Dirty carburetor? it will only start and run with choke fully open and engine idles slowly then a little more quickly and back down again and repeats

Your question isn't detail enough such as.

Q: It starts up wide open with the trigger depress fully but when I release the trigger it dies.

A: The carburetor idle jet (Picture A item #27) is dirty/plugged. the carburetor needs to be taken apart and cleaned or it just needs to be turned out a bit to flow fuel or the idle screw that sets the idle speed (Picture B) on the carburetor need to be screwed in to open the flap a bit more. Your carburetor may not be like either of these two carburetors but close enough. If you need to figure out which carburetor you have to locate the idle jet look search for picture of your carburetor and that tells all such as in picture A.

Another couple things. In picture B you see the spring on the throttle shaft that closes the flap to idle. If that area is really dirty the flap could be stuck wide open. Make sure the it is free to open and close. If the chainsaw has had a lot of use the engine could be wore to the point at idle the piston isn't making enough compression for the engine to stay running even if you turn the idle speed screw (Picture B) in as far as you can.

Procedure #1-

1. Be sure the hook on the bimetallic spring contacts the choke shaft lever.
2. Install the cover and retainer and lightly tighten the screws.
3. Turn the ceramic element to align the mark on it with the middle mark on the housing.

~~~ Procedure #2-

1. The engine must be cold to make this adjustment.
2. Remove the air cleaner.
3. Find the automatic choke. It's the round thing on the upper right-hand of the carburetor. There is a wire coming to it from the positive side of the coil.
Note: The automatic choke is a round ceramic thing with the heating element wound inside of it. (The ceramic part may be covered with metal so that it looks just like the rest of the carburetor.) The choke is held in position by a triangular ring clamp and three screws so it can be loosened and rotated for adjustment.

4. Pull the throttle arm on the left side of the carburetor down to free the little step arm (cam) that the screw at the top of the throttle lever rests on.
Note: This stepped "cam" is connected to the butterfly valve inside the throat of the carburetor by a shaft that extends all the way into the automatic choke. With the engine cold, the butterfly valve should be closed. As the engine warms up, the coil inside the automatic choke uncoils, opens the butterfly valve, and moves the cam to reduce the idle speed.

5. Release the throttle arm so that the return spring snaps it back. The little screw at the top of the throttle arm (again, with the engine cold) should now rest on the top step of the stepped cam. This sets the hi-idle, which is needed together with the choke on a cold engine to provide sufficient idle speed to keep the engine running until it warms up fully.
6. Loosen the three screws on the right side that hold the choke in place.
7. Keep your eye on the butterfly valve in the carburetor throat.
8. Turn the choke element clockwise (viewed from the right) until the butterfly is standing straight up, then turn the choke counterclockwise (viewed from the right) until the choke butterfly fully closes (barely -- not too tightly), then tighten the three screws that hold the choke in place.
Note: This is important; the automatic choke may be assembled wrong and not catching the hook on the coil spring at all.

9. Start the engine with the air cleaner off. As the engine warms up, make sure that the butterfly opens until it is standing straight up (full open) when the engine is fully warm. If it doesn't, readjust the choke until you get it right.
Note: The engine is now warm, so you won't be able to adjust the choke per the foregoing. Note the position of the notch on the side of the choke relative to the three little ridges on the body of the carburetor. If the butterfly is too far closed with the engine warm, turn the choke clockwise just a bit to straighten it up. The notch on the choke should never be too far outside of the three ridges on the body of the carburetor. If you are not able to adjust the choke using these method, something may be sticking, or perhaps the coil spring inside the canister is broken, or perhaps the wire has fallen off of the contact on the canister so that it is not getting power from the battery properly.

Your engine probably has an automatic choke device that adjusts automatically with engine temperature. To set the choke when the engine is cold you have to depress the accelerator to the floor and release it completely. You shouldn't have to depress the accelerator again to start the engine, turn the key the engine should start. As the engine begins to warm up it will also begin to initially run faster then it will start to run rough. At this point you have to press the accelerator and release it again. The auto choke mechanism should allow the choke butterfly to open. There may be a number of stops in the system and the choke butterfly will stop again still not fully open and only become fully open when the engine is at operating temperature. Hope this answers you question, Rgs. Bob

