Is the fuel fresh?

Is the fuel tank empty?

Is the shut-off valve closed?

Is the fuel diluted with water?

Is the fuel line or inlet screen blocked?

Is the fuel tank cap clogged or un-vented?

Is the carburetor blocked?

Is the engine flooded?

Is the fuel solenoid (if equipped) functioning properly?

Is the spark plug fouled?

Is the spark plug gap set incorrectly?

Is the spark plug lead faulty?

Is the kill switch shorted?

Is the flywheel key damaged?

Did the engine stop all-of-the-sudden after striking an object?

Fuel can become gummy inside your engine when left standing over long periods of time, especially in the cold. Once certain parts of the engine are gummed up, the engine must be disassembled and cleaned in order to function properly.

The best technique is to run your mower dry after that last mowing job. If you have to drain the tank, put the old gas into your car’s gas tank; it will not hurt the car and is probably the best environmental approach to dealing with waste gas.

Most small engines have a single paper filter element. The filter provides engine protection, but when clogged, it prevents the engine from breathing. Air filters are relatively inexpensive and easy to replace.

The best approach is to keep a spare air filter on hand and replace per the manufacturer intruction manual. In a pinch, a dirty air filter can be gently cleared of major obstructions with compressed air or a small brush.

Generally, plan on replacing the filter every two years - more often for heavy use.

Running a four stroke engine with low or no oil is a sure way to cause serious damange. Most lawn mower and small engine manufacturers recommend changing the oil annually - usually in the spring. For lightly used tools, the oil change can be extended to every other year but no longer.

Keep an eye out for significant oil loss. If oil is leaking, it will combine will dust and dirt to form an oily crust on the engine.

The most traditional of mowers don't pollute—and you won't need to store gas or a power cord. Pushing them turns a series of curved blades.

Because they have no engine, they're also quiet and inexpensive, relatively safe, and require little upkeep beyond blade adjustments and sharpening.

They only cut swaths are 14 to 18 inches wide, cutting tends to be uneven, and most can't cut taller than 1½ inches or trim closer than 3 inches around obstacles. They don't disburse clippings like a rotary mower, so you'll need a bag (or a rake) for the clippings.

Electric walk-behind mowers are good for small, level lawns. They use an electric motor to turn a rotating blade, but not to drive the wheels; you provide the push.

Cordless mowers weigh up to 30 pounds more than corded models, but they can roam farther from a power outlet and free you from a cord. Cords are a potential problem on lawns with trees and other obstacles. Today's cordless models run longer per charge than previous models.

Cord and cordless types start with push-button ease, produce no exhaust emissions, and require little upkeep beyond blade sharpening. Most offer a side or rear bag and a mulching mode that cuts clippings finely enough that they settle within the lawn and fertilize it as they decompose.

The best electric mowers perform as well as gas models, but they can't match the best gas machines in tall or thick grass and weeds. Electrics typically cut an 18- to 20-inch swath.

Gas mowers come in push and self-propelled models. All have a four-stroke engine with a 160- to 190-cubic-centimeter displacement, a measure that has replaced horsepower ratings on many models. Some also list torque, or twisting force.

Most cut a 21- or 22-inch swath, can handle long or thick grass and weeds, and can bag, side-discharge, and mulch clippings.

Self-propelled models are best for most lawns and blend ease and performance. But gas mowers are noisy and produce exhaust emissions—though today's models emit less than those of the past—and the engine requires regular tune-ups and oil changes..

These front-engine machines often cost less than the older rear-engine riders.

Most mow a 42- to 48-inch swath and can bag, mulch, and side-discharge clippings. Some cut an even wider swath and offer four-wheel steering for tighter turns. All accept snow throwers and other tools.

These machines typically create significant exhaust emissions and require about 4-by-6 feet of storage space. Add-on features are often pricey and hard to install.

These riding mowers are similar to the ones landscapers use, with a rear-engine and rear-wheel steering. Maneuverability is their strong suit.

Twin steering levers let you power the two rear wheels individually; with one wheel in forward and the other in reverse, they can turn circles in one place. They can side-discharge, bag, and mulch clippings.

They usualy mow a 42- to 48-inch swath. But they cost more than most tractors and typically don't cut as well. Their rear-steering wheels can tear up grass during turns.

They can lose traction and be hard to steer and control on hills, and their lever controls for steering and ground speed require practice. For a large lawn with lots of obstacles, consider a four-wheel-steer tractor, which steers via a familiar steering wheel.