550xp Mark 1 squish band cavitaiton

[NPKenny]

2015 Husqvarna 550XP. Muffler mod and smoothed ports. No port timing changes and flywheel is not advanced.

Built about 18 months ago with a meteor piston after purchasing it non-running. I had the carburetor flashed and had to replace a pto side transfer cover o-ring (thanks @cus_deluxe ). Base gasket was retained and squish was over 0.20" as a I recall.

It dies in the cut and won't idle as soon as it is warmed up. I pulled it down to pressure and vacuum test. The Decomp leaks like crazy. With it plugged there is no pressure or vacuum leak for a sustained period.

Pulled the cylinder and found the deep marks that look like cavitation around the squish band. I have had another saw I have bought like this, but not one I have built do this.

Here are the pictures. I'm looking for help on the diagnosis. Would a bad decomp cause this? I'm leery to blame the carb as it ran quite nicely for many tanks, until it didn't.

 

[Woodslasher]

Looks like pre-detonation or something was bouncing around in there.

 

[cuinrearview]

Looks like water in the gas to me. Or super damp air?

 

[NPKenny]

It has the appearance of pump cavitation. Damp air is out, all of the cutting has been in Arizona.

Water in the fuel would not be impossible. I’ve had fuel problems before with temperatures here.

 

[huskihl]

Bad /old gas. The good stuff has evaporated out and the octane is now too low.

@davidwyby noticed his running funny last week in Southern California

 

[Lee H]

To me, It looks like severe detonation, Can a leaking decomp cause this, Yes it is possible cuz it can be drawing in air
causing a lean mix in the combustion chamber. JMO.

 

[davidwyby]

Bad gas made my saw surge at idle. That decomp being in the top makes me want to blame it.

 

[Stump Shot]

In the picture the decomp hole appears to me to be plugged. If that is indeed the case then I'm going to throw out there this possibility... too tight of squish band clearance along with good old H2O in the fuel.

 

[NPKenny]

In the picture the decomp hole appears to me to be plugged. If that is indeed the case then I'm going to throw out there this possibility... too tight of squish band clearance along with good old H2O in the fuel.

It is plugged in the picture. I tested with the decomp, found it to be a huge leak, plugged the decomp, and retested. No pressure or vacuum leak once it was plugged.

I pulled the cylinder off and found the piston and cylinder in this condition.

 

[Ketchup]

My 394 died like that. Piston eroded in the squish band, top of jug appeared to be hammered by little rocks. Saw ran great until it died mid cut. I pulled the muffler, saw the snaggletooth piston rim and knew I was hosed.

Intake block was cracked. Bad gas could have been involved as well. My decomp had been blocked for years.

 

[rogue60]

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?

 

[davidwyby]

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?

When my spark plug thread insert came loose it ran lean.

 

[rogue60]

Had a bit of a look around the net can't really find an answer for my above question.
Was hoping just to get an understanding of what/how air gets pulled in through a decomp in my mind I can't put together when vacuum happens pulling air in through the decomp not saying it doesn't.
I've had massive what I'd call compression leaks is 2 Strokes over the years but it never seemed to damage anything not saying it can't just I've never seen it.
I had a 066 with messed up sparkplug threads every so often the plug would work its way loose and eventually fall out and the saw would stop. I could hear it start leaking up to the point the plug would fall out. I'd just put the plug back in tighten it best I could and continue using the saw until it happened again.
Ran it daily for months like that when I finally got around to putting a new cylinder on it there was zero damage to the piston or the old cylinder apart from the damaged threads.
I've had a few 2T's with leaking head gaskets over the years from mowers to bikes all showed no damage form continuing to run them like this. One old air cooled bike I had with a leaking head gasket didn't seem to miss a beat road it around for years like that sounded like an exhaust leak but from between the head and cylinder lol when I finally put a new head gasket on couldn't find any obvious damage to anything.

Anyway looking around the net I did just find this very interesting read all about detonation in 2T's
https://www.klemmvintage.com/camerondeto.htm

 

[Moparmyway]

In my eyes, it’s probably not sucking air, however it IS blowing out what should be a perfect mixture of fuel/air vapors. If enough of them get out before ignition, it’s lean. A great test here would be to run the same fuel after a replacement piston is installed with the decomp plugged.

