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The engine runs rough under a load

Any engine related problem that can be described "runs rough" can have a variety of common causes. Common Causes: Dirty air filter Weak ignition spark Worn or fouled spark plug Improper carb adjustment Valves needing adjustment If a full tune up (including valve adjustments) has been performed recently and the condition is only noticed when the engine is under a work load, the problem is likely to be caused by trash in the carb or an improper adjustment. If equipped, the main jet is most often located on the bottom of the carb. Note that most newer engines will not have an adjustable main jet. Basic Main Jet Adjustment

With the engine warmed up and running at the recommended idle speed, locate the main mixture screw. Step 1. Slowly turn the main jet adjustment screw in until the engine starts to miss or the RPM just starts to "fall off".

Step 2. Turn it back out until the engine again starts to miss, hesitate or "fall off".

Step 3. Finally, again turn the mixture screw back in clockwise until you reach approximately between both extremes.

Step 4. - Now put the engine under a normal work load to see if the problem has been corrected and make any final adjustments by turning the mixture screw no more than 1/4 turn (in or out) to reach the optimum adjustment.

If an adjustment doesn't help or if none of the other potential problems are found, you'll most likely need to clean the carb and fuel system.