High Quality Chainsaw Bars Husqvarna Toys

Diagnosing what caused burnt piston/scoring?

CrystalRiver1

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Hey Studs,

When you inherit a saw with a toasted piston and or scored up cylinder how do you determine what caused the damage?
I know many folks say stuff like this, "loaned to Wilbur and he straight gassed it---it was running super lean, or bad air leak...etc, etc."
Yet, if you have no such report...what methods/troubleshooting do use to walk down what caused the damage? My finances are not like Ft. Knox:greedy:, and if I can save some dough by DIY...I will!
Perhaps I've been postponing the inevitable by NEVER doing a project saw...:qmiedo::lunchacos::hmmm:
I see some excellent DIY rebuild video's on YouTube...BUT these cats already know what killed the saws!!!

Here are pics of my toasted 026 and all I can tell you is this:
Bought it and it ran like a psycho ape for about 10 minutes, noticed it had an extra dull chain, shut it off---let it sit for about 7 months, put fuel/mix in it...it would pop---never fire up---checked the spark-plug and it extra black and the new plug was the exact fit via the Stihl manual, yet was loose as a goose...wire coming from plug was raggedy as a wino's mouth!!

Kept popping, but never fired up....took to the local Stihl shop---$35 later---Burnt piston!
Here are ze pics of great sorrow and horror!
I open to any & all feedback.
3 Dog.jpg
Piston 2 dog.jpg
wire.jpg
 
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Stump Shot

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In this case "It ran like a psycho ape for 10 minutes" is part of the clue to what happened. It shouldn't run like that, so a lean condition is indicated. Unless the carburetor was severely maladjusted or malfunctioning, the most likely culprit is a vacuum leak. Which can be found out by further testing with a pressure/vacuum test of the crankcase.
Usually a straight gassed piston will have scoring all over it, not just the exhaust side like a leaned out one. Also check the gas in the tank by pouring in a clear glass jar, then you can see if it is gin clear or if it has color that oil is mixed in it. Hard to tell? Pour known straight gas in another jar and compare.
 

MustangMike

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Like Stump says, straight gassed will usually be on both sides of the piston, heat failure/lean may be on just the exhaust side.

Always check the simple stuff first … I got a burned MS440 that had the Hi screw set 1/4 turn out … replaced the piston, cleaned the cylinder, and it ran just great! Also, make sure that the plug is tight and that the decomp in not leaking … I once got a MS460 with no compression … decomp was bad! A tree guy brought a saw to me that would not run right … plug was loose!

If the cause is not obvious, or is the saw has a lot of use (the 440 looked like new), then always do a vac / pressure test.

Also, even if you tested, always pay attention to how the saw behaves … there is no replacement for common sense. If something does not sound right, look for the problem. Carbs, impulse lines, tank vents, air leaks and electrical problems are all fairly common. (bad plugs, bad wires/switches or coils that don't work when they get hot).
 

Homelite750

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we had a new husqvarna 395 with 36" bar come back less than a week that was scored. Gas was good, no air leaks and tuned properly. The chain was super dull and the guy reefed on the saw so hard the rear handle was hitting back of the saw. The heat created from running a dull chain killed that saw.
 

pwheel

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Local dealer gave a blown MS460 to me a couple of years ago. Exhaust side scored and bearings ruined with aluminum bits. When I got it all back together, I found that the high speed needle on the carb was tuned way lean. Bingo. Previous owner probably tweaked it too far on a hot day and cooked it.
 

pwheel

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This stock MS460 has torque, no bad habits, and can run a 32" bar w/skip chain, but it's no hot rod. If the previous owner wanted a screamer, he had the wrong saw.
 

Al Smith

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Surprising how some just forgot the basics or never knew to begin with .Crank them down,make them scream and wonder why they cook a piston .It's all summed up,it takes fuel to make power .Noise is just noise .
 

