MS 250 Recoil Sporadically Locking

[GatorBait]

Hello all, thanks for taking the time to read my post. This is my first post, but I think I'll be back. I run a small credit union in Daytona Beach. But I'm a DIY guy and a hustler and I've recently discovered that I can buy Stihl saws and resell them for $100 or more. Thats worth my time. So here I am diving into fixing up saws.

I have this MS250. Paid $100, guy said it won't start and he doesn't have time to fix it. That's right up my alley. I pulled the carb off, took it mostly apart, cleaned it out, and reinstalled it. I got the saw running! I've already made at least $75! But here's my problem:

1. Half the time when I pull start it, it tries to lock up on me. I've already had to replace the pawl once due to this. I'm confident the issue is not in the recoil. When I take the spark plug out, it pulls over beautifully. The issue is only with the plug in. I've also noticed that when the choke is on, the saw still pulls over beautifully. Its only when I put it in the "run" position that I start to get that super hard hesitation. Its enough that it will maul the pawl if you keep doing it.
2. But if I persist... pull it a couple times on choke, then couple times in run, she starts up. The first time I got her running, had a lot of smoke. She winds up good... sounds perfect at high RPMs. But when you let her get into lower RPMs, she starts to die. You have to keep the RPMs high to keep her running. If the RPMs get too low and you give her throttle she dies.
3. The issue does repeat itself. This wasn't just the first time I got her running. After I had her running a few times... the issue will still repeat itself... if you pull her in the "run" position, she wants to lock up. You can only clear it out by taking out the plug and/or pulling her over on choke.

Is this hydro lock? Is this too much fuel, causing this issue? Is her inability to idle also tied into this hard starting?

I greatly appreciate the feedback. I'm learning so much and really enjoying taking a saw that someone doesn't know how to fix, or isn't willing to put the time in to fix and making it run. And like I said, when I can do this stuff in an hour on a Saturday and make $100 I'm all about it. I'm just needing to learn a little more to get better!

 

[thompsoncustom]

I would pull the flywheel and see if the flywheel key is broke. Since it pulls over nicely with the choke on you can rule out hydrolocking since that is the position that gives you the most fuel.

Your idling issue could also be because of a sheared key but it could also just be the carb is not set right.

 

[coffeebrk]

What does the piston and cylinder look like?
If you are going to get involved with repair a good investment is a borescope/endoscope camera. Very inexpensive and worthwhile.

 

[GatorBait]

I would pull the flywheel and see if the flywheel key is broke. Since it pulls over nicely with the choke on you can rule out hydrolocking since that is the position that gives you the most fuel.

Your idling issue could also be because of a sheared key but it could also just be the carb is not set right.

Pulling the flywheel and inspecting the flywheel key is not something I've done yet. Is that a straightforward task? One reason I only own Stihl equipment myself and intend to only try to flip Stihl stuff is because its quality and so far, for a guy who runs a bank, is pretty straightforward to work on. So I can look into that. I'm sure there's a video on YouTube of taking them off. Steve's Small Engine Saloon does a good job with videos, assuming he's giving correct advice.

I has the same thought you did... that choking the engine gives it more fuel, counterintuitive to a hydro issue. But I'm looking forward to diving into it a little bit more. The saw it yellowed, and someone has messed with the carb and the recoil rope... so its got some miles on it. But I don't think its near end of life. I think if I keep at it, I can get it running nicely again. Then maybe I have a $250 saw on my hands! Thanks for your help!

 

[GatorBait]

What does the piston and cylinder look like?
If you are going to get involved with repair a good investment is a borescope/endoscope camera. Very inexpensive and worthwhile.

I just watched a YouTube video on how to inspect the cylinder and piston, via spark plug or muffler and seemed very self explanatory. I gave it a quick glance last night via spark plug and it looked fine. So I think I have an engine that can run really nice and strong again. But I agree with you, it would be nice to have one of those tools. Can you suggest a decent one at a good value? Certainly don't need a high end one, as this is still just a hobby right now. Thank you!

 

[pioneerguy600]

I just watched a YouTube video on how to inspect the cylinder and piston, via spark plug or muffler and seemed very self explanatory. I gave it a quick glance last night via spark plug and it looked fine. So I think I have an engine that can run really nice and strong again. But I agree with you, it would be nice to have one of those tools. Can you suggest a decent one at a good value? Certainly don't need a high end one, as this is still just a hobby right now. Thank you!

