"If you use a little common sense, it shouldn't take more than half an hour to figure out the general cause of why a small engine won't run, or why it won't run right."
That's according to Josh Potter, the corporate service manager for our dealership. To prove his point, he recently spent a day working his way through a pile of non-running lawn mowers, ATVs and snowblowers that had baffled other mechanics at one of our stores. By the end of the day, all the machines were running, or torn down and their ailments precisely diagnosed, ready for repairs.
His diagnostic procedure starts with the simple stuff: First check for fuel, then check fuel quality. Modern gasoline degrades rapidly if not used within 4 to 6 weeks. Many poor running small engines are magically repaired simply by refilling their tanks with fresh fuel.
Second step: check for spark at the spark plug. Avoid the old farmer's trick of removing the spark plug, grounding it against the cylinder head and turning over the engine to watch for a spark at the electrode. Aside from a potential explosion risk if gas fumes are leaking from the spark plug hole, the computerized circuitry of modern small engine ignitions can be damaged by that sort of crude grounding. A simple spark plug tester is the best way to test for spark at the plug. If there's no spark, work backward from the spark plug through the electrical system till the fault is identified. Also--while the spark plug is out, sniff its tip, or turn over the engine and sniff the air near the spark plug hole for gasoline fumes that indicate fuel is getting to the engine.
Third step: If lack of gas fumes from the spark plug hole hints that fuel isn't reaching the engine, remove the air cleaner and spray carb cleaner into the throat of the carburetor while cranking the motor. Forcing a flammable into the combustion cycle should make the engine at least fire, and possibly run for a moment. If it will at least fire on the flammable spray it's time to explore the carburetor and fuel system for problems. Gummed or plugged jets, stuck floats and other problems caused by poor quality or degraded fuel are prime suspects.
Fourth step: If tests have proven that fuel is getting to the engine, and that adequate spark is present, check compression with a compression tester. If compression is low, it's time to pull the cylinder head and check for damaged rings, stuck/damaged valves, or other maladies related to poor compression.
If the small engine starts but runs poorly, Potter's two rules of thumb are: "Backfiring through the intake/carb means an ignition problem--bad plug, timing problems, something out of adjustment with the ignition. Backfiring out the exhaust indicates a fuel problem--too rich, too lean, poor quality fuel."
if you've got the correct tools, all those diagnostic steps should take less than half an hour. Potter's K.I.S.S. (Keep It Simple, Stupid) diagnostic strategy works on any small gasoline engine, motorcycle, ATV, and on a lot of older, carbureted gasoline engines in tractors and vehicles on farms.