Raj
TreeHouser
Ouch Al!
Changed the decomp. Same damn problem. Cold weather issue?? What else can it be, FFS?
Changed the decomp. Same damn problem. Cold weather issue?? What else can it be, FFS?
Worn out Decompression Valve
- Thread starter SeanKroll
- Start date
- Replies 93
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SeanKroll
Treehouser
- Thread Starter Thread Starter
- #77
I've been wondering about this jerk-back phenomenon
Most things point to premature ignition and backfiring (offset flywheel with sheared key, coil starting to fail, etc).
https://arbtalk.co.uk/forums/topic/58124-pull-cord-kickback-when-starting-a-stihl-ms-660/
https://www.arboristsite.com/community/threads/kick-back-pulling-is-the-timming-out.98742/
Most things point to premature ignition and backfiring (offset flywheel with sheared key, coil starting to fail, etc).
https://arbtalk.co.uk/forums/topic/58124-pull-cord-kickback-when-starting-a-stihl-ms-660/
https://www.arboristsite.com/community/threads/kick-back-pulling-is-the-timming-out.98742/
Raj
TreeHouser
I hope not!
Magnus
TreeHouser
Larger saws I pull with two fingers so I don't hold to hard and get arm broken.
Jonsered XF was known for braking of under arms.
I had enough pain to listen so two fingers it is.
If handle don't allow this I hold it further out on finders and not in hand.
If they backfier it pulls handle out indread of breaking my arm.
If decompression valve can't stay open it may need replacing or cleaning.
Carbon buildup can cause high compression.
Not often I see that here on saws in use as most run Aspen fuel. It leave no carbon buildup at all... Ever.
Jonsered XF was known for braking of under arms.
I had enough pain to listen so two fingers it is.
If handle don't allow this I hold it further out on finders and not in hand.
If they backfier it pulls handle out indread of breaking my arm.
If decompression valve can't stay open it may need replacing or cleaning.
Carbon buildup can cause high compression.
Not often I see that here on saws in use as most run Aspen fuel. It leave no carbon buildup at all... Ever.
Raj
TreeHouser
A new valve didn't solve the problem. Seems to be worse when cold.
Nutball
TreeHouser
Are you just now getting it broken in? Compression is now at it's best?
Cold can make oil thicker and increase compression by lessening the amount of gas blowing past the rings.
Cold can make oil thicker and increase compression by lessening the amount of gas blowing past the rings.
Raj
TreeHouser
Or I'm getting weaker!
Edit. I checked the flywheel, I can see the entire keyseat on the shaft. Checked the muffler, all clear. Backed the coil off. Added a redneck D handle. Got it running. Had a decent milling job, but the rain cancelled it.
Edit. I checked the flywheel, I can see the entire keyseat on the shaft. Checked the muffler, all clear. Backed the coil off. Added a redneck D handle. Got it running. Had a decent milling job, but the rain cancelled it.
Magnus
TreeHouser
I have sold HVA decomp valves to a hearley builder here. They broke starters as the comp was a tad high.
starting with decomp valves saved starter and happier custemers.
He must have bought 50 or so by now. Its two every build...
starting with decomp valves saved starter and happier custemers.
He must have bought 50 or so by now. Its two every build...
Merle Nelson
TreeHouser
Ha, that's interesting.
Magnus, I have one sticking open on a Husky 372XP I just bought used from a customer. Would you tend to think in terms of replace it, or is there a lube that should loosen it up. (Powerhead probably only has 30 hours or less on it.)
(I pull it closed with my fingers after start up.)
Thanks in advance for your thoughts.
Magnus, I have one sticking open on a Husky 372XP I just bought used from a customer. Would you tend to think in terms of replace it, or is there a lube that should loosen it up. (Powerhead probably only has 30 hours or less on it.)
(I pull it closed with my fingers after start up.)
Thanks in advance for your thoughts.
Magnus
TreeHouser
Wash it in petrol. I bet its sticky from oil.
Magnus
TreeHouser
If valve is defective it will loose power, run as plug was loose.
In itself it can't destroy engine as it can't fall in as hole in to cylinder is just 1,5mm in diameter.
In itself it can't destroy engine as it can't fall in as hole in to cylinder is just 1,5mm in diameter.
Merle Nelson
TreeHouser
Okay thanks, I'll do that.
