Stihl MS250 rebuild.

[ArlyA]

Here is a fuel story that might not apply to you Dodge. We take our machines into the field to be used by volunteers so with each operator, we've give them some extra fuel in a red 22oz camp fuel bottles. Since we had 2 and 4 stroke machines with us, I'd mark some with a large "MIXED FUEL" to help keep them straight. Helping us we had a super smart university employee who'd never make a mistake in his life, fill the bottles and as you might guess, put straight fuel in the ones marked MIXED FUEL. I happen to fill my machine 1st that day and noted the fuel was not the correct color and ran around finding the miss filled bottles.. It happens to even the really smart folks. :laughing:

 

[ericm979]

Other two stroke assembly tips: check the port edges and gently round the sharp ones. Very gently if it's nikasil instead of an iron liner. On Chinese saw cylinders I will clean up the ports to remove any dingleberries or other weird stuff and make them symmetrical. I use small stones to round the edges.

Piston skirt edged get slightly rounded (on like a 1mm radius) if they're sharp. I want oil to stay on the cylinder wall not get scraped off.

I measure the piston and cylinder to check for the correct clearance. I have had manufacturers ship mis-marked parts.
I already mentioned checking the ring gaps.

Assemble with generous squirts of two stroke oil on the bearings and bore.

I check the squish clearance after it's assembled. You can't do much to reduce it on a clamshell engine but it'd be good to know if it's too tight. You could file down the piston crown in that case.

 

[ruffdog]

Thanks for all the answers. How do you pressure test? Plates to cover up the ports?

Yes, plug intake, exhaust, and pulse line.

Also....3 bond 1184.

 

[dodge man]

I got the new parts in today, hopefully get it put together this weekend.

 

[dodge man]

All back together. I have not started it yet, I wanted to let the RTV dry overnight. Took me about 4 to 5 hours to get the piston installed on the old rod and crank and get it put back together. One problem, I got one leftover torx screw, there are several that hold parts together, such as the handle. I looked the saw over good and can not find where it goes. Ericm was nice enough to send me several manuals a few months back and I looked through them and could not find where it goes. The pic is the saw after it is back together and the leftover screw in the tray.

 

[ruffdog]

Silicone sealers are not fuel proof....keep the screw for next time....

 

[ericm979]

Yea, you need something like Yamabond #4 or Dirko to seal crankcases.

Did I send you the parts list? It shows the size and length of the screws.

 

[ArlyA]

According to a google search concerning rtv gasket chemical.

RTV works quite well for gaskets because it has very low shrinkage when cured, and the material remains compliant without being hard or brittle. It is also resistant to gas, diesel, oil, or other automotive fluids, so it won't disintegrate over time when exposed.

 

[dodge man]

Yea, you need something like Yamabond #4 or Dirko to seal crankcases.

Did I send you the parts list? It shows the size and length of the screws.

You sent a parts list but it didn’t list the size of the screws or the length. There was a lot of torx, maybe 12, and they were all the same except one. I do not think any are critical except the 4 that hold the crankcase on the cylinder and in the saw.

The Hiway kit had nicely rounded ports, I did not need to do any smoothing.

The kit also had a warning about fixing what went wrong the first time. One of things was using ethanol gas. At one time I ran ethanol at 10% but it was 91 octane. Then I switched to pure gas at 87 octane. A couple of years ago a station started carrying 91 octane pure gas.

 

[ruffdog]

According to a google search concerning rtv gasket chemical.

Oh....Okay........:laughing:

https://441py33rout1ptjxn2lupv31-wpengine.netdna-ssl.com/wp-content/uploads/tech_docs/tds/81160.pdf

Chemical / Solvent Resistance
The product retains effective properties in contact with automotive fluids, such as motor oil, transmission fluids, alcohol, and antifreeze solutions. Note: Not recommended for parts in contact with gasoline.

 

[dodge man]

Well the good news is it runs and I figured where the missing screw went. The bad news is it will not idle and the chain is over oiling badly.

For the idle it will idle nicely if I do it with the trigger a little bit and the chain is not turning. I assume I just need to adjust the screws some.

The chain over oiling I am looking for ideas. When I put it back together I was more worried it would not oil.

 

[ericm979]

Ethanol did not cause your piston seizure. It gets blamed for a lot of problems that it does not cause. Stihl calls for 89 octane. I'd run 91 octane E10 over 87 octane E0. Two strokes don't handle detonation well. Just a little can result in piston seizure. And unlike with four strokes, you can't hear it. Your piston did not look like it suffered from detonation but it's better to be safe. Your new piston and cylinder may have higher compression or not as good a squish area, either of which would make it more sensitive to detonation.

