The 3 most common part(s) or condition(s) which cause the symptom Small engine runs poorly are listed below starting with the most likely. Check or test each item and watch any available videos.
RepairClinic Item # 1727769
If the small engine runs poorly, the carburetor might be clogged or have bad fuel in the float bowl. If old fuel was left in the small engine for a long time some of the volatile ingredients may have evaporated, leaving a thicker, stickier product that is more like varnish or shellac. This sticky fuel can clog up the small jets and ports in the carburetor and it can be difficult to burn making the small engine run poorly. The only solution is to drain the old fuel from the float bowl and thoroughly clean the carburetor with carburetor cleaner. If that isn't effective, replace the entire carburetor.
RepairClinic Item # 1863243 Aftermarket (Non-OEM) part
If the small engine runs poorly the spark plug might be defective. All small engines have an ignition coil. The coil is the device that provides spark for the spark plug(s). If the coil is bad power won't get to the spark plug(s). The coil is essentially a small, induction generator. It is mounted next to the flywheel. The flywheel has one or more strong, embedded magnets. As the flywheel spins, the magnet(s) zip past the induction coil, this action of a magnet passing past a coil induces a voltage. Depending on the design of the coil and the strength of the magnets, a stronger or weaker spark can be created and delivered to the spark plug(s). Each time the flywheel spins around a spark is generated. The best way to test a spark plug is to use a spark plug tester. The spark plug should have a strong, visible spark between the electrodes when the engine is cranking. If it doesn't, replace it.
RepairClinic Item # 1659173
If the small engine runs poorly the flywheel key might have sheared in half or partially. The flywheel key is a small metal shim which fits into a slot in the crankshaft and engages with the flywheel. If the engine stops abruptly due to a problem with the equipment the flywheel key may shear. The only way to know for sure if this is the problem is to remove the flywheel and inspect the flywheel key.