Marc-Antoine
TreeHouser
Not in my 066 and ms440. The hole is drilled full diameter through the cylinder wall. Both décomp valves have a full access to the inside and their obturator is about flush with the dome's surface. If the little retaining rod breaks, the obturator will make a mess.
Magnus
TreeHouser
Loosing compression is not like a air leak. You still have lubrication and correct fuel mix.
It will loose power and could have bad idle. I did this a few times.
It feels hollow and sounds different. Put it to wood and its powerless.
The 357 and most non X-torq saws with decomp at side has a tiny hole in cylinder wall about as high as it can get.
Hole leads to a chamber were valve sit.
The xtorq has decomp valve in top of expansion chamber in a big hole and no channel or hole to lead to valve.
Perhaps this is same on the Stihls? Don't see so much Stihls here.
I don't see how a broken valve can cause seizure. It would be very easy to know something is wrong and stop running.
I guess anything is possible but you really have put some effort to it.
On the saws were hole valve go in compression chamber it still will not happen much to it as flame will not eat material as it expand. If it did we would not have aluminum combustion surfaces as there is on piston and cylinder.
The only thing coated/sleeved is were piston travel to reduce wear. The rest is exposed.
Oil used could cause carbon buildup, corrosion etc, but thats not common today.
When I see broken decomp valves its mostly broken from outside forces. A few had worn out spring, but I have never seen a engine blow up from a decomp valve yet.
It will loose power and could have bad idle. I did this a few times.
It feels hollow and sounds different. Put it to wood and its powerless.
The 357 and most non X-torq saws with decomp at side has a tiny hole in cylinder wall about as high as it can get.
Hole leads to a chamber were valve sit.
The xtorq has decomp valve in top of expansion chamber in a big hole and no channel or hole to lead to valve.
Perhaps this is same on the Stihls? Don't see so much Stihls here.
I don't see how a broken valve can cause seizure. It would be very easy to know something is wrong and stop running.
I guess anything is possible but you really have put some effort to it.
On the saws were hole valve go in compression chamber it still will not happen much to it as flame will not eat material as it expand. If it did we would not have aluminum combustion surfaces as there is on piston and cylinder.
The only thing coated/sleeved is were piston travel to reduce wear. The rest is exposed.
Oil used could cause carbon buildup, corrosion etc, but thats not common today.
When I see broken decomp valves its mostly broken from outside forces. A few had worn out spring, but I have never seen a engine blow up from a decomp valve yet.
SeanKroll
Treehouser
- Thread Starter Thread Starter
- #93
I don't know.
Just what I've hear about de-comp valves around here. Might not be true.
I've experience a loose plug in a saw, and a decomp valve not shutting properly (bent). I didn't notice any damage overall.
I don't understand how it would run into a lean condition, but that's what the Stihl dealer said, and what i've heard about pro fallers and decomp valves.
Just what I've hear about de-comp valves around here. Might not be true.
I've experience a loose plug in a saw, and a decomp valve not shutting properly (bent). I didn't notice any damage overall.
I don't understand how it would run into a lean condition, but that's what the Stihl dealer said, and what i've heard about pro fallers and decomp valves.
Magnus
TreeHouser
Dealers are often told what to say and think even. Brands teach them what to say if product fail.
Often its operator or fuel, settings was popular to put blame on until they removed setting screws.
Now they find something else to blame.
Lean running saws is saws that run hot from not enough fuel.
It can be caused by air leaks that affect pressures, but that is under piston ring.
What happen above piston ring with pressures rarely case fails.
I am no brand dealer and self taught. If they go on about stuff as they do with those that are depending on their product, I just sit and listen. Sort thru the BS and sometimes get a comment or question I respond to as short as I can.
I could help and do sometimes. They bring the stuff here when it takes more than ten minuets to figure out and fix.
They rarely ask what is done, just pay and go back to store.
Often its operator or fuel, settings was popular to put blame on until they removed setting screws.
Now they find something else to blame.
Lean running saws is saws that run hot from not enough fuel.
It can be caused by air leaks that affect pressures, but that is under piston ring.
What happen above piston ring with pressures rarely case fails.
I am no brand dealer and self taught. If they go on about stuff as they do with those that are depending on their product, I just sit and listen. Sort thru the BS and sometimes get a comment or question I respond to as short as I can.
I could help and do sometimes. They bring the stuff here when it takes more than ten minuets to figure out and fix.
They rarely ask what is done, just pay and go back to store.
