Worn out Decompression Valve

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.
 
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.
 
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.


20181029_111254.jpg
 
  • Thread Starter Thread Starter
  • #83
What?

The guy who did/ does Granberg demos says milling in the rain keep the saw cool. Kevin, I think is his name.
 
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...
 
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.
 
  • Thread Starter Thread Starter
  • #86
If I had a defective valve that can burn up the saw, I'd buy a new one. They are cheap and easy to replace. First sign of trouble, I would not put duct tape a on tire.
 
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.
 
  • Thread Starter Thread Starter
  • #90
What happens with the air leak? Any leaning out?

I've always heard people talk about a faulty decomp valve burning the saws up, so some pro fellers put a bolt in the hole.

A feller's old 460 I have came with a decomp valve.
 
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.
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.
 
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.
 
  • 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.
 
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.
 
Back
Top