The carburettor on a Stihl chainsaw contains a metering section, a mixing section and an attached fuel pump with diaphragm. When the engine stops performing properly, the culprit is usually the carburettor. Dirty or old carburettors and fuel systems will cause the engine to stop receiving the proper mixture of air and fuel. When this happens you will start noticing a wide array of problems. When troubleshooting the carburettor, it is best to isolate the problem by the identifying the symptoms you experience during operation.
Start your Stihl chainsaw and, if you can, let it warm up to proper operating temperatures. Try to throttle the saw and test what happens. Locate the engine symptom you are experiencing, such as poor acceleration, engine dying or carburettor flooding, to troubleshoot your chainsaw's specific problem.
Remove and clean the air filter if your chainsaw runs but does so poorly. Check the linkage on the throttle cable and make sure it is properly attached to the trigger and the carburettor. Drain and clean the entire fuel tank.
Remove the pickup body from the end of the fuel line. Remove the fuel line from the engine and replace with new, clean parts. Make sure when reattaching the new fuel lines that they are properly sealed with the carburettor. Use a sealant if necessary.
Check and clean the fuel vent on the tank. Replace if it's dirty or clogged. Refill the Stihl with freshly mixed two-cycle fuel and try to run the saw again. If it's running a little better than before, adjust the high, low and idle speeds on the carburettor screws. If it still is running just as poorly as before, remove, disassemble and clean the carburettor.
Remove the carburettor if you haven't already. Disassemble the carburettor. Replace the fuel inlet needle. Remove the inlet control lever, replace with a new lever and make sure it is flush with the top edge of the housing if your carburettor is flooding easily.
Remove and replace the diaphragm in the fuel pump if your engine is stalling while trying to start the chainsaw. Fit a new diaphragm into the metering section of the carburettor. Fit new gaskets onto both of the new diaphragms.
Remove and replace the fuel pickup body and main and purge fuel hoses if you haven't already from the previous troubleshooting.
Readjust the "LA" or idle screw if your saw will not idle. Readjust the "L" speed screw to accommodate the "LA" screw adjustment. Remove, disassemble and clean the carburettor parts if you haven't from the previous troubleshooting.
Clean out the jet bores thoroughly if your engine stalls when idling. Blow out each valve and replace the filters for each. Clean or replace the valve jet if idling problems persist. If idling problem, acceleration or dropping load problems persists you likely have an air leak either on the carburettor or on the engine.
Hook up the removed and reassembled carburettor to the pressure gauge. Submerge the carburettor in fuel and pump up the pressure to 11.60 psi. Watch the needle, if the pressure drops, the carburettor has an air leak. If you can't find the air leak on the carburettor, you will need to replace the carburettor.
If no air leak is present, you have an air leak in the crankcase, cylinder or around the intake manifold. Hook up a compression gauge and test the engine for air leaks.