Stihl MS250 floods crankcase when not running

[c20z24]

Hello! This is my first post on the forum, although it appears I joined it back in 2001.
I hope this is the correct place to post the question.

I bought my MS250 new about 6 years ago and have run about 10 tanks of gas (total) through it. I have always run 92 octane ethanol free gas with Stihl synthetic oil. Last fall when i tried to start the saw for the first time in about a year, it would run at idle, but stall if I tried to open the throttle. After cleaning the carb it ran well, but was prone to flooding.

Yesterday when I tried to start it (first time since January), it wouldn't pull over. After pulling the spark plug I found the crankcase full of gas. Has anyone else had this issue before? I'm wondering if I have a sticky needle valve in the carb or if I have a plugged fuel tank vent.

Thanks for looking.

 

[pops21]

Needs a full carb rebuild with all the hardware and not just the gaskets. The needle is not sealing I believe.

 

[Thommo]

I would check the metering needle in the carb. I have seen the viton tip of the needles tear from being stuck to the seat after sitting for a few months.

 

[c20z24]

Thanks for the replies. I'll pick up a kit and give it a full rebuild.

 

[pops21]

Keep us updated. Make SURE you put the kit in just like the old one came out. If not it will give you problems.

 

[c20z24]

Not much progress to report yet. I stopped at the local Stihl dealer yesterday, but didn't know which style carb I had, so I didn't pick up the kit. When I got home I found that I had the Zama carb and ordered a kit from Bailey's for $13 ($7 less than the dealer). With any luck, I'll be able to put it together later in the week.

 

[R. Flournoy]

greetings ,
hope you got the 1123 up and running but if not heres another thought. When you cleaned the carb before what did you clean it with? Why I ask is...the construction and function of the carb is dependent on small check valves pressed into the body. The check valves are easily damaged first off by ethanol enhanced road fuels . as well as commercial solvents (i.e. brakleen , carb n choke cleaner ect.) Compressed air though the check valves or even through the throat if blown though a check valve will push the internal parts of the check valve right out or deform them at the least. If the check valve in the high speed circuit is compromised fuel weeps out any time there is fuel in the system causing the engine to flood. With that being said on smaller body carbs the check valves are too small to be commonly serviced (read as available) You can replace the inlet needle , lever , spring diaphrams , ect. but the carb will still flood . If you have a Stihl dealer near by most complete carbs for current models are affordable(under $50) Unless I can pressure and vac check all circuits of the carb cant say for sure thats whats your issue is but like i said another thought.

 

[hardpan]

R.Flournoy
Welcome. You sound like an asset to this site. What is your recommended procedure for carb cleaning? I am constantly amazed at the number of common cleaners and solvents that a chainsaw carb can't handle. I've done a couple of the nono's you mentioned above. Of course total replacement was the result.

 

[c20z24]

R.Flournoy - Thanks for the reply. I did use a carb-cleaner, but not brake cleaner. I also used compressed air to dry the fluid after it soaks. I've rebuilt dozens of 2-stroke carbs before, but never one on a Stihl. I've used the same cleaner and compressed air on the others and they have always worked well afterward.

What type of cleaner do you recommend? Are the check valves serviceable?

If I were to buy a replacement, is Walbro a better choice than Zama? This saw has never been a good starter (4-5 pulls if it hasn't been run in a day or two). I'm wondering if the carb might have been part of the problem. Most of my other equipment (saws, trimmers, etc) tend to start after 2-3 pulls unless they have been sitting for months.

 

[Wood Doctor]

Hard to believe but the low-speed carburetor set screw may be too far open. If so, that will cause the carb to flood much too easily. When that set screw is too far open, I've seen fuel blown right out the exhaust and through the decomp valve. The carb itself is usually OK and begging for an adjustment.

 

[hamish]

Needle not seating or failed tank vent.

 

[R. Flournoy]

Cleaning two stroke carbs...all of this is IMHO...to begin with you can't blame Stih , Husqvarna , Echo or even Zama or Walbro for fuel quality issues!!! You as the operator are responsible for keeping fresh (within a month from the pump if it's got ethanol) 89 octane or better fuel in the fuel system. I don't care who made it modern road fuels tear it to pieces if it sits there!!! Moto mix if awesome btw. That aside I think it goes without saying that the bigger the carb the easier to clean. Also the bigger the price tag to buy a complete one too so you would be more likely to spend time and money to clean and rebuild. You can clean the outside off good with what ever cleaner you want just put your finger over the throat on each side and spray it clean and blow it dry. Now you can pull the pump side cover( the one with one screw in the center on most modern carbs) need to check the screen and normally replace,it's in most all rebuild kits. Clean what ever is in the hole out with a pick of a small screwdriver before removing the screen to keep it from going deeper in the carb when you pull the screen. Now pull the metering side cover. Take out the pivot screw and inlet needle , spring and fulcrum. Take out whatever mixture adjustment screws you have. I'm sure there's another topic on here that discusses or cusses rather limiter ca


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[R. Flournoy]

