An Autotune Husqvarna Model 550 challenge

[Karla]

I purchased a Husqvarna Model 550XP Autotune (Model year 2015) which is in beautiful condition, and looks like it has only been run once or twice. The cylinder, piston, and piston ring is severly scored 180 degrees around the exhaust side with aluminum transfer, no real grooves but scored. The cylinder exhaust port and muffler have Zero carbon deposits, looks like they were cleaned with carb cleaner, and the piston only has a lite coating of carbon on top. I found the decompression valve stuck open with some carbon on outside seating area of valve, valve was very difficult to close believe it may have stayed open and not closed. Also found the clutch had been overheated, due to burn color/ bluing of metal run very hot, and found the Air intake filter was filthy, the rest of the chainsaw was really clean like new. I have completely disassembled the Model 550Xp, and tested everything, bearings, air seals, carburetor functional operation, fuel lines, and did a complete vacuum/ pressure test of engine, etc, etc.

Obviously I am going to re-build the top end of this chainsaw, since I really like the Model 550XP and currently own its sister a Model 545xp, But I don't want this re-occurrence on a brand new cylinder, piston and piston ring.
So what are your thoughts, and any help will be appreciated ???

PS. My thought - With a stuck open decompression valve, and running the engine with Autotune, the engine ran very lean and scored the cylinder, but just a guess? But then why no carbon in the exhaust ?

 

[pro94lt]

How did you close the decompression release?

 

[fordf150]

Pics would be helpful.

Just to recap

Passed pressure and vac test.
Decompression valve was stuck open.
Clutch/ clutch drum was blue from heat.
Air filter filthy.

How did you close the decompression valve?
Check for codes on the autotune?
Oil residue on piston and underside of piston?
Did you check the remaining fuel in the tank and carb?

Just guessing from the info given but i would say straight gas or very lean mix

 

[moondoggie]

My 562 had a leaky decomp I didn't know about till it scorched thepiston 107 hours on it when i found out. The piston was black top to bottom. The saw was replaced under warranty.

 

[Karla]

Yes-It passed all of the tests and everything else looked clean, including the fuel filter which was pristine and looked like brand new, I really tested everything, which I did as I took it apart. Oh, the cylinder exhaust port looked too clean, along with the muffler. I tried to take a shop towel, and wipe out the exhaust port and not a smudge of carbon on the shop towel, It could have been a factory or third party re-furbish because it was shipped in one of those boxes, so not sure what they may have done to it also.

I removed the decompression valve which was very hard to push on the bottom of the valve to close it, I tried cleaning it and manually cycling it open and closed and very hard to push in both directions. I built a test fixture for testing decompression valves, and tested it with a new decompression valve, and a 100 Lb compressor, and it appeared to work fine, I was able to manually open it, and close it with air pressure. When I tried the old one or original to the cylinder decompression valve, I could manually open it but difficult, and 100 lb air pressure would not close it, so I assumed it must have stayed open during running the chainsaw, not sure with the Autotune If that would work. (Note: I need more Autotune experience). I have Not purchased the Autotune diagnostic test kit as of yet, but will probably in the near future when the new "CST" Autotune comes out. Oil in the crankcase, and on the piston, I really can't tell because I have made a habbit of spraying "LMT" cleaner which contains oil into the engine thru the Spark plug hole before I pull an engine over, my thought process was to always make it easier to initially pull over the engine, and reduce any additional cylinder damage If the engine is really not too bad, versus cranking on a dry piston and cylinder to see what will happen, Lol. Pictures - The engine is completely apart, and I have already done some cleaning. so Not sure what the pictures will show, but it really is a pretty chainsaw, and I can include an as purchased picture, I originally purchased it for parts, but now after seeing its condition I definitely want to rebuild the top end (parts are on order, and waiting).

See uploaded as purchased picture. I am just trying to really rationalize and understand what went wrong, I may even try to contact the seller to see If he Knows for sure, like he ran straight gas in it or something, but he is under No obligation to tell me.

Thanks for all your comments, and suggestions, and your votes - I know its difficult to diagnose the original symptom of the real cause, I call it the forensic diagnostics, but I enjoy that part the most,

 

[Karla]

Moondoggie, Thanks for your reply. I am wondering If that maybe the same case here. I need more Autotune experience, but I am also trying to understand how the engine could run with a wide open decompression valve, unless the Autotune compensates for the air loss thru the carburetor and makes an adjustment to lean the fuel, Which would then act much like an air seal leak and could lean the engine with high RPM's and then score the cylinder and piston, really not sure !

 

[Clearwater]

I have a Husky 555 AT. Is there a sound or power diffence that the operator should hear or feel that would warn of the stuck compression release?

Thanks, Dave

 

[moondoggie]

The auto tune compensated for the air leak. There was one burned spot inside of the jug directly below the decomp.

