Small-Engine Diagnostics

February 26, 2010 09:53 AM

Ever have a planter, sprayer or combine sit idle because a small gasoline-powered engine on a transfer pump or auger refused to start? Five simple steps can often help diagnose small-engine problems.

"If you use a little common sense, it shouldn't take more than half an hour to figure out why a gas-powered engine won't run or why it won't run right,” says Josh Potter, corporate service manager for Van Wall Equipment in Madrid, Iowa.

1. A fresh tank of fuel cures many problems. "Gas more than six or eight months old may not start as easily, or may sputter or miss a little once you get it running,” says Chuck Barnes, a fuel salesman with Heartland Co-op in Des Moines, Iowa.

If fuel quality is good, Potter checks the fuel flow to the carburetor. Plugged screens at the fuel tank outlet, obstructions in the fuel line, a plugged fuel filter or gummed/plugged needle valve(s) in the carburetor are obvious problems but often tricky to diagnose. Loosen the drain plug on the carburetor float bowl to check for fuel flow from the tank to the carburetor.

2. Check for fuel from the carburetor in the cylinder. To see if fuel is getting to the combustion chamber, crank the engine a few times with the choke on, then remove the spark plug and sniff the tip. A gasoline odor implies that a little fuel is getting to the cylinder.

If fuel flow to the combustion chamber is suspected, remove the air cleaner and squirt carburetor cleaner into the carb intake while cranking the engine. Forcing a flammable aerosol into the combustion cycle should make the engine fire and run for a second or two. If the engine fires and attempts to run on carb cleaner, and it's been proven that fuel is getting to the carburetor, check for plugged jets, stuck floats or other carb-related problems.

3. Check for spark at the spark plug. Spark plug testers available at most auto parts stores allow plugs to be tested safely. If there's no spark at the plug, work backward through the plug wire, the coil and the ignition module to diagnose the problem.

4. Check engine compression. According to Joe Loss, technical support staff member with Briggs & Stratton, small gasoline engines need a minimum of 85 psi cylinder pressure on the compression stroke to start and run properly. If tests prove fuel is getting to the engine and the spark plug is firing, check engine compression.

"If you don't have a compression tester, put your thumb over the spark plug hole and crank the engine,” Loss says. "You'll have a little trouble holding your thumb on there if compression is good. If there's only a weak puff of air out of the spark plug hole when the piston comes up on compression [stroke], you'd better check for damaged rings, a stuck valve or something that's killing your compression.”

5. Listen for the pops of backfiring. "Backfiring through the intake or carb means an ignition problem,” Potter says. "Check for a bad spark plug, timing problems, something out of adjustment or wrong with the ignition.

"Backfiring out the exhaust indicates a fuel mixture problem: too rich, too lean, poor quality or old fuel. Try a tank of new fuel, then adjust the fuel mixture screw. Never overlook a clogged air filter. Always try the easy stuff first.”

You can e-mail Dan Anderson at

Back to news



Spell Check

No comments have been posted to this News Article

Corn College TV Education Series


Get nearly 8 hours of educational video with Farm Journal's top agronomists. Produced in the field and neatly organized by topic, from spring prep to post-harvest. Order now!


Market Data provided by
Brought to you by Beyer