Please Diagnose Hard-Starting Stihl MS250

[WarmGuy]

I've had my MS250 for 6.5 years. I've used it for a total of 74 hours within that time.

I last used it one year and three months ago. At that time I ran it dry.

Here's how it was today (fresh gas):

I put it in the full choke, pulled the cord a few times until it "burped." It then took many pulls before I got it started, and it died almost immediately. For a long time, it would start, and then immediately die. It would die if I let it idle, and it would die if I goosed the throttle.

After a long time of this, I could finally get it to run okay, with careful blips of the throttle.

During the first 20 minutes of use, I needed to get it up to full speed before starting a cut, or it would die.

After that, it ran perfectly.

Note that it has always been a bit like this, even when new. It's just worse when I haven't run it for a while.

1. Is there something wrong that can be fixed?

2. Is there some starting procedure that will get it warmed up sooner?

Thanks,

Al

 

[dougand3]

Could be a fuel delivery problem. Carb diaphragms stiff, Carb dirty, Fuel line cracked, Fuel filter partially clogged. Also, make sure air filter is clean. After checking fuel lines and filters, I'd try the easy carb route....An aftermarket MS250 carb is ~$15 on ebay. You can rebuild your carb with an $8-12 kit...but when a new factory new one is so cheap, it's not worth the trouble.

 

[CrufflerJJ]

My MS250 has been fussy about starting ever since I've owned it. It floods VERY easily. If you pull with the saw on full choke until it tries to fire, you may or may not ever hear it fire. When I ran my saw a week or two ago, I pulled on full choke about 4 times until it started to fire, then switched to half choke & pulled until it started. As in your case, the saw died almost immediately. I think it did this twice. I put it back on full choke, pulled once, then back to half choke, pulled and it started & ran fine.

Finicky, thy name is MS250.

 

[DougA]

Pull the carb and the air filter out and check them both. I get sawdust inside the air filter and it works it's way into the carb. Usually there's a few small pieces in the carb gaskets and that will cause the same problem you are experiencing.

I've got two 026's but the carb is similar if not the same. The carb is cheap to replace with a grey market one but easy to pull apart and clean.

 

[Jon1270]

The nice thing about chainsaws is that they're mechanically very simple machines, and it's easy to narrow down where the problem is because there aren't all that many components to begin with.

First, it has to have compression. Assuming you don't have a compression gauge, what happens if you hold the saw up by only the starter handle? Does it hang there for a few seconds, then drop a few inches, hang for a few more seconds, drop a few more inches... or does it drop quickly? If you remove the muffler. what does the surface of the piston visible through the exhaust port look like? Can you see any vertical scratches on it?

While you've got the muffler off, check the spark arrestor screen at the outlet. If it's clogged with carbon, remove the screen and burn it clean with a propane torch, put it all back together and see whether it runs better.

If cleaning the spark arrestor doesn't help and if compression is good, i.e. no damage visible on the piston and the saw drops only slowly and in increments when lifted by the starter handle, then the next thing to check is whether the spark plug is firing. Your saw runs, albeit badly, so chances are your ignition system is fine.

That leaves fuel and air delivery. The air filter might be clogged with fine dust after lots of very dusty cutting and a lack of routine filter cleaning. The engine might be getting too much air due to an air leak, but this is unlikely in a six year old saw unless it's been tampered with. The carburetor might not be able to get enough fuel because of a clogged fuel filter, leaking or collapsed fuel line, or damaged impulse hose. If there are no air leaks, the air and fuel filters are clean, and the fuel and impulse lines are in good condition and there isn't mechanical damage to the choke or throttle linkage (again, unlikely in a saw that hasn't been tampered with) then that leaves the carburetor itself. It might just need adjustment, or it might need cleaning and replacement of wear parts (or the complete replacement suggested above).

One thing that bears emphasizing here is that your longstanding habit of laboriously coaxing it to run, blipping the throttle, revving to full speed before starting a cut just to prevent stalling etc., is a very bad idea with any 2-stroke motor. When a saw (or weedwacker, blower, hedge trimmer, etc.) is not running correctly and you don't know why, assume there's a significant danger that the reason it's not running right is that the engine is getting too much air relative to the amount of fuel it's getting, i.e. it's running lean. Running a 2-cycle engine lean is the surest, quickest way to cause serious engine damage. When your saw isn't running right, STOP RUNNING IT unless you are cutting your way out of a burning building, and get it fixed one way or another. The fact that you've continued to run it this way for years means you've been risking trashing the motor, so check that compression.