Gas with ethanol goes stale quicker than E0. To combat that, use a fuel stabilizer and empty the tank and run the carb dry if the saw's going to be sitting for months. E0 goes stale too, so it won't save you from needing to do that.

 

[dodge man]

Yeah, I did not really buy into the ethanol causing the problem. I suspect in spite of me not knowing how, the saw did not have enough or any oil in the fuel at some time.

I got it tuned up and idling good now. It accelerates really good to, but I have yet to cut with it.

How would I rate rebuilding a saw on a difficulty scale? Not hard but not beginner stuff either.

 

[yomax4]

Yeah, I did not really buy into the ethanol causing the problem. I suspect in spite of me not knowing how, the saw did not have enough or any oil in the fuel at some time.

I got it tuned up and idling good now. It accelerates really good to, but I have yet to cut with it.

How would I rate rebuilding a saw on a difficulty scale? Not hard but not beginner stuff either.

It's a fun project and you really get to know your saw inside and out. I did this with an 034 Super last year. The saws power degrades slowly over time and once rebuilt it's a pretty good wow factor.

 

[dodge man]

I sure did get to know it inside out. I am sure if I did it again it would take half the time.

I used it today, roughly a tank of fuel. It may break in a little better and make more power but I was a little disappointed in the power. Not the best chain so that did not help. The problem is I bought a MS261 a few months ago. It is rated at 4hp and the MS250 at 3hp. A very noticeable difference when cutting wood. I remember when I got the 261 I was not that impressed but now that I have the 250 running I realize how much better it is. The 261 weighs only about half a pound more than the 250.

I think it is all sorted out. I researched the over oiling problem and emptied the oil tank and blew air into it, seems ok now. There is no adjustment for the Oiler. Thanks for all the input from everybody.

 

[sd455dan]

I sure did get to know it inside out. I am sure if I did it again it would take half the time.

I used it today, roughly a tank of fuel. It may break in a little better and make more power but I was a little disappointed in the power. Not the best chain so that did not help. The problem is I bought a MS261 a few months ago. It is rated at 4hp and the MS250 at 3hp. A very noticeable difference when cutting wood. I remember when I got the 261 I was not that impressed but now that I have the 250 running I realize how much better it is. The 261 weighs only about half a pound more than the 250.

I think it is all sorted out. I researched the over oiling problem and emptied the oil tank and blew air into it, seems ok now. There is no adjustment for the Oiler. Thanks for all the input from everybody.

Don't give up on the Power level just yet...


When I went through and did some mods to my Husky 350's just running a few tanks of fuel made a noticeable difference in power and compression. ,
I measured ~ 20 PSI increase in cold crank compression between reassembly and after breaking the saws back in.

It was even obvious just pulling the pull start cord after a bit of running time.

Modifying the squish and putting a flat top piston and new ring + breaking in one of the Husky's took it from 105 psi to 167 psi

Muffler modding it further improved the power. I may even get into doing some porting eventually, but for now pretty satisfied with the power.

If you get bored with the saw maybe the Stihl 250 can be hopped up a bit to. Muffler mods seem to wake up many saws
just something to consider..

ps. Congrats, on successfully rebuilding your saw- 2 running saws beats having only 1 runner any day:thumbsup:

 

[ericm979]

A lot of Chinese cylinders have smaller ports and/or shorter port timing than Stihl. That can reduce performance. They can also have more squish clearance. If squish is too great that will reduce performance and make them more succeptible to detonation.

With a clamshell engine like this it's hard to change squish clearance. You can't just mill the cylinder base because the cylinder is also the top half of the crankcase. People have done it but it takes some serious machinist skills.

Also it will take a few tanks of gas to break in and get the best seal from the rings.

 

[dodge man]

I actually have a third saw, I can not even remember what kind, Homelite maybe. I have used it maybe 3 time in the last 15 years to get a pinched saw loose. It is a piece of junk, no power, tiny chain, I do not take care of it, but it always seems to start.

Just looking at the Hiway replacement engine I got, it seems like a quality piece, when comparing to the old one it seemed the same, but without taking measurements hard to say.

 

[dodge man]

So I have two chains for my 250 and put a different one on and sharpened it. It cut much better, it actually did better than I expected. In the pic the saw is sitting on the piece I cut in half yesterday and the other one I cut in half today. I think yesterday I might have hit a rock first thing and likely dulled the chain. I cut these in half so I can get them up and on my splitter and handle then easier.

 

[ArlyA]

Geee Dave, We have more chains than you.