 

[flyinlow]

I have had a very similar experience with my 550xp. I purchased it from a small shop that repairs utility company chainsaws. He had traded for the saw in as new condition but not running. He assumed the AT was toast. I paid for the saw knowing I had a good chance of being burned. But---- the bottom line is that the decomp valve was leaking badly and the previous owner had mixed the fuel with an excessive amount of oil. The cyl. and piston were pristine and I was lucky enough to have a mint AT ! The decomp. valve is not an exclusively Husky problem as I have 3 Stihls that have plugs in the decomp. hole for the exact reason. My 550xp only had 105#of compression until I installed the plug. I have no idea how anyone could get this saw started with the valve stuck wide open. The 550xp now has about 5 hrs on it and is gaining power as it breaks in. Someone needs to build a really high quality valve and do us all a favor. I believe the AT and MT saws are far better equipment than most people think. I have 4 of them and this is the only problem I have had. And....... who needs a decompression valve on a 50cc saw!

 

[moondoggie]

I have had a very similar experience with my 550xp. I purchased it from a small shop that repairs utility company chainsaws. He had traded for the saw in as new condition but not running. He assumed the AT was toast. I paid for the saw knowing I had a good chance of being burned. But---- the bottom line is that the decomp valve was leaking badly and the previous owner had mixed the fuel with an excessive amount of oil. The cyl. and piston were pristine and I was lucky enough to have a mint AT ! The decomp. valve is not an exclusively Husky problem as I have 3 Stihls that have plugs in the decomp. hole for the exact reason. My 550xp only had 105#of compression until I installed the plug. I have no idea how anyone could get this saw started with the valve stuck wide open. The 550xp now has about 5 hrs on it and is gaining power as it breaks in. Someone needs to build a really high quality valve and do us all a favor. I believe the AT and MT saws are far better equipment than most people think. I have 4 of them and this is the only problem I have had. And....... who needs a decompression valve on a 50cc saw!

I haven't used the decomp once on the replacement 562.

 

[Karla]

Moondoggie, Thanks again. I just looked at the piston top, and there is definitely a darker layer of carbon which outlines the decompression valve under where the decompression valve would be in the cylinder, but the total carbon buildup on the piston is ever so slight, it looks more like a carbon wash If you took carbon cleaner with carbon in it and painted the top of the piston. I don't really see a burn mark on the piston top, but the carbon marking the outline is slightly darker.

Flyinlow, Thank you for you Input, and I agree about 50 cc engines and smaller are certainly easier to pull, and I have been kind of thinking about plugging the decompression, but I also have a few new decompression valves, so I may end up replacing it. Totally not sure which way to go with that one, and If I don't eat my Wheaties in the morning or just having a kind of tired day, It is nice to have the easier pull with the decompression valve, and I can feel the difference. In either case it will get replaced with a new one or plugged when I get my new cylinder assembly kit. The old one or original to this engine, I'm keeping for show and tell, and If I were the Original owner, and had the original receipt. I would probably be looking for a warranty repair. Lol.

 

[Karla]

Dave, I would be curious to the answer of your question. I'm thinking with Autotune the chainsaw would keep trying to run, probably at a reduced horsepower, but maybe a higher speed as when an engine goes lean. If you were cutting wood, I doubt you would hear anything different really not sure, but performance would definitely diminish. It would be interesting to try but, I like to experiment but not sure I would be thrilled to burn up a cylinder to find out, my budget is sort of restricted. I guess one could intentionally fail a decompression valve in the open position, and then see what happens with an Autotune, I would be curious , Lol.

 

[Old2stroke]

When you find a saw with an overheated clutch, it means the operator has been loading the saw down to the point where the clutch has started to slip. Low rpm under heavy load means the flywheel fan can't deliver enough air to keep the cylinder temp safe for the amount of power it is producing. There's a reason people say to keep the rpm up when cutting. Rather than autotune, it would be nice if the manufactures would build a system to sense cylinder head temperature and shut the saw off when it gets too high.

 

[Chris-PA]

Well, I guess it has to be able to start with an open decomp valve, or what would be the use of them?! And if it can start it can run, even if poorly.

 

[Karla]

Fordf150,
Just to answer your question, I went out to my torn apart engine, and checked the fuel tank and I was almost sure there was no fuel in the tank when they mailed it to me, However I turned the tank over onto a clear glass clean pickle jar, and to my surprise I got about 2 Tbls to maybe 1/4 cup of fuel. I know I did this before and there was nothing there, but anyhow I looked at the fuel, and it smelled good, and looked like chainsaw fuel, very clean, and a lite tinge of green to it, and feels like oil in the fuel, so I am now assuming it probably had the correct fuel mixture in it. Unless of course the third party chainsaw repair or refurbishment shop dumped the fuel, and filled it with their own mix before testing or trying the chainsaw, and then emptied it again before shipping it to me, I don't know. Oh, I also on occasion, especially If an individual tells me they don't know how long the fuel has been in the chainsaw that I'm about to repair, I will dump his fuel and put my fuel in along with a new fuel filter, If its a suspected fuel or carburetor won't start problem, I think its better to know what you have and start from there. Lol.