 

[AK13]

Sorry, nothing helpful to add, but how on earth do you know that you've used your saw for 74 hours of time? Do you log your time every time you pull the string on it?

I'm just curious how you could know since there is no hour meter on it.

 

[CrufflerJJ]

I wondered the same thing about the OP.

As an aside, my MS250 worked nicely (no hassles at all) for me in cutting up some Ash today. Still need to split/stack it for proper drying.

 

[WarmGuy]

Sorry, nothing helpful to add, but how on earth do you know that you've used your saw for 74 hours of time? Do you log your time every time you pull the string on it?

I'm just curious how you could know since there is no hour meter on it.

Yes, that's exactly what I do. I have a notebook with a one page checklist and activity log. I find it pretty useful. I attached it.

 

[WarmGuy]

Thanks for the help. The hang-from-the-starter test showed that the compression is fine. The spark arrestor screen was pristine.

Well, today I removed the air intake and looked in the carb. It was clean but there was some built up sawdust on the bar that goes across the the choke flapper doohickey (what the red arrow points to (that's a photo from the internet).

I can't imagine that made a difference, but I put things together, and it started up just as it should, and ran perfectly right from the start. I was careful to only pull the starter one time with it on full choke.

So, which do you think is true:

1. Because it was sitting idle for a year it had particular problems yesterday, and running it through a full tank of gas fixed that.
2. Removing that small amount of sawdust fixed it.
3. It worked better today because I only pulled the starter once on full choke. That is, yesterday it was flooded???

Thanks,

Al

 

[DougA]

I don't think a bit of sawdust on the choke would make any difference but that does pretty well guarantee I was correct that you have sawdust in your carb. If it's there, it's inside too.
Nor do I think flooding it would affect performance once it was running. Once the engine is running, carb can't still be flooded.
One year idle? Now that is not good. I can't exactly recall but I think you are supposed to take off the plug, spray some wd40 in the piston and crank it gently once, then put the plug back in loosely.

I'm betting you will still have recurring problems until the carb is clean. Sometimes, the dirt gets pushed out by rough handling. The membranes on the carb are very thin and any dirt would affect performance. If you unhook the gas line and undo the linkages, the carb just pulls straight out over the two long bolts shown in your photo. Sometimes they feel stuck but just wiggle a bit, then pull.

Take apart carefully, note where everything came from in your notebook, then rinse off the parts with gas and reassemble. If you do that and find no sawdust, I'll buy you a beer.

 

[dougand3]

Nor do I think flooding it would affect performance once it was running. Once the engine is running, carb can't still be flooded.

+1

I'd bet your carb needs attn, too.

 

[WarmGuy]

Well I guess I'll get a new carb from Amazon. Seems like a no-brainer.

And it's super simple to replace the carb, right? Like this:

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[Jon1270]

I'd try running it a few more times before ordering a new carb, because it might just be fine now. Who knows, something (like the choke) might've been hung up somehow, and gotten freed thanks to your inspection today.

 

[DougA]

And it's super simple to replace the carb, right?

Yes, it is pretty easy. If you are buying one anyway, buy a new air filter at the same time, They are cheap and worth having a spare one.
Once you switch out the carb, take an hour and take apart the old carb, clean it out and then you'll have it ready to replace if needed. Carbs can wear out but that is uncommon.

Last year I replaced the piston & cylinder on my old 026. It sat there for a month while I got up the nerves to do it and once I got working on it, everything was quite easy. I put in a new oversized Meteor piston/cylinder and the only problem I had was breaking one of the rings trying to fit the d**** piston back in. The first ring was simple, the second was impossible. I broke down and bought a ring compressor tool and the real problem was that the rings are machined to fit EXACTLY. Once you see how they fit, you understand how they get so much compression out of that tiny motor.

Suggest you check out HL Supply. Be careful with Chinese parts. Some have luck with them, some hate them. Even the Chinese carbs sometimes are not machined to tolerances needed. If you got a Walbro, they are good.
http://www.hlsproparts.com/stihl-replacement-parts-s/79.htm

 

[Firewood Bandit]

Carb sounds lean on the low side. I'd really want to check the WOT unloaded RPM too. These come real lean from the factory.

The reason it is so temperamental is it needs to be richer when it's colder. The lack of acceleration is a clue.

Overall, lean = death to chainsaws.

 

[DougA]

WOT = wide open throttle (I had to look it up)

 

[WarmGuy]

A few more hours of running, and it's behaving perfectly.