MS660 Decompressor valve pops too soon

[Nathan_Anthony]

Issue as stated above.
I installed a Big Bore kit on that saw and the valve pops every time I pull the cord now even with the spark plug disconnected.
It's better with a chinese valve, but I still have to keep it pressed when starting.
I even tried it with a new valve from Stihl, but it's just worse with that.
Could it be that there's just too much compression?
Are there any modifications I could do to the valve?
Maybe just drill more holes?

 

[rupedoggy]

yeah

 

[RED-85-Z51]

My Blue G660 does it too, ita got a ported top end, but it kicks on the first pull cold and is running on the 3rd so its not bad.

Sent from my LM-G820 using Tapatalk

 

[Nathan_Anthony]

Well, guess I'm just gonna try to drill then
to be fair, I rarely use it since I got my MS200t wich is tremendously fun for firewood, I know it's not meant for that task, but it's just super handy and good enough for smaller stuff.

 

[U&A]

Take it out and Put a bolt in it. Make sure its short enough


[emoji2373]

Sent while firmly grasping my redline lubed RAM [emoji231]

 

[Nathan_Anthony]

Update
I think it worked.
I can yank it a lot harder til it pops now and it still pops(obviously) if the saw fires
can't really start it since it's the middle of the night

why would I put a bolt in it?
work smarter, not harder ;P

 

[drf255]

Buy a Husky 395 blue button OEM valve if your current one doesn’t work. It’s got bigger vent holes and they can be drilled larger. I use them on ported 80+Cc saws with great success.

 

[U&A]

Update
I think it worked.
I can yank it a lot harder til it pops now and it still pops(obviously) if the saw fires
can't really start it since it's the middle of the night

why would I put a bolt in it?
work smarter, not harder ;P

Everyone is different [emoji1303]

Glad you got it working man


Sent while firmly grasping my redline lubed RAM [emoji231]

 

[ChoppyChoppy]

I've always found saws really hard to start using the compression release.

It's interesting lots of snowmobiles don't have any compression release. The Summit 800 I had, I couldn't pull it quick and hard enough to start if it was cold.
Thankfully it had electric start.

 

[67L36Driver]

The bleed hole in the Stihl valve is .025” diameter.

Drill it to about .040” and then to .060” depending on your results.

Stihl 046 and larger need .060” for this geezer.

I wouldn’t use the China made item (two .070” holes [emoji2960]) for nothing. [emoji90]

 

[Nathan_Anthony]

The bleed hole in the Stihl valve is .025” diameter.

Drill it to about .040” and then to .060” depending on your results.

Stihl 046 and larger need .060” for this geezer.

I wouldn’t use the China made item (two .070” holes [emoji2960]) for nothing. [emoji90]

the bleed holes on my Stihl valve are actually 0.06", the valve has weaker retention than the chinese one
the chinese valve had one hole with about 0.08"
I recon that with the larger hole you have, even tho the overall crossection is smaller, a lower amount of skin friction
why would you not use a chinese valve?
are there any risks involved?

it just feels kinda wrong to modify the expensive Stihl valve, altho I think that drilling it out to 0.07"-0.08" on both sides would propably do the trick, but I'd also drill about 0.12" to about 90% of the depth to reduce skin friction and give it a better chance, I did something similar to the chinese valve for the 2th hole
but there's some uncertainty due to the weaker retention of the stihl valve

 

[albert]

Ditch the china valve. Good way to wreck a saw. Get oem and drill if needed.

 

[arto_wa]

I had a hard time starting MS660 because the OEM Stihl decompression valve would pop up too soon and it was really tough to pull the starter rope.

Some of the chinese decop valves have supposedly failed and the knob has fallen off, resulting in the valve plunger dropping to the cylinder and causing damage.
No idea how many cases like that - could be just one or two but bad news travel fast?

This Husqvarna OEM valve solved the problem in my case.

 

[pioneerguy600]

Its not just a couple of the China decomp valves that have failed, their problem is that when the valve stem heats up the plastic button releases/ falls off and as said the valve will fall into the cylinder. If one wants to run/use one then pay close attention to the button becoming loose.

 

[rupedoggy]

How can the valve fall into the cylinder? All the cylinders I have seen the hole is smaller than the head of the deco valve. Maybe I have not seen cheap Chinese ones. Mike

 

[67L36Driver]

How can the valve fall into the cylinder? All the cylinders I have seen the hole is smaller than the head of the deco valve. Maybe I have not seen cheap Chinese ones. Mike

Most Stihls actually, OE or aftermarket, nothing to ‘catch’ the valve stem.

I’ve had two lose their plastic knob and fall into the cylinder.

One running at idle, dinged up the top of the piston and ended up in the muffler.

The other on the bench not running fortunately.

[emoji2959][emoji2959][emoji2959][emoji2959]

 

[pioneerguy600]

They fall directly in the cylinder, the hole in the Stihl cylinder is 10 mm dia. drilled straight through generally from the top of the combustion chamber.

 

[Nathan_Anthony]

so what you mean to tell me is that all these valves are subject to this type of failure?
I didn't see any with a metal knob tho, so maybe just drill trough the plastic cap and the stem and insert a pin or some wire?
and how is the valve just supposed to drop in there unless you push it, there's always positive pressure in the cylinder while running

 

[pioneerguy600]

so what you mean to tell me is that all these valves are subject to this type of failure?
I didn't see any with a metal knob tho, so maybe just drill trough the plastic cap and the stem and insert a pin or some wire?
and how is the valve just supposed to drop in there unless you push it, there's always positive pressure in the cylinder while running

Not on the down stroke/exhaust stroke.

 

[Nathan_Anthony]

Not on the down stroke/exhaust stroke.

why not?
the fresh mix is being forced in and the exhaust gases are being forced out by that.
the only place with negative pressure is the crankcase