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Hey Studs,

When you inherit a saw with a toasted piston and or scored up cylinder how do you determine what caused the damage?
I know many folks say stuff like this, "loaned to Wilbur and he straight gassed it---it was running super lean, or bad air leak...etc, etc."
Yet, if you have no such report...what methods/troubleshooting do use to walk down what caused the damage? My finances are not like Ft. Knox:greedy:, and if I can same some dough by DIY...I will!
Perhaps I've been postponing the inevitable by NEVER doing a project saw...:qmiedo::lunchacos::hmmm:
I see some excellent DIY rebuild video's on YouTube...BUT these cats already know what killed the saws!!!

Here are pics of my toasted 026 and all I can tell you is this:
Bought it and it ran like a psycho ape for about 10 minutes, noticed it had an extra dull chain, shut it off---let it sit for about 7 months, put fuel/mix in it...it would pop---never fire up---checked the spark-plug and it extra black and the new plug was the exact fit via the Stihl manual, yet was loose as a goose...wire coming from plug was raggedy as a wino's mouth!!

Kept popping, but never fired up....took to the local Stihl shop---$35 later---Burnt piston!
Here are ze pics of great sorrow and horror!
I open to any & all feedback.
View attachment 194511
View attachment 194512
View attachment 194513

Well Reg she`s toast, what are your plans for it?
 

Al Smith

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Look closely at the pictures and note the carbon build up on the exhaust port .I may be mistaken but it appears to me at one time it was either extremely rich and/or likely had a plugged up muffler .If so it would back stuff the loose carbon right back in on the piston .Pyrolitic carbon is about like blasting sand .Which is not to say it wasn't cooked by either lean run or straight gas .Just an observation after the fact .
 

CrystalRiver1

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When they come in and say," the saw was never working better, it was really screaming and cutting like a banshee" ! , now it won`t even start.
Well Reg she`s toast, what are your plans for it?

Well Basher, I've been pondering on a couple of possibilities.
Trade it in on another saw from a reputable seller here or close by.
Hang onto to it for a project down the road. :rolleyes:
Package it up with a 038Mag carcass, some other spare parts and sell it here or Facebook/Craigslist.
I'm finishing up my Masters currently and really don't have time to work on it personally, besides everybody and his kid brother wants freaking $300 plus to rebuild it! I've heard prices from $300 up to $750 for installing a piston/cylinder kit! NOT!
 

Al Smith

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With regards to the 046/Ms 460 .from my observation it's a solid reliable mid sized saw .Like any removable cylinder Stihl they can be modified for improvements .
Just about any thing can be repaired .Due to the fact ,as I understand it was a freebie .It would be an ideal example to try a persons hand at it maybe finding a donor or after market parts .You're not going to learn any younger if that's what you want to do .
 

Basher

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Well Basher, I've been pondering on a couple of possibilities.
Trade it in on another saw from a reputable seller here or close by.
Hang onto to it for a project down the road. :rolleyes:
Package it up with a 038Mag carcass, some other spare parts and sell it here or Facebook/Craigslist.
I'm finishing up my Masters currently and really don't have time to work on it personally, besides everybody and his kid brother wants freaking $300 plus to rebuild it! I've heard prices from $300 up to $750 for installing a piston/cylinder kit! NOT!


Another option, you mail it to me, I will rebuild it bottom to top mechanically and send it back to ya. I have tons of 026 parts in my inventory just itching to move along. Send me a PM sometime.
 

Al Smith

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Geeze the thread jumped all over the place .I reread it and the original post talked about an 026 then shifted to an 046 .Now here's some info FWIW .The 026 lists three different muffler systems .A fact I didn't know until Kevin(Cut for fun) pointed it out to me .One is highly restrictive .That in itself could cause problems should the carb settings get out of kelter ,screwed up .
I landed an 024 which is nearly the same model at a dealer having been traded in with such a muffler system .It was not in the shape this one appears to be,crispy critters .The piston was slightly scotched on the exhaust side .All I did was gut the muffler and retune the carb and nothing else that I remember .I've had no problem in any way from it .I think I've had it for at least 5-6 years ,never once failed me .
 
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