Stihl MS 250

woodbutcher

Well-known Member
I read a post on a Stihl forum about using 87 octane gas for the fuel mix in this saw. The manual states that using less than 89 can cause the engine to seize. I've used the cheapest gas in mine since I bought it new 15 years ago. It hasn't seized, but it has gotten really hard to pull the rope, so I'm wondering if that might be the cause. I can't find any other problem. If I use some 93 in it, will it heal itself?
Butch
 
is the rope pulley plastic? Did you check how freely the pulley spins? If it still runs I doubt the piston & cylinder have been scored seriously. They don't run at all then. You can take the muffler off & look for piston/ring damage.
 
Symptoms of seized cylinder are, as you say hard to turn through and also poor/no idle, loss of power.

If you haven't already, strip down the saw to the point you can eliminate any other problems, like starter, coil dragging the flywheel, clutch problems, etc.

Turn the engine through by hand, plug out, and feel for tight spots.

You can also peek into the spark plug hole with a flashlight, preferably the "bore scope", light on a thin stem you can actually put inside the cylinder. Look on the exhaust side of the cylinder wall for black and blue vertical scrapes.

Also check for up and down play on the crank shaft. This can also cause binding due to crank bearing failure.

Obviously, none of this will go away by simply changing the fuel. These engines are rebuildable though.

Did 87 octane fuel cause this? I personally doubt it. Other factors, like running too lean, too much oil, not enough oil, packed up cooling fins, and just plain ol' worn out will do the same.
 
It's such a hard pull that the rope broke, and I had to replace it. Yes, I believe the pulley is plastic, but it spins freely. It ran fine for one day of cutting and trimming a week ago. This problem came up before that. But since that day, I haven't been able to get it to run.
Butch
 
Pull the muffler - that's the easiest way to see the piston and cylinder wall. I have always used 87 with no problems. One of the reasons Stihl recommends the higher octane is that the quality is generally more consistent. I only mix 1 gal at a time, and use an old Coleman fuel can to store.
 
Hello woodbutcher,

When the pull rope gets old it will wrap around itself and go between two loops that will make the pulling effort pretty hard. Make sure the rope is the right size and rewinds on top of the rope not in between. Most Stihl manuals specify 89 octane, and/or super if that is not available. So 91 or 93 octane should not hurt the saw

Guido.
 
i think it was a ms 250 that the owner complained the same symptoms as you.
when I took it apart the plastic pulley was split and dragging because it was wider . still worked but made it real hard to pull over. something to check.
 
I have a MS250 and have always used 87 oct I was also given a small Joensoreds that says to use 90 + oct. so I thought I might as well run 93 in all the 445 Husky the stilhl and the joens. all use 50 to 1 but my old echo uses 20 to 1 so I will still need 2 seperate cans. My daughter has the MS250 with (what I call girly start) LO.
 
Have you ever thought about useing OPTI-2? One mix for all your saws and weed eaters. My Stihl dealer go me started on it over ten years ago when I bought my MS170. At the time I had 4 differant fuel cans. Since than one can takes care of all of them and Stihl will honor warranty using OPTI-2.I use it in Stihl, Echo and a 26 year old Husky.
 
I bought my MS250 eight years ago and it pulled through hard from day one. I even took it back several times and they said there was nothing wrong with it but said it had more compression than what they normally come from factory with and told me to use it. It runs great no lack of power but have replaced several starter ropes.
 
Seems counter-intuitive that a fuel issue would cause the saw to increase compression and then pull harder. If anything the compression should reduce over time and it should roll over easier.

Only exception I could think of would be a clogged muffler that would reduce the ability to breathe properly. That would build over time but I can't believe if it was clogged to that point it would even run.

My guess would be that you have binding in the starter mechanisim or possibly a bad bearing in the engine. You did however say the engine rolls over freely with the plug removed and that would rule out the bearing.

I have a 2-year old MS250 and that has serious compression. I have to step on the handle when I pull it but it starts easily. I am using the Stihl synthetic oil that should be giving me an edge.

Octane rating only applies to the fuel's resistance to pre-ignition (knocking). It has nothing to do with the energy content of the fuel and/or any cleaning ability of it.
 
I have a 250, the first couple cold pulls on the rope tend to do funny things, but then it rolls over easier. Good thing too, it usually takes 6-8 pulls to start.
 

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