Need advice on piston/cylinder state (550xp)

[McPherson]

Hi folks,

Been long time since I went here, was pretty busy and now 2 kids (2 years old and 1 month old).
Just bought a second saw for limber (572XP), so far loving it!

My smaller saw (550XP, MY 2012 with about 40 hours of use) had some hot start issues so I did a compression test, the reading was 90 psi. Damn.

Decided to open to case to see what's going on.


Intake side
https://photos.app.goo.gl/6J7FYJ2cGZmQvhpZ6

Exhaust side
https://photos.app.goo.gl/KaY7FJyaxnS4nGbJA

Cylinder bore
https://photos.app.goo.gl/FBEtiWGJnAaE6cFG8

No alum transfer on the cylinder walls. Ring end gap 0,013" above the exhaust port, 0,015" in center and under. Can see some blowby on the piston under the ring. Can still see machining cross-marks on the bore.

So, what should I do? Change piston and ring? Ring only? Change nothing, put it back and beat the hell out of it? What could cause the blowby and the poor compression?
Will do a pressure test too to check the crank seals, probably change them anyway since the saw is already in pieces.

Thanks for the help!

Francis

 

[kyle1!]

Are you sure your compression tester is working properly? Hot start problems are caused by fuel/bad carb on the 550xp. I have a 550xp and it acts up now/then

 

[McPherson]

Pretty much. I must admit I should have tested it on my brand new 572 before to be absolutely sure... But the test gauge isn't very old too and from a good brand. I'll try it tomorrow.
Beside the scratch on the exhaust side, everything seems okay. Will buy new base and exhaust gaskets and crank seals and wait few replies here before putting it back together.

 

[Mad Professor]

P/C look decent. A little wear on piston skirt and that blemish on ex side. Check P/C clearance, if that's still tight, I'd just put a ring in it since you are in there.

Not sure of your compression tester reading? Checking another saw is a good idea. How does it feel pulling the recoil?

 

[McPherson]

Are you sure your compression tester is working properly? Hot start problems are caused by fuel/bad carb on the 550xp. I have a 550xp and it acts up now/then

P/C look decent. A little wear on piston skirt and that blemish on ex side. Check P/C clearance, if that's still tight, I'd just put a ring in it since you are in there.

Not sure of your compression tester reading? Checking another saw is a good idea. How does it feel pulling the recoil?

So I did a little test this morning with my compression gauge. On my brand new 572 (two tanks so far), reading was 110 psi with an adaptor, and 140 psi direct. I had to check my 550 with an adaptor since the gauge doesn't screw directly.

My gauge is similar to that one:
https://www.innova.com/product/detail/3614/Compression-Tester-Plus-6-piece-kit#keyFeaturesSection

I've also checked P & C clearance, but only have access to a mitutoyo digital caliper for now. Not the best, I know. I had 0,0015" bore value higher than the piston. Can't take that for granted, but should be OK.

How should I have tested the compression, knowing the adaptor can screw my value?

 

[Mad Professor]

So I did a little test this morning with my compression gauge. On my brand new 572 (two tanks so far), reading was 110 psi with an adaptor, and 140 psi direct. I had to check my 550 with an adaptor since the gauge doesn't screw directly.

My gauge is similar to that one:
https://www.innova.com/product/detail/3614/Compression-Tester-Plus-6-piece-kit#keyFeaturesSection

I've also checked P & C clearance, but only have access to a mitutoyo digital caliper for now. Not the best, I know. I had 0,0015" bore value higher than the piston. Can't take that for granted, but should be OK.

How should I have tested the compression, knowing the adaptor can screw my value?

Be careful not to mar the piston, use some oil, but feeler gauges used as go- no go- gauges will get you +/- 0.001" P/C clearance.

 

[irhunter]

Other than the hot-start issue, how was the saw running? If it was OK in wood, I think your 90 psi is a compression gauge issue.

If I had it all apart, and was unsure about the compression...I don't know. I could break either way: new ring right now or button it up and re-test.

Roy

 

[McPherson]

Other than the hot-start issue, how was the saw running? If it was OK in wood, I think your 90 psi is a compression gauge issue.

If I had it all apart, and was unsure about the compression...I don't know. I could break either way: new ring right now or button it up and re-test.

Roy

The saw behavior wasn't so bad, on the last cuts the saw almost died in a cut but was fine after.
I'll definitely change the ring since i'm there.