Sorry phone slipped while I was typing. Anyhow there's more details on limiter caps and there removal elsewhere on this site Here's the important step get LOW VOC Brakleen You can now follow the fuel path all the way through where the fuel line hooks on look at how the flaps are on the diaphram on the pump and you can see how fuel has to go through the little holes in the body then spray through where the screen was and up into the metering chamber. Now spray through the holes the mixture screw came out of and watch it come through the main nozzle in the center of the throat when you spray through the h side and out the little holes in the top of the throat when spraying through the L If it's much dirtier or corroded than that the chances of survival are slim. Remember to check the play on the throttle shaft before you spend a lot of time with one too. There's an accelerator pump in a bore the works of the shaft in a lot of carbs so wear on the shaft and body can cause headaches too. On bigger saws with bigger carbs I'd say ms290 Stihl and bigger (Walbro hd21 carb I believe) the high speed check valve and all welch plugs are available so you can strip the carb punch out the welch plugs and check valves clean it good with low voc or a sonic cleaner and reassemble. Can be done but not for the novice. Not trying to sound like a carb salesman but a good stihl parts person can find you an affordable carb for most saws newer than mid '90s or so. If they use media cat they can see all the options and price them out for you


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[hardpan]

I notice you never mention "rodding" any ports with anything like a torch tip cleaner or wire from a wire brush. Probably a dangerous move? Low VOC Brakleen sounds like the safe route and a good tip. Thanks

 

[c20z24]

Thanks for the suggestions. Here is what I did prior to the flooding:

-less than 10 tanks of fuel total through the saw since new
-only 92 octane ethanol free fuel (Stihl brand 50:1 synthetic oil)
-the same fuel is used in my other 2 stroke equipment
-fresh fuel is mixed if it hasn't been used up within 6 months
-The saw was stored for up to 12 months at a time with fuel, which I believe was the initial cause of the problem
-I cleaned the carb with low VOC Gumout Carb-Choke Cleaner
-No visible dirt or contamination was found inside the carb or on the filter screen
-When the carb was reassembled without any new parts, the saw ran great but was prone to flooding
-The carb kit should arrive later this week, hopefully revealing the problem to be a bad needle valve...

 

[lwn9186]

Make sure you have the diaphragm and gasket are in the proper order on the needle side or you can have flooding.

 

[R. Flournoy]

Sounds like you're doing right when it comes to fuel quality. But there is a big difference between carb n choke cleaner and Brakleen no matter low VOC or not. The solvent in each is different example...if you overspray carb n choke cleaner on your hand it's cold and feels oily if you run it between your fingers. Do the same thing with Brakleen and it's warm and feels watery to the touch I know it's kinda splitting hairs but there's a difference and it matters to the check valves inside a carb. Rebuild the carb , make sure your orient the gaskets and diaphragms properly and set the inlet needle height start the saw with the mix screws at 1 and 1 and let it warm up now lightly seat the L screw to closed ad see if the saw still runs. If it does your high speed check valve is bad and is leaking fuel at idle. If it's just the needle not sealing then a new needle should take care of it. As to "rodding" or drilling out carbs if you're gonna change port timing or height or something else internal of the engine then I could see drilling out a carb but on a stock saw you can open up the H screw to the point the saw won't take any more fuel anyhow so I don't see where drilling the carb out to flow more fuel would gain you anything but a head ache IMHO that being said I also don't believe in modifying exhaust beyond taking a a spark arrester t two. That engine was designed to run with a certain amount of back pressure. Too many people praise their gutted drilled out muffler but then cuss whoever made the saw because they scored the top end all to pieces.


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[c20z24]

Well it took a bit longer than I thought for the kit to arrive, but it finally got here earlier this week. On Wednesday I installed it. The started with 5-6 pulls and ran well. I cut some light brush for 10-15 minutes before shutting it off. I let it sit for 3 days. Today I started it again. It took 4-5 pulls on full choke but no flooding issues.

While I was waiting for the rebuild kit to arrive I was able to locate a tech bulletin from Zama on how to rebuild their carbs. They specifically recommend using carb cleaner as a cleaning agent. Brake cleaner may also be effective, but I've decided to stick with what the manufacturer recommends.

During the rebuild I discovered a couple of potential causes for the flooding. First (and most obvious), the fuel pump gasket was not correctly aligned on one of the locating pins. Evidently I did this when I cleaned it last. The other was a light indentation on the needle valve. Either/both may have caused the flooding issue.

Thanks again for all the suggestions. Hopefully it will stay fixed this time!

 

[Wood Doctor]

... During the rebuild I discovered a couple of potential causes for the flooding. First (and most obvious), the fuel pump gasket was not correctly aligned on one of the locating pins. Evidently I did this when I cleaned it last. The other was a light indentation on the needle valve. Either/both may have caused the flooding issue.

Thanks again for all the suggestions. Hopefully it will stay fixed this time!

Sounds like somebody tried to rebuild the carb but did not know how to do it. Happens all the time.

 

[griffonks]

Most of the time I don't open the carb until I have a full repair kit for it anymore. After having two Walbro's in a row with bad check valves I use an ultrasonic cleaner with a little Dawn dish washing soap in water and like 20 pounds of air pressure to gently blow out passages.

A Mityvac is really handy for diagnosing carb problems, tank vents, fuel line and impulse hose leaks and crankcase leaks... easily worth the money for an 8500.