A couple years ago I purchased a Stihl MS250 chainsaw to cut down a few trees around my property. I regretfully admit to allowing old gas sit in the carburetor and tank for several months while it was not used. As a result, the internal diaphragm and fuel pump has suffered, rendering it unable.
After allowing your chainsaw to sit for several months, one day you try to start it. You pull and pull and pull until you are blue in the face. The saw may kick over a time or two but does not continue to run. What you may be experiencing here is a fuel delivery problem.
Since there are several possibilities here, it is very important to properly diagnose the problem prior to moving forward. Understanding why your 2 stroke small engine is not running is usually a fairly simple process.
How to determine if carburetor diaphragm is bad
Cover the basics
Returning to the basic principles of the internal combustion chamber engine, when your engine refuses to run, the issue basically boils down to one of two things – fuel or fire. If you are missing one or the other, your engine will not run or run very briefly. In order to diagnose the problem, you should always start with the basics. Here are the first steps that you should take:
- Remove and clean or replace your air filter. Any engine needs fresh air in order to run. If your air filter is clogged, the engine will become choked out and run rich, if it runs at all. In most cases, you can simply remove the filter and blow it out good with compressed air to remove any lodged particles of saw dust or dirt. Do not wash the MS250 filter with water, only use compressed air.
- If your saw has been sitting for a while, dump the old gas out and replace with fresh gasoline. For this particular model, Stihl recommends 89 octane gas mixed with 2 cycle oil in a 50:1 ratio. The most common gasoline that most use in their car is the 87 octane. Splurge a little and get the mid grade gasoline, your saw will thank you. Also, use a quality 2 cycle oil. Since I rarely ever use my saw, I mix 1/2 gallon of gasoline with 1.28 oz of oil. This is about 2.5 tablespoons. Likewise, if you are mixing a whole gallon, add 5 whole tablespoons of oil to one gallon of gas. Here is a helpful gas/oil calculator that you may want to use. Stihl’s can be very picky when it comes to the correct fuel mixture. Don’t use that old 32:1 mixture that you have laying around.
- Check your fuel pickup and replace as necessary. Inside your fuel tank, you will find a black fuel hose and on the end there is a small fuel filter. These can often become clogged with dust and debris, starving the carburetor of fuel. Use a stiff wire with a hook on the end to fish out the tube and filter. Once removed, clean it out with carburetor cleaner and compressed air. Look closely at the filter in the sunlight and inspect for contaminants inside. If the filter is ragged or there is trapped debris inside, replace with a new one. They are fairly cheap and need to be replaced every so many hours.
- Use the wrench that came with your saw to remove the spark plug and inspect it. If it is in fairly good shape you may just want to give it a good cleaning and check the gap. Clean with a piece of fine sandpaper to remove carbon deposits around the electrode. When finished cleaning, used carb cleaner and give it a good spray followed by a generous few puffs of compressed air to clean out anything that is lodged in there. Close the plug and gap to .020″. Next, test your spark by grounding the threads of the plug to the engine block and giving the rope a few pulls (ensure that the ignition switch is on). You of course don’t want your fingers anywhere near the spark plug as you could get a nasty shock. If you have a good spark, carefully screw the plug back in and tighten until snug. Replace the plug wire.
- Check the carburetor adjustments. Vibration could have thrown these out of specs. On the MS 250 there are 3 adjustment screws located on the right side of the saw near the carb. The High (h) Low (l) and the idle (la). Screw all three all the way in. Turn the Low screw out one full turn, High out 3/4 a turn and back the LA off a turn or so. You will be able to adjust the idle speed once you get get started. At this point you just want to ensure that the idle is not turned down, cutting off the fuel supply.
Now, you have a fresh source of fuel, adjusted carburetor, clear airways, and a good spark. Try starting it. There is a good chance that your saw will run properly. I would say about 50% of the time one of the above 5 is the issue. Still no luck? Continue reading.
How to remove the Zama carburetor
Once you have completed all tasks above and your saw is still not working, its time to start taking things apart. With spark, fuel, air and compression, your problem most likely will be in the carburetor. Here are the steps required for removal.
- Remove the access cover to reveal the air filter. Remove the filter.
- Disconnect the kill switch wire from the kill-choke lever and snap the right side of the lever out of its socket. This is require to remove the choke linkage as the carb comes out.
- Snap out the linkage from the accelerator lever. I believe you will have to pull the accelerator trigger and use a screwdriver to pop it out.
- Remove the 2 8mm bolts holding the carb in.
- Remove the fuel line by gently pulling it from the nipple.
- Work the entire unit out carefully while removing the choke lever simultaneously.
- Plug the fuel supply line to prevent gas from leaking out.
Inspect fuel line
A common problem with this particular model is a cracked or rotted fuel line. This would cause a fuel delivery problem, making it an important factor to eliminate. Removing the fuel line is quite simple. As before, you will want to fish the fuel filter out and remove it. From here, simply pull the fuel line out from the top. This is of course done with the carb taken off. Dry the line off and bend several different directions while looking for signs of aging. Replace the line if any issues are present.
How can I tell if diaphragm or pump is bad?
The only way to tell for sure is to do a pressure test which involves pressurizing the fuel inlet nipple and watching for a drop in pressure. At this point, it is best to rebuild or replace your carb. The kit will come with everything that you need to complete this task including the pump and diaphragm.
Cleaning your carburetor
Now that you have made it this far, you have 2 choices. You can purchase a rebuild kit for $10 on ebay, clean it up good, put it back together and hope that you removed the obstruction or you can take the easy route and but an new carb. These are available on Ebay for $20 to $30.
The $20 ones are usually the Chinese knock offs while the $30 are typically the real Zama. Being the tightwad that I am, I decided to save $20 and risk it by rebuilding the carb.
Most 2 stroke carbs are the same and I already wrote a post on rebuilding a small engine carburetor. There are basically 2 screws on the diaphragm side and 1 screw on the fuel pump side. Remove these, taking note of which order the gasket, diaphragm and plastic pump goes. Follow the cleaning instructions in my other post.
Once everything is clean, replaced, and back together, assemble your saw in reverse order. She should crank right up if you followed all instructions. If not, you may have a compression issue, which will need to be checked with a compression gauge. Don’t forget to adjust your idle screw by turning it clockwise until the chain stops spinning while idling.
How does the linkage go on the MS250
So you took your saw apart and can’t figure out how to get the linkage back on correctly. This happens to me all the time. One way to solve this is by taking a picture as you take it apart. Luckily for you, I have done this for you.