extremely high compression problem, MS461

[crotchclimber]

One of our 461s is very difficult to start due to extremely high compression. Decomp valve pops right back out even though it hasn't fired. I took the sparkplug out and it turns over easily. My coworker thinks soaking some decarbonizer in the cylinder is the first thing we should try. We run Stihl Ultra synthetic oil at 50:1 . Any suggestions what is going on?

 

[alexcagle]

Might just be that your decompression valve is bad. It shouldn't shut until the force of combustion forces it closed. They can wear, and get weak.
Aftermarket decompression valves are notorious for failure.

 

[crotchclimber]

Might just be that your decompression valve is bad. It shouldn't shut until the force of combustion forces it closed.

I think you misunderstood. The compression is so high that we can barely pull the rope without using the decompression valve and when we do it usually pops the valve because the compression is so high.

 

[fwgsaw]

I think you misunderstood. The compression is so high that we can barely pull the rope without using the decompression valve and when we do it usually pops the valve because the compression is so high.

A compression test would prove your theory otherwise your just guessing it's the compression that's causing you to barely pull it over.

 

[crotchclimber]

A compression test would prove your theory otherwise your just guessing it's the compression that's causing you to barely pull it over.

We don't own a compression gauge. It's just very different from our three other 461s and nearly impossible to start. Cranks over easily without the sparkplug in.

 

[rupedoggy]

Hummmm. Try leaving the switch off and see if it still does the same thing. If not it is a timing problem.
If it does have that much compression, I bet it runs fantastic, if you once get it started. Mike

 

[Artemis]

What mods, if any, have you done to the saw? I've seen another thread where someone removed the base gasket, decked the cylinder, and had a custom pop-up piston which resulted in 240psi approx. If the saw is stock then it's hard to say.

 

[Conquistador3]

If the saw runs more or less well once started, it's most likely carbon buildup. Easy to check: remove the muffler and check the the exhaust port and the piston top for carbon buildup.

Forget using those "decarbonizer" things, carbon buildup needs to be manually removed. Just remember to use something less hard than your piston and cylinder.

 

[alexcagle]

Once the deco is depressed, how hard is it to pull it back out using two fingers?

- and yes, I do understand what you're talking about. I had a 3120K Husqvarna that would pop the deco out about halfway through the pull, which would violently yank the rope out of your hands.
- The kind of pain which felt like it took your soul, skin, and tendons with it......

A new deco solved the mystery.....

 

[CR888]

I don't use Ultra but those who have over time have reported built up problems. There may be carbon in the decomp, piston crown, exhaust port or top of chamber. Pull the cylinder and take a look. Even remove muffler and shine some light in there. I would not try to run the saw until further inspection. Forget de-carbonizer.... scotch brite pad on die grinder for piston crown and whatever to clean ports/cylinder.

 

[Big_Wood]

sounds the same as my old 461. my ignition timing was just in the right place to make it hard to start. a way to test is turn the kill switch on and then pull. if you can pull easier then before then it's the timing of your spark. you can advance and retard it anyway you like to make it easier or just live with it. i lived with it but man is it ever hard on the recoil. i had a decomp plug and squish set to .018 on mine too and i could start it but i had to pull it over top dead center before i pulled on it otherwise it was rip the cord right out of my hand and hurt like a mo'fo! here i was in the woods with no decomp to add to it and work needing done so i had to get it running each and every tank for 2 weeks straight! lol wrist and hand was hurt'n after! i actually found losing the elastostart handle helped significantly. then i realized that running a husky helped even more significantly so that's what i do. i think i remember member old-cat retarding the timing on one of his customers 461's for this same reason. just odd how some have it and some don't as i've ran some that were as easy to start as a stock husky 372. guess it doesn't mean it is your issue though.

 

[alexcagle]

sounds the same as my old 461. my ignition timing was just in the right place to make it hard to start. a way to test is turn the kill switch on and then pull. if you can pull easier then before then it's the timing of your spark. you can advance and retard it anyway you like to make it easier or just live with it. i lived with it but man is it ever hard on the recoil. i had a decomp plug and squish set to .018 on mine too and i could start it but i had to pull it over top dead center before i pulled on it otherwise it was rip the cord right out of my hand and hurt like a mo'fo! here i was in the woods with no decomp to add to it and work needing done so i had to get it running each and every tank for 2 weeks straight! lol wrist and hand was hurt'n after! i actually found losing the elastostart handle helped significantly. then i realized that running a husky helped even more significantly so that's what i do. i think i remember member old-cat retarding the timing on one of his customers 461's for this same reason. just odd how some have it and some don't as i've ran some that were as easy to start as a stock husky 372. guess it doesn't mean it is your issue though.

you meant turn it off right?

 

[Big_Wood]

you meant turn it off right?

when a kill switch is turned on it kills the spark. if the spark is killed it's not gonna rip the cord out of your hand like it would if the kill switch was turned off allowing the ignition to spark. make sense? lol

 

[alexcagle]

Ok.
As long as "on" means the saw is "off".
or "auf" means the saw is "aus".

 

[mohick]

Drill a couple holes in top of the piston< that should take of your rubber arm friends

 

[Full Chisel]

Hard startin' Stihl...dime a dozen!

The 461 I used when I was working for a state park had stupid high compression. I don't usually bother with decomps on saws but I had to start using it on that saw by the end of the day. And I hate the elastostart handles...they give you a sloppy pull. Another answer to a problem that didn't exist.

 

[Big_Wood]

Hard startin' Stihl...dime a dozen!

The 461 I used when I was working for a state park had stupid high compression. I don't usually bother with decomps on saws but I had to start using it on that saw by the end of the day. And I hate the elastostart handles...they give you a sloppy pull. Another answer to a problem that didn't exist.

elastostart on saws like that can ruin a guys day right quick! imagine burning 7-8 tanks a day on a ported one with no decomp. man was my hands hurt'n and it's been the only time i've been tempted to refuel running hahahahaha decided that wasn't a smart idea though. the elastostart just absorbs your power and the saw snaps back at you ripping it from you hands. sold that saw after one shift just like my 661 lol

 

[fearofpavement]

First thing I would do is trade starter recoils with another saw and see if the problem moves.

 

[67L36Driver]

Just drill the decomp's bleed port larger. Go up in size until it starts comfortable.
I had to do that with my MS880.

 

[crotchclimber]

I looked at the saw again today. I narrowed it down to the timing problem. The decomp was weak, so I swapped it out with a better one from a parts saw. It still popped right away though when in run. When the switch was in off or when the spark plug wire was disconnected the saw cranked normally. This saw is stock. So how did the timing get off and how do I retard it?