure I have when I built a saw and couldn't find one in my box o junk. LOL

They(decomps) do make it much easier on starter components.

 

[tickbitintn]

They(decomps) do make it much easier on starter components.

And old joints and limbs!!
Lol

Sent from my XT1635-01 using Tapatalk

 

[Nutball]

I shouldn't have said "never". I'm sure I have when I built a saw and couldn't find one in my box o junk. LOL

I was about to say the 2019 MMWS GTG raffle saw had the decomp plugged. It may have been because it was a big bore on a 6400.

 

[Mastermind]

I was about to say the 2019 MMWS GTG raffle saw had the decomp plugged. It may have been because it was a big bore on a 6400.

It was because the top cover and decomp location didn't match.....or because I didn't have one for it.

Point is.....I don't plug the decomp because I worry about them leaking.

 

[av8or3]

This just occurred to me. I repaired a motorcycle engine once where the wrong plugs were put in it. It ran so hot it burned holes in both pistons. The spark plug insulators were coated with aluminum. It was obvious to me what it was. I wonder if you’ve studied your plugs? It looks like detonation or pre-ingnition to me FWIW. It looks so bad that it could be that something was bouncing around in there. Very interesting damage for sure.