For a more specific answer, you should post the applicable elements of Make/Model/Year/Engine Model/Transmission Model/Deck Size, and hours on meter if you have one. A short description of the events preceding the beginning of the problem is also helpful.
Several possibilities for small engine backfire:
- Needs new plugs. Start with new plugs.
- Shear pin under flywheel is slightly damaged throwing off the timing. Remove flywheel examine shear pin for any damage. Just go ahead and replace it; if it even looks nicked or sliced.
- Coil, or spark plug wire, is going bad. Inside the Coil are two windings of wire and no moving parts. With shorted (melted together inside) windings you can still get a spark, but it will be weak. With open (burnt/broken) windings, you get no spark at all. There are also electronic components like diodes which may be failing. Coils are subjected to heat and vibration and do fail on occasion. Another coil consideration is the spark plug wire. Back in the day, we would replace all our spark plug wires when we gave our car a tune up; because spark plug wires do fail. The only way to replace the spark plug wire on a mower is to replace the Ignition Coil.
Note: Coils may not fail completely at first. They can be complicit in the situation where a mower starts and runs for a while.They may cause backfire. As it heats up the coil parts expand and cause the engine to shut down. It does not re-start until the machine has cooled enough to allow the coil to cool. This can be 30 minute to several hours; perhaps it starts again the next day only to repeat the failure.
Short of a well running engine, with no special equipment, the only field-test for a coil and the spark plug wire output is to visually assess the color of the spark at the end of the spark plug. Alternatively, you may use a spark tester. A spark tester gauges the strength of spark by making it jump a bigger gap. This test calls for a gap in the range of 4.2 mm (0.166 in). It's important to note that setting a test gap beyond 5.0 mm (0.200) could damage the ignition components. Briggs and Stratton manuals, for the newer engines, specifically state that you should not test for spark with the spark plug out. I do not know why this is. They specify you should use a spark tester.
B&S notwithstanding (at your own risk), with a new plug attached to the plug wire hold the threaded part against a ground point or wrap some bare copper wire around the threads and fasten the wire to the engine frame or other good ground. Make your shop or garage dark so you can see the spark easily, then crank the engine. The flywheel must spin rapidly (at least 350 RPM). The strength of the spark is revealed in the color. A red or yellow spark is weak and probably will not spark in the cylinder. A blue or white spark is strong and has enough voltage to fight across the spark plug gap even under pressure within the cylinder.
Note: a good strong spark also makes a strong snapping noise as it fires.
- Needs a valve adjustment or a valve job. This is high on the list of the most common reasons for backfire. Start with adjustment.
- The other thing that causes backfire is fuel mixture too lean. On a small engine this is not usually an adjustment problem. It may be the fuel system/carburetor is dirty or partially clogged. It may also be an air leak in the gaskets of the intake manifold. It might be low fuel pressure from a bad pump or clogged fuel lines.The electronic fuel shut off valve may be malfunctioning. If nothing else solves the backfire, check fuel flow through the pump and lines, remove the carburetor and clean or rebuild it. This is the least likely of the options so save it for last.
Here is a link to a great essay on magnetos and spark plugs.
Village Science Magneto Spark Plugs
For Twin Cylinder Engines
A power balance, or cylinder balance, test should be run on a twin cylinder engine. If compression is good and cylinder leakage is minimal (in other words if the engine is not worn out, doesn't have too many hours), but a cylinder is misfiring or shows up weak in a power balance test, it indicates a fuel delivery or ignition problem (fouled spark plug or bad plug wire).
If an engine is hard starting, runs rough, misses or lacks power, a cylinder balance test will determine if both cylinders are operating to their full potential. You will need an engine tachometer, a spark tester, and an insulated screwdriver.
- Using the spark tester, with the plugs in, start engine and verify spark is equally good on both cylinders.
- Remove spark tester.
- Connect engine tachometer
- Run engine at top no load speed. Note rpm.
- Ground one cylinder spark plug with screwdriver. Note rpm.
- Ground other cylinder with screwdriver. Note rpm.
The difference in RPM between the cylinders should be less than 75 rpm. If the difference between the two cylinders is greater than 75 rpm, the cylinder with the least rpm loss is the weakest and is a potential problem.