Worn out Decompression Valve

SeanKroll

Treehouser
Joined
Oct 13, 2016
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12,252
Location
Olympia, WA
The dealer mechanic told me that my ms660 decomp valve was leaking, resulting in a lean condition.
Not in any way noticeable when disassembled. Snapped open, snapped closed. Maybe it needs to be harder to pop closed. Dunno. Didn't know it could be seemingly fine and be faulty.

The piston ring was broken and it scored the cylinder.


I've seen a decomp valve rattle when the saw ran, and replaced it. I saw a bent decomp valve once, and replaced it.

I got a new decomp valve for my older 460. Figured it's better to change in advance for $16 than the quoted $550 from the dealer for a rebuilding the 660 that blew up.

Lesson learned.
 
I haven't heard of that situation either, Sean. Not in more than 35 years of chainsaw use.

I rather suspect your dealer is blowing smoke, but I certainly could be wrong...it has happened before:).
 
If it closes its closed positive pressure will keep it closed and if spring is good it will hold it closed when there is negative pressure.
Easy to test. Stick it in your cookie trap and suck. If you can't get it to open its likely ok.
I seen many valves gone bad over the years. This was a common excuse to put blame on when they cooked and measures were taken to make sure this was taken out of equation.
Haven't seen them go bad as often the past 15 years or so. Not much issues at all since they scrapped the auto decomp systems.

Chances are better it ran too hot/lean for other reasons. Fuel, adjustment, carb, seals, lines, dull chains, bad cooling.. A lot more common issues.
 
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  • #7
I wondered if they were blowing smoke, too. I can't see anything wrong with it. I figured I'd get some good experienced advice here. I agree its likely failed from running lean, just not sure why. Seems like diagnosing a seemingly good decomp

I wondered if it had the carb go bad, and run lean. Its probably the original carb.

I am not the only one who runs it. Hard to know what happened. 40psi compression and no starting, so I gave up on troubleshooting.


I wonder what happens if its rebuilt without changing the carb diaphram and it blows up again?

I was thinking carb in the first place, or ignition, but the ring is definitely broken and the cylinder is scored too deeply to hone out, at least by the dealer.



I was thinking a saw builder would be the way to go, unless I could do it stock, in-house. I have expert advice here, but don't frequently have time, patience, motivation, etc, all at once.

Since my ms 361 is shot. I might try to learn on it, first. Its got a ton of miles on it.
does the cylinder wear normally, too much from normal use, or just damage? Do the rings effectively take the wear?


Any good videos about rebuilding?
 
..... If you were to throw it out, I'd be a good dump....

I can't point to a specific repair video, there are a lot of them.
 
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  • #9
Carb is off, but don't know if it was opened. I'll be interested to see if the diaphragm is damaged.
 
Basically it ran too hot without enough lubrication. With a two cycle, the fuel is the lubrication so a lean condition will not only make the engine run hot but it will also have reduced lubrication.

A few basics I've learned over the last 35 years:

Run a high quality synthetic mix oil. Regular 'orange bottle' Stihl oil is junk. If you must use Stihl oil, run the silver bottle synthetic. Echo oil is also very good. A high quality synthetic oil will give you more margin for error with lubrication needs.

Today's saws all run way too lean as delivered when you buy them new. If you do not modify them, they will run lean and they will usually die an early death. I have a dozen saws that are 10-20 years old and all of them have been modified in some manner in order to not run lean.

Sharp saws run cooler than dull saws. If your saw is not sharp, do not keep pushing it. Stop and sharpen it. Otherwise it will die an early death.
 
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  • #11
Thanks.

Always Silver HP ultra.


I cut sharp, unless I'm root pruning, but that's not 660 work, clearly. I have employees, so...


Skimmed a video. Doesn't look that hard.


What kind of replacement cylinder is recommended?



This saw was woods-ported at Madsen's a long time ago, when new or newish, which helps it run cool.
 
Never use mine either.
Except on the MS880.
I just ain't man enough to start that one without.

One of our MS441s spit the whole valve out last week.
Saw shop guy said that was a first for him.
 
Today's saws all run way too lean as delivered when you buy them new. If you do not modify them, they will run lean and they will usually die an early death. I have a dozen saws that are 10-20 years old and all of them have been modified in some manner in order to not run lean.

I've never modified a saw and ours in the 70 cubic centimeter class live for about 3-4 years.
That is close to 5000 hours of run time.

Maybe using alkylate fuel is a factor.
 
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  • #16
Carb hasn't been opened.



Sometimes, when someone needs a dying tree removed, they'll ask why its dying. If i don't have time, I'll tell them I could diagnose that for them if they'd like, but ultimately, their tree has to get taken down (the one lone tree that won't have disease vector issues type of thing).

I can imagine that in the course of rebuilding saw, they would clean the carb, install a diaphram, and adjust it. Might have said decomp valve leak even though it was another cause of lean conditions, with all bases covered by the repair, without charging for additional time to diagnose all possible causes of lean conditions, as it was at the repair-estimate stage.
 
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  • #18
I don't mind paying more the better quality on a money-making tool.

Seems like the time to ship to a builder.

I'll start a thread.
 
I run 40:1 mix. I have it in my head if the saw runs lean for some reason the 40:1 will keep it lubed. Don't know if I'm right.....??
 
IMO find out why it scored in the first place. Seals, carb tune, decomp etc etc to make it go lean with air leak or whatever.

Pull top and see how bad is cylinder. If OEM cylinder lets see how bad it is. Most can be cleaned up and then use a OEM or Meteor piston kit. JMO

Then you have maybe 50-70 in piston and gasket in it, depending on $ to fix reason why it did it in first place.

Bunch of stihl techs in my group and website if help needed.
 
You can replace the decomp valve with a solid plug if you don’t use it. Totally personal preference. New piston and cylinder is super easy but make sure you have a ring compressor or it can be a pita.
 
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