If your Stihl weed trimmer starts to smoke, you should stop the engine immediately as this is generally a serious problem; do not try to run the trimmer until you have fully addressed the problem and fixed the broken parts. If you cannot find the cause of your problem, you will need to take it into a Stihl service dealer to get it fixed.
Once the fuel gets ignited in the chamber, all of those heated exhaust fumes must get pumped out of the engine to keep from overheating. These fumes pour out of the exhaust port and then the muffler. After several seasons of use, thick layers of carbon can build up around the walls of the muffler and around the opening of the exhaust port. If this carbon gets reignited by new exhaust fumes, your muffler and engine will start smoking. You must follow the cleaning schedule for your Stihl as you should clean out the muffler with a brush and rag every 30 to 40 hours of use.
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Improper Fuel Mixture
The fuel mixture on a Stihl weed trimmer is 50:1 gas to oil. If you've measured this mixture wrong, your fuel may be causing your engine to smoke. If it's smoking, it means you've added to much oil to the mixture, causing it to be too rich for the engine. When the mixture is too rich, the excess oil will get burned off in the internal engine and start smoking, but it's better than too lean of a mixture, which will cause your engine to run too hot and can blow out your piston. Immediately dump out any bad gas and remix the fuel according to the specification in your model's operator's manual.
Leaking Gas or Oil
The mixed fuel used in the Stihl two-cycle weed trimmers contains gas and oil. This gas must flow from the tank to the carburetor and from the carburetor to the cylinder and then back to the tank. If a faulty gas hose is leaking or isn't connected properly, a small amount of gas may leak out and hit a heated area of the engine. This gas will burn and cause the engine to smoke. You must replace the fuel lines every season and, if you reinstalled, you need to use a press fluid to help the lines stick to the ports on the carburetor.
Gradually fuel impurities will build up in the walls of the carburetor, suffocating the engine. If the carburetor gets really dirty, gas will come out in other places, hitting the engine and burning. Gas must mix and flow through the carburetor unobstructed. You will need a Stihl service mechanic to clean and service the carburetor to stop the smoking.