I’ll put my $$ where my idea is too ……. send me the saw, I’ll see if we can true up the band and set squish, replace the piston, plug the decomp, then you run it like you stole it and report back to us.

Hows that sound ?

 

[Motorka]

Piston & Cylinder Damage – Detonation will often cause
what appears like erosion damage at the top of the piston above
the exhaust port. That “erosion” is actually melting of the piston
caused by the extreme heat of detonation. Detonation pressure
spikes can break pistons, break ring lands, damage bearings and
even break cylinder mounting flanges. Detonation damage is most
likely to occur on larger handheld 2-Stroke engines around 50cc or larger.

Page 27 of Engine Failure Analysis Booklet.pdf

 

[Stump Shot]

It is plugged in the picture. I tested with the decomp, found it to be a huge leak, plugged the decomp, and retested. No pressure or vacuum leak once it was plugged.

I pulled the cylinder off and found the piston and cylinder in this condition.

It's kinda normal to have to plug off a decomp for pressure/vac testing as most do leak some. It would have to be stuck open to cause a catastrophic failure that the carburetor couldn't tune its way out of.
I still keep my assessment the same maybe would add something with the oil in the fuel putting out big droplets into the cylinder. At any rate, my point is, whatever it was that made the craters, fuel, oil or water it couldn't have happened if the squish tolerance is right. I know you say it was at .020" and that should be wonderful for a 50cc motor. However the true measurement can be much different than what you measured. For the solder trick to work it needs to be a soft solder and a diameter just larger than to be squished. Also needs to be ran over about twenty or thirty times until no resistance is felt with your hand on the flywheel. If the ratio is off it will only take the solder down so far and quit as the motor will cam over before enough pressure can be applied to squish the solder any longer.
For the fix, get yourself a new piston and re-machine the cylinder to a true .020" or a little more even will be fine, but no less. Put new fuel filter and fuel mix in a new spark plug and you should be good to go again.

 

[huskihl]

In my eyes, it’s probably not sucking air, however it IS blowing out what should be a perfect mixture of fuel/air vapors. If enough of them get out before ignition, it’s lean. A great test here would be to run the same fuel after a replacement piston is installed with the decomp plugged.

I’ll put my $$ where my idea is too ……. send me the saw, I’ll see if we can true up the band and set squish, replace the piston, plug the decomp, then you run it like you stole it and report back to us.

Hows that sound ?

If it’s a perfect mixture, it should be the same as running at say, 96? or 99?% full throttle. I don’t see it causing anything detrimental.

While cutting firewood, normal tune on a ported saw, I’ve pushed the decomp in and continued cutting. The tune didn’t change, just had mildly less power. So I don’t buy the “decomp caused a scored cylinder “ theory

 

[rogue60]

I've had detonation in a stock ms660 not as bad as the OP's saw.
I don't remember anything about how the tune was reacting only thing I remember was the excessive heat coming off the saw it was running very hot I new something was up. Turned out to be stale fuel was low octane to start with, ended up brought less fuel at a time so I'd use it up before it went off after that.

 

[NPKenny]

In my eyes, it’s probably not sucking air, however it IS blowing out what should be a perfect mixture of fuel/air vapors. If enough of them get out before ignition, it’s lean. A great test here would be to run the same fuel after a replacement piston is installed with the decomp plugged.

I’ll put my $$ where my idea is too ……. send me the saw, I’ll see if we can true up the band and set squish, replace the piston, plug the decomp, then you run it like you stole it and report back to us.

Hows that sound ?

I’ll cover shipping both ways, piston and parts, and what you feel is appropriate for your efforts.

I’m as interested as anyone.

Let’s document the process.

 

[NPKenny]

While cutting firewood, normal tune on a ported saw, I’ve pushed the decomp in and continued cutting. The tune didn’t change, just had mildly less power. So I don’t buy the “decomp caused a scored cylinder “ theory

I’ve had episodes with this saw over the last 6 months. I’ve got plenty of others to cut with, so if it isn’t running right, I grab another and go. Each time this saw was not running right, I kept wishing I could tune it a bit and keep cutting. It always felt like it should tune up and just never did.

I’ve tried the small mixture screw in a few different positions with zero impact.