Less costly to just remove the muffler and look in through the exhaust port, all the front side of the piston can be seen by rotating the crank via the recoil and it is most often the exhaust side of the piston that gets damaged on chainsaws that are run too lean. One can see the intake side of the cylinder through the exhaust port with the assistance of a light source, flashlight etc. Bore scopes are more of a gimmick for inspecting chainsaw cylinders due to the easy removal of the muffler on most all models of modern saws.

 

[GatorBait]

Less costly to just remove the muffler and look in through the exhaust port, all the front side of the piston can be seen by rotating the crank via the recoil and it is most often the exhaust side of the piston that gets damaged on chainsaws that are run too lean. One can see the intake side of the cylinder through the exhaust port with the assistance of a light source, flashlight etc. Bore scopes are more of a gimmick for inspecting chainsaw cylinders due to the easy removal of the muffler on most all models of modern saws.

That makes sense. Belive it or not, I've yet to take off a muffler. To date, I've always just assumed carb issues and attacked that. But moving forward, I've learned that the piston/cylinder are very important to inspect before investing time and money... so I imagine I'll be removing my first muffler very soon. Probably never a bad idea to clean it up anyways. Some of the saws I've bought, the high mileage ones, have seemed to have been missing a screw. Are those easy to get? I'd imagine so. Would the effect how the saw runs if it only has one screw? Or is it more structural than anything? Thanks for the advice!

 

[coffeebrk]

The MS250 is a hard to start saw even when everything is OK.
Fuel filter
Air filter
Put a kit in the carb ( is the carb oem and upon inspection are parts missing?)
How many turns out for each jet ?
That smoke you mentioned is probably carb, and having to feather the throttle is too.
Pull flywheel and check key
Definitely and properly measure coil gap, I use the Stihl tool especially on MS250's.

 

[GatorBait]

The MS250 is a hard to start saw even when everything is OK.
Fuel filter
Air filter
Put a kit in the carb ( is the carb oem and upon inspection are parts missing?)
How many turns out for each jet ?
That smoke you mentioned is probably carb, and having to feather the throttle is too.
Pull flywheel and check key
Definitely and properly measure coil gap, I use the Stihl tool especially on MS250's.

The Carb is a Zama, so I would guess is OEM. However, I know its been messed with because one of the jet plastic covers are missing. But for my 3 months of experience, it looked complete. Now here's an odd thing. On the MS 250 there are markings on the side cover about how many turns for each jet. Well, most of the one's I've worked on are like 1 turn and 3/4 turn. Well this saw clearly says 1/4 turn for one of the jets (I can't remember H or L). So I did what the saw said and its only out 1/4 turn. But that seems awfully little for the rule of thumb one one full turn for each, give or take. Why would this saw be different from others?!

 

[pioneerguy600]

That makes sense. Belive it or not, I've yet to take off a muffler. To date, I've always just assumed carb issues and attacked that. But moving forward, I've learned that the piston/cylinder are very important to inspect before investing time and money... so I imagine I'll be removing my first muffler very soon. Probably never a bad idea to clean it up anyways. Some of the saws I've bought, the high mileage ones, have seemed to have been missing a screw. Are those easy to get? I'd imagine so. Would the effect how the saw runs if it only has one screw? Or is it more structural than anything? Thanks for the advice!

The screws for the muffler are an easy replacement, if a Stihl saw,the dealer can get them but also check eBay, plenty of sellers on there with OEM and AM screws, other makes are even easier, online sellers and eBay you will find them. Most modern saws use metric thread screws but they can even be found at good hardware stores, sizes vary from saw to saw for diameter, length and thread pitch so try to find the size and pitch before going looking, often a Google search will find the info in IPL pages or just a direct inquiry or just ask on here, there are plenty of guys that work on these saws enough to have that info recorded or off the top of their head. It is not a good idea to run one of these plastic framed saws with muffler bolts loose or missing, the escaping hot exhaust gases will melt the plastic. The saw may run ok with a screw missing but the damage to the plastic will occur quickly.

 

[pioneerguy600]

The Carb is a Zama, so I would guess is OEM. However, I know its been messed with because one of the jet plastic covers are missing. But for my 3 months of experience, it looked complete. Now here's an odd thing. On the MS 250 there are markings on the side cover about how many turns for each jet. Well, most of the one's I've worked on are like 1 turn and 3/4 turn. Well this saw clearly says 1/4 turn for one of the jets (I can't remember H or L). So I did what the saw said and its only out 1/4 turn. But that seems awfully little for the rule of thumb one one full turn for each, give or take. Why would this saw be different from others?!

The one quarter turn ones are carbs with the limiters on them, that is all the limiters allow the screws to be turned, it is not where the screws are set at from the loosely seated and backed off setting.

 

[coffeebrk]

The Carb is a Zama, so I would guess is OEM. However, I know its been messed with because one of the jet plastic covers are missing.

