I have a 6 year old Stihl MS-250 that idles much too fast, so much so that the chain won't stop moving. So far I have rebuild the Zama RB 105 carburetor with a new kit, replaced all of the rubber parts(fuel line and filter and impulse line) and even tried one of those Chinese Zama knock-off replacement carbs that I bought on eBay, all to no avail. If I tighten the L(low speed) adjustment to less than a full turn to slow it down, the engine dies. Hi speed performance is fine. The LA adjustment is backed off all the way, but no matter where I have the L and the LA adjustment, I cannot get the idle speed down to where the chain stops moving and the engine keeps running and idles properly. This MS-250 has a lot of mileage on it - it's a great saw for limbing take-downs because of its light weight. I have not replaced the boot that connects the engine manifold to the carburetor or the crankshaft seals. The boot appears to be in good shape - no cracks or stiffness. The engine is in good shape - no bearing problems or piston wear that I can detect. I have not done a compression and vacuum test because I lack the tools to do so, but the engine power and torque has not diminished or changed over the years. I bought another 250 to keep going, but I would like to fix this one properly. Any suggestions?
Idles too fast - Stihl MS-250
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LonestarStihl
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Try letting it run and turn it on its sides. Sounds like an air leak. That'll make it pertnear impossible to get a good tune on. See if it races or stalls with clutch or starter side up. Let it sit a moment on each side
LonestarStihl
Addicted to ArboristSite
I just dealt with same thing 2 days ago on 041 super.
It sounds likely that a crank seal is worn out, or an air leak of some sort (loose lower cap, torn intake boot) has developed on it.
I would take it to a shop that has the pressure/vacuum testing equipment & have it tested, and don't run it anymore.
Make sure you find out exactly where the leak(s) are that they found & then you can fix it yourself to save $$.
I would take it to a shop that has the pressure/vacuum testing equipment & have it tested, and don't run it anymore.
Make sure you find out exactly where the leak(s) are that they found & then you can fix it yourself to save $$.
LonestarStihl
Addicted to ArboristSite
Or my dealer taught me you can fill the cylinder with fuel mix after stripping it down to where you can see the seals. You'll see the fuel leak out if you have one.
Good home remedy.
At first I thought you were going to say strip it down to save the good parts, fill the crankcase up with mix, light a match to it, then throw it in a ditch!
At first I thought you were going to say strip it down to save the good parts, fill the crankcase up with mix, light a match to it, then throw it in a ditch!
LonestarStihl
Addicted to ArboristSite
Haha I thought about it. But it's a Stihl not a husky
lol. I'm sure if you did hold a lighter around the seals as soon as the fuel came through it would be a good indicator 
Running it on its side, up or down, doesn't seem to make any difference. I figured that it was probably an air leak, but I was trying to avoid pulling the flywheel and clutch and separating the clamshell. But if I have to, I might as well replace the intake boot and both crankshaft seals. To reseal the engine, what do you recommend? Is Permatex black RTV OK or something else? Apart from the two crankshaft seals and the intake boot to the carburetor and the seal around the clamshell, are there any other potential air leak sources? Thanks for responding so promptly.
Is it one of the easy start ones with the decomp? If so, give her a good what fer.
No, the MS-250 has no easy start or decompression, which is something Stihl changed on the MS-250C and some other newer models, new features which make the newer models inferior, IMHO, to the original MS-250, which they recently discontinued.
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Have you looked at the clutch drum bearing, and/or the clutch?
LoveStihlQuality
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+2
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That'll teach me to read the entire post... 3rd vote on clutch springs, especially on a high mileage saw.
LoveStihlQuality
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Whatever you find, be sure to post up the cure
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You'd all be lucky if that was it, as him being a saw guy I'm sure he's already checked the drive out.
My vote is for pirate air matey!
Joel D'Angelo
ArboristSite Operative
Check metering valve
check the "cover washer" ( the big washer behind the clutch)is on correctly. if its on backwards it will cause the symptoms you describe.
pressure and vac test to confirm no air leaks
pressure and vac test to confirm no air leaks
Joel D'Angelo
ArboristSite Operative
When I said metering valve I meant the fulcrum that controls fuel flow. 2X now I have had an otherwise good saw that wouldn't take a tune and it turned out the thing was bent wrong allowing to much fuel.
