Small engine timing is completely mechanical. The coil is mounted closely to the flywheel. The flywheel is the large circle with cooling fins. The flywheel is on the opposite side of the engine from the output shaft. The flywheel has magnets mounted in it. As the flywheel magnets pass the coil they induce a current. The coil then discharges to the spark plug. The relationship of where the coil sits and where the flywheel magnets are, dictates the timing. The coil can be moved slightly to change the timing. Move the coil in the direction the flywheel moves and the timing will be retarded and opposite to advance timing. The only other factor that affects the timing is the flywheel. Occasionally the flywheel can shear it’s key on the crankshaft. The key locates the flywheel in the correct rotational spot to insure correct timing. If the key is sheared the timing can be anywhere, and the engine may spark but not run. The key is square and can be viewed from the end of the shaft. The key will have a cutout in the shaft and the flywheel. Offset keys purposely rotate the flywheel to advance timing.
Small engines are very touchy with their tuning. EPA regulations have made small engines run much leaner than they have had to in the past. This will make the engines run cleaner and be more fuel efficient, but generally don’t run as well. Also if the carburetor has any build up in it the engine will be too lean and will surge. To compound the problem most new carburetors are non adjustable due to EPA regulations. Most small engine carbs have a high speed and low speed circuit to control the fuel. When you hear an lawn related engine surging it is almost always lean. If the engine is older, the carb screws on it’s side may be adjustable. You need to locate the main circuit or high speed screw that usually has the letter H along side it. This screw is a fuel control screw that will have a spring under it. As you turn the screw out it delivers more fuel to the engine. Don’t turn the screw more than 1/8 of a turn at a time then wait a few minutes before changing again. The engine should smooth out and the setting should stay there.
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A lawn mower engine engine is relatively easy to repair. These small engines have small engine parts and are easy to move around after being uninstalled. Common Briggs and Stratton, Kohler and Tecumseh style engines come in just about every type of lawn vehicles or yard tools made. Small horsepower models all the way to appx. 20 hp v twins, are all of similar style. There are two common types of 4 stroke lawnmower engines.The L-head, which were the older style engines and overhead valve engines. L-head engines did well for their purpose but can’t keep up with tightening emissions regulations.
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