Uh-oh

 

[pioneerguy600]

I have never trusted a carb set at the factory and have seen many saws come in with smoked pistons due to the factory setting being too lean, these saws mostly had limiters installed at the factory and in most cases there was only about a quarter turn richer or leaner allowed by the limiters. All the saws I own or work on get the stops trimmed off the limiters and then set to a good safe tune. I run my own ported saws a bit on the edge but I know how and when to change the tune to fit the conditions at hand, the screwdriver is always close at hand.

 

[GatorBait]

I have never trusted a carb set at the factory and have seen many saws come in with smoked pistons due to the factory setting being too lean, these saws mostly had limiters installed at the factory and in most cases there was only about a quarter turn richer or leaner allowed by the limiters. All the saws I own or work on get the stops trimmed off the limiters and then set to a good safe tune. I run my own ported saws a bit on the edge but I know how and when to change the tune to fit the conditions at hand, the screwdriver is always close at hand.

Here is a question: because the limiter caps only allow little movement... and because when I've cleaned a carb I've wanted to be "thorough" and get a fresh start, so I've taken the screws out completely, cleaned them and put them back in... I've had to pry the caps off. I've yet to get them off without damaging them. Do you need to keep the caps on to to keep the screws in place? Due to the vibration and the engine running, can the screws back out, thus causing the saw to run poorly? I guess what I'm getting at is, do you need limiter caps on to keep the L and H screws at the right setting? Or is it fine to run the saw without them?

 

[pioneerguy600]

In my case I just remove the tabs that catch on the stops, either a very sharp mini chisel when on the saw or just an exacto knife if the carb is off the saw will allow the shank of the limiter to stay on but still allow the needles to be turned wherever needed. I cannot see where the plastic limiters are actually needed to hold the screws in place,they don`t contact any surface to provide locking in place. I leave them on as they are easier to get the tuning screwdriver into them when the saw is running. On some saws like my Efco 56 I heated the blade of an old screwdriver and pushed the heated blade deeper into the plastic limiter cap to make it easier to get my regular tuning screwdriver into the slot and keep it from slipping on a running engine.

 

[thompsoncustom]

I've ran them without caps before and never had any problems with them moving.

As far as the flywheel goes there is a little tool that you can get for 10-15 bucks that screws on to the flywheel that really makes removing them easy.

 

[GatorBait]

I've ran them without caps before and never had any problems with them moving.

As far as the flywheel goes there is a little tool that you can get for 10-15 bucks that screws on to the flywheel that really makes removing them easy.

I watched some YouTube videos on that last night. It looks straight forward, but I wanted to make sure it wasn't a reverse thread, or that you didn't have to lock the piston, like when replacing a sprocket. But yeah, guys were beating on the shaft and I'd rather avoid doing that if I could! Could you send a link to one. One guy had a big flat round Stihl tool, which I could not find online. Other tools looked like nothing more than a deep well socket. What tool do you think works best/easiest?

 

[pioneerguy600]

One of the most common reasons I encounter with saws locking up when trying to start a saw that has been sitting for a while is the carb has leaked fuel through it into the cylinder. If one can test the carb with a pressure and vac test it often as not shows a slow leak through the metering needle, its not shutting off the fuel completely. To test out that theory remove the sparkplug and pull the engine over rapidly, something like 10 -12 times, it helps more if the saw can be stood up on end,bar up, some prefer to turn the saw upside down but I can clear them with just the bar up. Clean and reinstall the sparkplug ,then try pulling the engine over rapidly with the choke in the off position. Most times the engine will turn over without the hitch or lockup and start on the 3 - 5 pulls over like normal

 

[GatorBait]

One of the most common reasons I encounter with saws locking up when trying to start a saw that has been sitting for a while is the carb has leaked fuel through it into the cylinder. If one can test the carb with a pressure and vac test it often as not shows a slow leak through the metering needle, its not shutting off the fuel completely. To test out that theory remove the sparkplug and pull the engine over rapidly, something like 10 -12 times, it helps more if the saw can be stood up on end,bar up, some prefer to turn the saw upside down but I can clear them with just the bar up. Clean and reinstall the sparkplug ,then try pulling the engine over rapidly with the choke in the off position. Most times the engine will turn over without the hitch or lockup and start on the 3 - 5 pulls over like normal

Thank you Pioneer, I'll give this a try this weekend, if not sooner!

 

[cookies]

take apart the starter and look for wear in the pulley and the shaft it rides on...these saws put a crap load of stress on their starters and if they wear any it cocks and binds up. Pawl problems are a symptom of it cocking.