I also took some pictures today of both intake and exhaust sides of the cylinder, and piston, and additional picture of the exhaust port but it was difficult to get a well lit picture to show how amazingly clean it was when I took it apart

Also If anyone want to change your vote for the Poll, with this additional Information, then that is OK, and I really appreciate the help.

 

[Karla]

Chris-PA, and Old2stroke, Thanks for your replies. I also did check the crankshaft and connecting rod for any signs of heating, and there were none, All of the heat was focused on the clutch, I also like the idea of having a temperature sensor on the engine.

When I run the saw for the first time after a re-build, I always take its temperature with Infra-red gun, both the exhaust, and cylinder. then I have something to compare for future tune-ups. These two stroke engines really do run hot.

Thanks for your help, and for your votes.

 

[fordf150]

@Karla thanks for pics.

you are right on the fuel...without knowing the story, fuel in the tank/carb is almost meaningless because that has become one of the first things i do in my shop...dump the fuel and put fresh fuel in customer saws(providing the compression feels good) and give them a quick go. fresh mix has solved an awful lot of repair bills. The catch to that is, I am the first one to look at the saw and dump the original fuel in a clear jar to examine before adding fresh mix. You dont have that luxury.

never had a stuck decomp valve, quite honestly i plug most of them on 80cc and smaller saws, and i dont know if a saw will run with one completely stuck open. everything i have ever messed with closes the decomp just pulling it over so i cant imagine an engine able to run with one stuck completely open. have had plenty of leaking ones and thats the reason i just plug them. dont think i have ever had a leaking one cause an issue though since they are usually very small leaks.

i still vote lean seize. if it were from the decomp the scoring would be heavier on the decomp side of the piston and its not. if it were overheating the scoring would be much heavier on the pto/exhaust side but it started on the exhaust side and worked its way around the piston. If the ring hadnt of stuck causing total lack of compression the scoring would have continued around to the intake side even more. another possibility(which still falls under the lean seize category) is high ethanol content fuel. have seen a few saws that by all accounts they would have been classified as straight gas/lean mix ratio without testing the fuel...20-25% ethanol will provide failures similar to straight gas and i have tested quite a bit of fuel at that rate coming from pumps labeled as less than 10%. Having a look at the data from the autotune would confirm/disprove that idea along with your idea of a stuck open decomp valve. both scenarios should have triggered fault codes/fuel settings inconsistent with gasoline.

here is the echo failure diagnoses book....it is pretty big file even though i compressed it some....pictures are really good in it along with explanations and cause/effect

 

[Karla]

Fordf150, Thank you so much it downloaded perfectly, now have lots of reading. I also really Luv the pictures definitely some great photography, and much better than mine LOL. I will start reading tomorrow. Thank you again.

 

[Old2stroke]

What I was trying to point out in my previous reply was that the saw was probably overheated by the way the operator was using it, and it seized due to overheating. The operator then opened the decomp valve when he tried to restart it, and it stuck open due to being overheated as well. I don't believe a saw with a stuck open decomp valve will start and run with enough power to cause this damage. An overheated engine is a fault that autotune won't compensate for.

 

[Karla]

Old2stroke, Thank you for the clarification, and yes I could see how that might happen.

Today I have also been messing with the decompression valve, so I took some more measurements and then built an excel spreadsheet to calculate the closing force on the seat of the de-compression valve, the program also lets you vary the compression values of the cylinder, and then tell you If you have enough compression to close the valve. So basically you take a (0 - 5) lb food scale, and press the decompression valve down onto the food scale, and watch the scale. Do this first in the open direction and then in the close direction, enter the numbers into the excel calculator and my program will tell you If a specific engine by cylinder diameter (mm) will have enough compression to close the valve. My old decompression valve took 1.5 lbs of force to open the valve and 2.75 lbs to close the valve, and If the compression in the cylinder was less than 130 lbs, which a scored cylinder would have less than 130 lbs, then there would be an insufficient margin to close the valve. Also my Excel program will tell what that margin would be. In any case, I'm Not putting the old de-compression valve back into the New cylinder assembly. I enjoy doing the mathematics, measurements, and calculating stuff like that, and Yes there is very little error, and the results were 100% repeatable. I did the same for a New decompression valve also, and it took 1.5 lbs open and 6 oz or .375 lbs close, and it also would close at a way less compression value (less than 30 lbs compression). Just way too much fun.

So maybe the cylinder starting to score previously, lowering the cylinders compression, and then the de-compression valve was opened, and could not be closed. So it maybe which came first, the scoring or the stuck open decompression valve, kind of like the chicken or the egg. LOL.

But obviously, I am going to re-build the engine, replace or substitute the decompression valve for a plug, and a new clutch, and finally get the carb tested with Autotune diagnostics to see If there are any fault codes, and check the history. It should tell me something, again its just a hobby and I am really focused on the forensic diagnostics, and Not so much on getting the engine running, I know I can do that. Lol again.