That puts me another question: You guys always do the auto-tune gets its data each time you use the saw? Since my use isn't regular (can use my saw in middle of summer to clean property, and the after in winter to do firewood, maybe 70*F difference). I usually get the saw going and doing some cuts, should ajust anyway? Maybe that's some of my issues...

 

[McPherson]

Be careful not to mar the piston, use some oil, but feeler gauges used as go- no go- gauges will get you +/- 0.001" P/C clearance.

Will try that trick. Thanks!

 

[cuinrearview]

Modern saws won't run with 90psi.

The wear on the intake side of the piston is consistent with what I have seen on these. All of the real fine dust in the carb box doesn't play well with the filtration system of the mk1s. The mark on the exhaust side looks like a single event, like some crud or a chunk of that carbon from the crown made it's way through. On a personal saw I'd just take the prof's advice, re-ring it- and run it.

 

[Mad Professor]

I'll agree the mark could be a carbon score. The top of the piston is pretty dirty with built up combustion material. Clean all of that, the combustion chamber and the ex port.

Also note the carbon above the ring. Mix or tuning is rich.

 

[RED-85-Z51]

If the ring groove is loaded with crap the ring wont act right...can cause a poor seal.

Sent from my LM-G820 using Tapatalk

 

[Mad Professor]

If the ring groove is loaded with crap the ring wont act right...can cause a poor seal.

Sent from my LM-G820 using Tapatalk

If you take an old ring and snap it. It makes a perfect scraper to clean out the groove(s).

 

[mountainlake]

To me it looks like the intake side is wore out, could be from debris or who know why. If its wore out it pushed the charge back though the carb rather than through the transfer ports making it hard to start. New piston and rings. Steve

 

[clint53]

It looks far better than the 550XP that was given to me.

 

[McPherson]

Modern saws won't run with 90psi.

The wear on the intake side of the piston is consistent with what I have seen on these. All of the real fine dust in the carb box doesn't play well with the filtration system of the mk1s. The mark on the exhaust side looks like a single event, like some crud or a chunk of that carbon from the crown made it's way through. On a personal saw I'd just take the prof's advice, re-ring it- and run it.

That's what i'm going to do. It's a personal saw and not for a living. Maybe if it runs great I would be tempted to sell it and buy an MK2, just because . Since I have my 572 which is an improved version, everything seems to go better with it...

I'll agree the mark could be a carbon score. The top of the piston is pretty dirty with built up combustion material. Clean all of that, the combustion chamber and the ex port.

Also note the carbon above the ring. Mix or tuning is rich.

I run stihl oil, 50:1 (in liters, not in gallons) and sometime Aspen premix fuel. Found that the saw behave MUCH better with aspen, no or very few hot start issue, starts better cold, better power. Again, maybe the way I use the saw (not ajusting it each time I take it) can be a cause of the behavior.

If you take an old ring and snap it. It makes a perfect scraper to clean out the groove(s).

Thanks for the tip, I was struggling getting that groove clean! I'll do that.



Someone told me Husky made a recall to change the carb to an AT12 and it solves the issue? Anyone heard about that?

 

[McPherson]

It looks far better than the 550XP that was given to me.

View attachment 873572

Ouch!!

 

[cuinrearview]

That's what i'm going to do. It's a personal saw and not for a living. Maybe if it runs great I would be tempted to sell it and buy an MK2, just because . Since I have my 572 which is an improved version, everything seems to go better with it...


I run stihl oil, 50:1 (in liters, not in gallons) and sometime Aspen premix fuel. Found that the saw behave MUCH better with aspen, no or very few hot start issue, starts better cold, better power. Again, maybe the way I use the saw (not ajusting it each time I take it) can be a cause of the behavior.



Thanks for the tip, I was struggling getting that groove clean! I'll do that.



Someone told me Husky made a recall to change the carb to an AT12 and it solves the issue? Anyone heard about that?

There were a few changes to carb. I thought it was the 55 that was the "good" setup. I've also read of folks figuring out little tricks to work around the issues. The "hidden" screw and what not. I re-built a 550 this summer that performs perfectly except for a stumble off idle once warm. I've got another to do. I think for the performance I could probably live with the stumble for what I'll have into it. The bottom end re-build will cost more than the rest of the saw.