STIHL ms 391 decompresser problem?

[Bushman-of-OZ]

Not sure if this is the correct forum but if not - administrator please move topic. My STIHL MS 391 is a fabulous saw - once it is going but starting is now near impossible. I suspect it is the decompression valve sticking that is making the recoil starter too hard to pull.

I found a replacement valve on a UK website but can't find anything in any of the on-line manuals about how to service or replace the valve unit.

Can anyone help with advice please?

 

[CR888]

A correct size socket over the decompression valve will allow you to remove it and check it. What happens when spark plug is removed and you pull the starter.....it should pull over easy/smooth as compression is not allowed. Have you removed muffler and look at exhaust port/piston crown for carbon build up? ls the choke opening/closing when you switch it on/off? A bit more info would help diagnose the issue. Our friends over the pond are sleeping now but l am sure will help more than l can when they arise. Could be one of 4 or 5 possible problems. Sit tight and we'll find out what going on. Check spark arrestor screen in muffler....it could be blocked with carbon and need cleaning with propane torch. Don't order any parts till others have chimmed in and we know EXACTLY what's going on. ln the meantime unscrew muffler bolts (4) with T-27 torx/scrench and have a peek inside, if you have another saw with same decompression valve you could swap them to rule that out. Ultimately YOU will solve this with help from others, a bit more info will help us get there.

 

[Bushman-of-OZ]

Thanks- did not want to experiment without some input from someone with experience of this brand. I have owned many chainsaws but this is my first Stihl.

The decompression valve has been removed, soaked in petrol, blown out with compressed air. Then I removed the spark plug - no carbon buildup and the gap was OK. The engine turned over easily with the plug out. Despite all that, the problem remains - regardless of the position of the decomp valve, the recoil starter is very hard to pull, so I really suspect a faulty valve.

As you suggest, I'll go the the dealer and borrow a spare valve to test if that fixes the problem. This robber wants over $80 for a new one, whereas one is available on line for $25 ex UK even after the exchange rate but I guess that is the result of being a captive market here

I'll let you know how I go

 

[CR888]

Check ignition key has not moved or been damaged on flywheel. This model is known for hard starting if ignition timing gets out of whack. Ebay will deliver a decomp valve for a LOT less than those prices. l might message a member who repairs these for a living to look at this thread. His guesses are usually better than mine.lol

 

[Terry Syd]

I found out that compression release valves are more complex than I initially thought. There seems to be an element of 'tuning' to them to a specific engine - spring tension, the size of the vent holes, engine displacement, compression.

The original factory compression valve is very likely 'tuned' to your engine, it probably just needs a good cleaning out with a solvent to dissolve the carbon. There's not much in them that can wear out or break. Let us know how the spare valve goes.

 

[Rockjock]

Check the part number and you will find it is the same valve for many saws. My 026 takes the same decomp as my 440 magnum I suspect even more share the same part number. a easy test to to place the valve in the closed position and try and blow air through it.. then the open position. It is like a light bulb it either works or does not, be it with a little leakage .

 

[Bushman-of-OZ]

Thanks Guys

The dealer/repair guy started the saw after about 6 or 7 pulls and claims there is nothing wrong with it. He is built like a gorilla and says he never used the decompression unit, but in my case, I'm 73 and have a crook back so I cannot apply as much pull on the starter - but then I surely should not have to.

Mind you, this guy has become a problem because he has more work than he can handle and really does not want to get involved. He has taken 12 months to try to fix my Sachs Dolmar that is also hard to start

I'm sure the advice from CR888 about the ignition timing might be right but finding anyone in this area to fix it ain't easy. They only want to sell new saws. Meanwhile, I'll remove the valve and soak it/blow it out again - that might help.

 

[CR888]

Aftermarket decomp valves on Ebay are $17.50 johno&jonno delivered and many many brands, models use same size. lf functioning correctly when pressed should make tuning over with pull start much easier. l don't have a 390/391 but hear they have 170-190psi compression off the shelf....that's a fair amount of compression for a 65cc farm saw! Maybe do a search on flywheel keys or ignition for ms 391 and look a a picture of a correct setup and compare that to yours to see if anything has moved.

 

[Bushman-of-OZ]

I think I have fixed the problem, after removing the valve again and this time blasting it with carburettor cleaner under pressure from an aerosol can, working the slide while this was being done. It was then relatively effortless to crank and started after 3 or 4 pulls. Hopefully, gunk inside the valve was the problem all along - we shall see

Thanks again for your advice

 

[7sleeper]

Great that it worked! My next assumption would have been a problem with the starter mechanism.

7

 

[CR888]

 

[a. palmer jr.]

Sometimes on the well used de-comp valves they get loose and go closed after one pull, especially on a big saw, I usually just replace them when this happens.

 

[Bushman-of-OZ]

Yes

It will be worthwhile getting a spare. I'll check Ebay as suggested

Alan

 

[a. palmer jr.]

Yes

It will be worthwhile getting a spare. I'll check Ebay as suggested

Alan

I imagine you can buy a bunch of them on ebay for what one costs at the Stihl dealer..

 

[Bushman-of-OZ]

Yes- $80 here is a ripoff but that's Australia for you - too small a population and dealers have a captive market. To be fair, this guy is not as bad as most.

 

[a. palmer jr.]

I think I paid under $15 for one here and I thought I paid too much.

 

[Anthony Campbell]

Not sure if this is the correct forum but if not - administrator please move topic. My STIHL MS 391 is a fabulous saw - once it is going but starting is now near impossible. I suspect it is the decompression valve sticking that is making the recoil starter too hard to pull.

I found a replacement valve on a UK website but can't find anything in any of the on-line manuals about how to service or replace the valve unit.

Can anyone help with advice please?

Mine gets gummy feeling and i beleive a shot of lube in the cylinder would help starting.The decompression valve will get plugged up on side vent hole and can be cleaned with brake cleaner.

 

[a. palmer jr.]

My 039 doesn't have a de-comp and is very hard to pull over. When I changed mine over from an 029 to an 039 I should have bought a cylinder that is equipped with a de-comp but never thought of it at the time.