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There is no fixed carburetor setting on these engines, just two screws to adjust low-end and high-end operation. Special tool may be required, there are about a half-dozen screw head standards. Check eBay for tool. Get the engine to run and idle fast (idle adjust) so it doesn't stall, adjust low-end screw so that engine doesn't hesitate or sputter when accelerated. Adjust high-end screw to run fast and smooth on full throtle. Readjust idle to normal.
With the filter removed check the choke valve. Make sure its connected to the linkage and working properly not opening and closing randomly. Now with these possible problems eliminated I would adjust the carburetor which is relative simple and the logical next step.
Locate the high and low carburetor adjustment screws. Some newer models may not have these adjusters. If there are none move on to cleaning the carburetor.
They should be on the left hand side near the handle. Look for 2 small holes. Look for an "H" and a "L" near the holes. Use a flashlight to see if the adjuster screws are there and determine the shape of the tool you will need to turn these. Older models may just require a long enough smaller flat head screwdriver.
You may have to buy a special tool made specially for Stihl saws (fairly cheap). Home Depot, Lowes or a Stihl dealer should sell these. Start saw. Find the "H" or high adjuster screw. Hold the saw on the ground firmly with one hand, insert the tool with other hand until it feels like its properly seated, squeeze the throttle and hold wide open (always hold the throttle wide open while adjusting the high RPM setting) then adjust the screw counterclockwise or clockwise until the saw reaches maximum RPM and runs smoothest with the least vibration.
Normally turning counterclockwise will result in higher RPM. Adjust it higher until it misfires, runs rough or burns to rich (too much fuel reduced RPM-bogging down). Then back it down slowly until the RPM begins to drop (restricting fuel flow). Now you know the spot you want is between those points. Open the adjuster slowly until it maxes out and runs smooth. It will sing and you have found the sweet spot.
You can adjust the "L" or low idle adjuster by releasing the throttle and adjust to the desired idle RPM (low as possible without stalling and smooth running). Do the same as the high RPM adjustment. Open the screw until the RPM is too high for idle or runs rough. Now back it down until it runs rough or stalls. Slowly open the adjuster until it idles smoothest at a reasonably low RPM. Rev it up then let it return to idle for a few seconds several times. If it does not level out and run smooth or almost stalls turn it slightly up and retest. Test it out in real conditions under a heavy load by cutting something big. If you are satisfied with the performance youre good.
If this does not solve the problem and/or there are no adjusters present then a dirty carburetor is likely the issue. Tanks gather sand and other trash that end up in the carburetor. Check the tank and if theres sand or trash empty it and rinse it out first. Remove the body/cover to get to the carburetor and carefully remove the linkage, fuel lines etc. Study the way each part is connected (take pictures or find the online manual if you have to) and what order they are removed as you will have to reassemble later. This can be tricky and tedious but dont rush or you will regret it. Careful dont break the plastic components. Remove the carb.
Take the carburetor apart carefully on a towel or large rag to prevent losing anything (small parts & springs). A diagram will be useful to know where small parts are located to avoid losing something that could fall or spring out. Carefully remove all possible gaskets and rubber seals without damaging them. If they are dried out or in rough shape you will want to replace them. No good to take this all apart and put back in damaged seals that may leak only to have to tear it all down again because its not running right and you are not sure of the cause (seals leaking or dirty carb). If you are lucky they may be standard sizes and easily replaced but you may have to find a Stihl dealer then hope they have in stock.
If rubber seals look OK apply a little oil (any kind) to condition them and help prevent pinching/tearing during reassembly. Use automobile type carburetor spray cleaner (or lacquer thinner, kerosine, WD40 etc) to clean inside and out. Spray through all of the holes and orifices. Dont get this in your eyes like I sometimes do. Good to use a torch tip cleaner or small wire/paper clip to unblock trash if theres sand or gunk present.
Use an air compressor and blower nozzle to blow through all the holes. Repeat the soaking and blowing process 2 or 3 times until your satisfied the holes are clean. Dont just go through the motions to clean it since its quite a bit of work to get the carb out usually and you want to be sure you have eliminated the possibility of a dirty carb. After its all back together adjust the "H" and "L" carburetor settings again.
There is no set adjustment on any chain saw. If it were that simple there
would be no need for adjustments. The problem is that most equipment
manufactured in the past 10 years can not be adjusted by the user. Carburetors
are now equipped with either adjustment limiters which limit the adjustment to
less than half a turn or with adjustment screws that require a special tool to
make the adjustment. More than likely you will have to take it to an authorized
dealer for those adjustments.
If you do have an adjustable carburetor,
then gently close both needles and open each on 1 full turn. This should allow
the engine to start and the final adjustments can be made at that point. If it
still will not start then open each needle 1/8 to 1/4 turn and try again. If
this still does not work then most likely your carburetor needs a good cleaning
and a rebuild kit.
The tool you need is available from Randy's Engine Repair, on his E-Bay store. The price is about $10., plus $3. shipping. Phone for Randy's is (937) 927-5665. It is usually busy.
Ebay item number is: 390025452838. Buy it online. I got mine about a week after I ordered it.