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This has worked for me on trimmers - also chain saws, etc: 1 - Identify the carb 3 adjusting screws: First is the idle speed adjustment - a mechanical stop contacting the throttle lever. Second is the low speed mixture adjustment. Often it's marked "L". (If not marked, it's the mixture screw NEAREST the engine on the carb.) Last is the high speed mixture adjustment. It usually marked "H". (If not marked it'll be the mixture screw NEAREST the air filter. Note - not all engines have an "H" adjustment - many newer ones have a fixed high speed orifice and are non-adjustable). 2 - Make sure you have FRESH gas and a clean air filter. Begin by starting the engine and letting it warm up a couple minutes. Make sure the choke is full open. 3 - Adjust the "L" screw 1/8 turn at a time until you find the spot the where the motor runs fastest. 4 - Now adjust the idle speed screw until you get your desired idle speed. 5 - Go back and re-adjust the "L" screw again for the smoothest idle. If you have no "H" screw, you are finished. If not... 6 - Quickly pull the throttle to full speed. If the motor stumbles or dies, it's too lean - open the "H" adjustment 1/8 turn at a time until the motor picks up immediately. If the motor picks up OK but smokes and runs rough at full speed it's too rich - close the "H" screw 1/8 turn at a time until the motor runs smoothly at full throttle. 7 - Finally, when you get the "H" set where you want it, go back once more and readjust the "L" screw for the best idle. Hope this helps! Bob PS - If the carb is completely out of adjusment, begin by turning the "L" and "H" screws LIGHTLY all the way in (do not force!). Then give each screw about 1-1/2 turns open. This will usually be right to get a motor started at least.
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