how to tell if MS250 is modded, broken flywheel key high compression

friend gave me his MS250 to fix, turned out the major problem was clutch drum bearing disintegrated.

However i remember using this same saw once 2,4, or 5 years ago, and remember wow this thing has a ton of compression trying to start, really have to commit pulling the starter cord but it would start fine and run great.

It has a sticker on it from the local shop he had either brought it to for service or bought it from- new, used, i don't know. In disassembling/cleaning I found the flywheel key was sheared - figuring likely due to the monster compression when starting. I now have a new flywheel on order no big deal. Other big thing I notice is rotating crank by hand with spark plug out there feels like a lot of piston {ring} stick where it is not easy and smooth to rotate the crank by hand... but i have no intention of pulling the cylinder apart at this point there is no obvious signs of overheat or scoring looking in from exhaust and motor does run very well.

questions are:

a) is it common / not uncommon for a shop to mod a saw and cause increased impression? Don't know if he bought this new, guessing might have been a used/recondition saw he got a price break on.
b) looking at the cylinder and crank pan, there is a thin obviously red (red/orange) line sticking out of gasket maker and it is hard like what RTV would be. I am not saying it is RTV, I don't know what it is. What does OEM Stihl use to seal the cylinder to crank pan, on an MS250?
c) is there supposed to be a base gasket between cylinder & crank pan on the MS250? Are they any saws that use a gasket there, or do all saws (stihl, husq, etc) use some curing gasket maker?

Increasing compression on a clamshell is “possible” but not something that people do. Is it hard to turn over all the way around? Or are you just feeling where the magnets pass the coil? These saws just use sealer, no gasket, as said its a clamshell, so the bottom of the cylinder covers top half of crank bearings, engine pan covers bottom half. Could be that seals/bearings were replaced or it developed a leak and needed resealed. I THINK normal oem stihl sealer is black, but i dont mess w creamsicles much.

Grey or red sealer is OEM. Never saw black.

cus_deluxe said:

Is it hard to turn over all the way around? Or are you just feeling where the magnets pass the coil?

Click to expand...

flywheel is removed, clutch disc is removed, spark plug is removed. Yes way harder to rotate crank no matter where piston is in it's stroke, way harder no where as smooth as a friend's used 029 i recently resealed. This is also after spraying fogging oil and pouring some 2-stroke oil in so it i know it's not because it is currently dry or unlubricated. With spark plug out & muffler removed &piston on down stroke it feels like 1/2 effort of compression stroke, stop and go stop and go stick not smooth.
I read online some other people complaining of MS250 or 025 saws having a lot of compression and being problematic to start, and there's either a 42mm or 42.5 mm piston in that model, wondering if this could be a manufacture problem?

It sounds like crank bearings are in need of replacing. If clutch and flywheel and plug are removed, it should turn over smoothly with very little effort. Youre sure piston is not scored?

only what i can see through exhaust port looks fine, spark plug fine

Looking at piston through exhaust port would be easy to tell if its scored. If its rough/catchy with all those things removed, then only thing i can think of is crank bearings. I assume oiler worm gear is off as well?

Could be the big end bearing, too. Easy saws to tear all the way down and rebuild if necessary.

Typical sealant is DirkoHT which is red.

Any luck?

It’s very difficult to increase comp on an MS 250. Possible maybe, but extremely uncommon for a typical saw Shop to do anything of that sort.

Just buy a compression tester and see what ya got.

Hi R7000,

I can't say that every one of the MS250 do this, but I have a complete MS250 engine that I swapped into an 021. I can tell you that I have to agree with you and this sucker is not easy to pull over. You have to be very careful and not just yank away willy nilly. Like you mentioned, you have to really commit to yakin it, no limp wrist or it will bite back. (okay.. sounds bad)

One thought that crossed my mind is I wonder if the starter pulley diameter on this saw is peculiarly small. I can tell you that that starter cord is not all that long and it is the original from when I bought the saw. Compression testing didn't yield anything abnormal.. somewhere in the neighborhood of 150 lbs.. if I recall.

If it’s a problem timing can always be retarded a bit. Performance will suffer a bit, but most wouldn’t notice

All the way from Australia here. I too own a Stihl MS250 saw. The saw was given to me by my dad who is in his 80's. He gave it to me because from the day he bought it new he had trouble pulling it over and he just detested starting it. I find, when starting from cold, that unless I am "committed" as mentioned above, the starter handle can wrench out of my hands too. Not a problem I have with either of my Husqvarna 350's nor my Tanaka. I thought the Stihl's issue may have been caused by a sheared key stuffing up the timing so I took it apart and it was as pristine in there as the day it was new. Maybe it's just a natural thing for MS250's.

i don't remember doing a compression test on this ms250 before removing the flywheel and then noticing the sheared flywheel key. But more than once the ignition was left in the off position trying to start, didn't start oh yeah flip on then started, and all those times cranking with ignition off it was hard. I don't know for how long the saw ran with a semi-sheared flywheel key and if and how much the timing might have been advanced causing increased starting problems.
But my solution for now since an aftermarket flywheel is around $20 is to just re-ring the piston and new crank seals, everything else internally looked fine and the cylinder looked excellent.

My MS250 is as hard to pull as those noted above. I'm not sure why it's such a beast. I've begun the cold start on the ground with my foot in the handle. Makes it a lot easier. Once its warm it's easy.