Stihl MS250 mixture screws?

[dodge man]

Is it easy to rebuild? I have rebuilt automatic transmissions and have the tools but are any speciality tool needed? I have thought about getting a MS251. I am not sure how old this saw is, maybe as old as 15 to 20 years old.

It has been several years ago but the spark plug was shot. You could actually pull the electrode all the way out of the plug. It still even ran but badly. Is detonation a problem on these? I also probably got the thing too hot a couple of times, cutting with a dull chain and wanting to get one more cut done. I am almost positive I did not run gas with no oil but it is always possible. One problem is for years I was running 87 octane but pure gas. The other choice was 93 octane with ethanol.

Thanks for all the help!

 

[mike48130]

It's toast. Keep running it till it dies then buy a cylinder and piston kit.

 

[ericm979]

Detonation is for sure a problem with two strokes. It can seize the piston really quick. You won't even hear it like you can on a four stroke. The piston top often shows pock marks when there was detonation. Usually the exhaust side of the piston seizes (it's the hot side). And the exhaust side skirt will look a lot like yours. But there's other things that can cause that besides detonation. Like running a lean fuel/air ratio or getting it hot from cutting with a dull chain and slipping the clutch a lot. Some people have melted the clutch side of the cases doing that.

I ran 87 octane for years then I finally read the manual and saw that Stihl says 89 octane. Oops. I didn't have any problems but maybe I was lucky. Now I fill up a 5 gallon can with 91 for the saws, because I don't know what the previous customer at the pump was pumping and the first gallon or two I get could be 87. All the gas I use is E10. E0 is hard to find and expensive here. If you rotate E10 gas every month or 6 weeks and don't let the saw sit for a long time with gas in it, it'll be fine.

I haven't rebuilt an auto trans but from reading I know they're a lot more complex than a two stroke engine. If you get a replacement Chinese motor it'll bolt right into the clamshells. Like this one: https://www.huztl.net/Engine-Motor-...iston-42-5MM-Crankshaft-Chainsaw-p228811.html

 

[dodge man]

I was also thinking my son when he was about 22 years old ran it quite a bit, cut maybe a cord of wood with it or a little more. If I ran it a little hard some, I am sure he probably worked it harder.

 

[ericm979]

Here's a thread from here about rebuilding an 025, which is the same as an MS250 except for the flippy caps: https://www.tractorbynet.com/forums/chainsaws/420802-rebuilding-stihl-025-advice-welcome.html

If you PM me an email address I can send the 025 shop and owners manuals.

 

[dfkrug]

Is it easy to rebuild? I have rebuilt automatic transmissions and have the tools but are any speciality tool needed?

I have had 5 of these MS250s, which I usually get in trade (all DOA).

I find them easy to work on, far easier than any auto trans. But I am an engine enthusiast. You do need decent Torx T27 wrenches, but that's all the special tools.

This photo is from one of the MS250s, which was just a year or so old. It had a broken flywheel fin, probably due to factory defect. Vibration eventually broke one of the crank bearings. You can remove your flywheel using a rope for a piston stopper. You do not need the Special Tool for that, unless you do a lot of them.

 

[dodge man]

What about the tools to pull a vacuum and pressurize the engine to look for leaks. It still runs pretty good but still won't quite idle correctly. It is probably on the way out though. I am probably gonna just get a new one, maybe a MS261. The last transmission I did was a 727 torque flite out of a 67 Dodge Cornet probably 20 years ago.

 

[ericm979]

The poor idle is probably due to the rings not sealing well due to piston damage. The ring lands get deformed from the damage, which makes the rings leak there, which means more heat for the piston, which causes more damage. Eventually it'll seize hard.

I built a lot of motorcycle engines without vacuum testing them. Just replace the seals when you replace the cylinder and piston, it's something that should be done anyhow. And use good quality sealant on the lower crankcase half.

I'm used to flywheels that are a taper fit on the crank and need a puller. It looks like the 250 flywheel does not have the threads for a puller. The rope for a piston stop works well. I use that when removing saw clutches. It's actually better than the official piston stops and it doesn't break and require you to disassemble the engine to get out.

 

[yomax4]

Great pics. Funny it even runs. Wow..

 

[dfkrug]

Great pics. Funny it even runs. Wow..

I have had a few that bad that would run, but poorly. Always 2-ring pistons. You can feel the compression resistance when pull-starting, so don't depend on that test to fully evaluate a saw.

As for vacuum testing the carbs and leak testing the crankcase, I do not do that test on every 2-stroke, but when I do, I use a general-purpose Mit-T-Vac tester, and sometimes I put a plate over intake or exhaust ports. You can also use rubber to seal them off underneath the muffler or carb. You can also look for a vacuum leak by spraying with carb cleaner while running, if you have access.

Some of the small saws have no place to attach a flywheel puller, so you protect the threads and whack the crank. They DO make a special tool for that, kinda like a really tall nut.