If the plugs are ok and your engine is not smoking after starting and running ..Sounds like either of three things..all with the carburetor .First the carb has a adjustment screw for air/gas mixture on the side if the carb where it attaches to the engine..try this start the engine and at about half throttle..now adjust the screw clockwise until the engine starts to miss and bogg down then back the other way until it starts missing and bogging again. Then slowly adjust back and forth until you hear the engine run the smoothest..give it more throttle if it starts to run rough do the same procedure until you get the results you want..if it is the mixture problem this will take care of it..if this doesn't it it may be your fuel jet in the bowl dirty or the blow float sticking..if you are not familiar with carbs best to have a professional look at it..you can drop the bowl by removing the bottom bowl and clean it out...check to see if the float easly moves up and down..replace the bowl and go from there..worse scenario new or rebuilt carb...good luck..

A complete replacement carburetor will just buy you another set of parts identical to the ones you already have. Nothing is worn out.

You should be able to clean the carburetor inside with SPRAY carburetor & fuel injector cleaner, after opening it up. Spray the cleaner through the jet and every passage you can find. Be careful about your eyes - some passages lead back toward your face, and carburetor cleaner is NO good for your eyes!

When it's clean, examine it to be absolutely sure everything's assembled correctly, especially the float valve ("needle and seat"). Put it back together and reinstall it on the engine.

Put a little FRESH gasoline (NEVER use old gasoline in an engine!) in the tank.

Now take off the fuel line (from the carburetor) and make sure fuel drains from it freely. Put it back on immediately and loosen the bowl. Fuel should also drain from the bowl freely.

Last, doublecheck your idle mix adjustment screw. If you toyed with it, you have to get it adjusted correctly again - a good starting point is SNUG (never TIGHT), then back it out counterclockwise about 1-1/2 turns. Once you get the engine started and running without the choke, it (the screw) should be adjusted until it's very slightly rich (counterclockwise) from the point where idle speed drops if you turn it clockwise any further. Said another way... get it idling without the choke, then turn that screw in (clockwise) 1/4-turn at a time until you can hear the idle speed drop. At that point, back it out (counterclockwise) about 1/4 turn.

If you CAN'T get it to idle without the choke, then just try backing out (counterclockwise) that screw 1/4-turn at a time. Each 1/4-turn should make it run just a little richer, necessitating less choke.

Kescorpio56:
Check your throttle and choke settings. Something is causing the throttle plate in the carburetor to remain open or the governor is improperly set. I would reset the air/fuel mixture screws to one and a quarter turns out and back the idle screw all the way out. Make sure that the throttle cable operates the throttle properly and check the governor setting and reset the governor setting if needed. Start the engine and adjust from there. When the engine starts, adjust the idle screw until the engine idles. Adjust the fuel/air mixture screws for the best idle. Then run the RPM up and adjust the fuel/air mixture screws for the best run/idle combination. Don't forget to adjust the governor if the engine runs too fast or too slowly.

turning Hi & Lo mixture screws in leans the mixture,out rich-ens it.turn the T screw in to increase idle speed the chain should slowly come to a stop at idle [chain brake off] adjust Low needle first until it accelerates smoothly then Hi needle for best 'revs'.then Lo again if necessary if you don't know where they were set before,start w/ screws 3/4 to 1 turn out from a lightly seated position

Before you start check the status of your choke. One that is stuck on will cause a high idle. If the choke checks out, proceed to the following:


You should have what's called a 2 needle carburetor. A larger screw adjustment for idle, and a smaller for high speed.

Start by adjusting your low speed (idle) needle until you reach the desired idle. I prefer about 850rpms. Do this with the engine running and adequately warmed so that your adjustments are not thrown off by the automatic choke.

Next slowly open up the high speed needle. As you open it your idle speed will gradually increase. Adust it in 1/8 turn increments, pausing after each adjustment to allow the changes to take affect. Keep opening it until you stop increasing your rpms, then take it back 1/8 of a turn.

Next readjust your idle mixture screw so that your idle is back down to 850rpms.

you have a dirty carburetor/carburetors. a good old fashioned carb cleaning will fix it up. here is a temporary fix that will get you going again. screw your idle mixture needles in ( counting the turns ) THEN SCREW THEM BACK OUT THE SAME NUMBER OF TURNS.

On older, carbureted engines, cold stalling (and hard starting) is most often due to an automatic choke that is sticking, misadjusted or broken. Cleaning the choke mechanism with aerosol carburetor cleaner may free up the choke allowing it to work properly again. If the choke housing as an electrical heating element, the element may not be receiving voltage when the key is on, or the element may have burned out (check resistance with an ohmmeter).

Other causes of stalling with a carburetor include an idle speed adjustment screw that is set too low (turn screw to increase idle speed rpm). The engine may stall if the idle fuel mixture screw(s) are not adjusted correctly or the idle mixture port(s) are dirty or clogged with fuel varnish deposits (clean the carburetor and readjust the idle mixture screws for smoothest idle). Stalling can also occur is there are vacuum leaks in the carburetor, under the carburetor (bad base gasket), or any vacuum hose connections to the carburetor or intake manifold