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Lawn Mowers
A CHECKLIST for dealing with gas-powered lawn mowers that won't start. In general, malfunctioning engines show symptoms that are easy to identify. Follow these instructions and you should be able to find the solution. However, some symptoms are so obscure that it takes a professional to spot the problem.
Note: Dirt is a major problem with mower engines because they work near ground level where spinning blades generate a whirlwind of dust. When working on your mower inspect all parts to make sure they are clean enough to operate properly.

If your engine isn't starting, check these common problems first:
Control Settings. Are the engine-throttle controls working properly?
• Move the control lever from "start" to 'fast" to "stop," checking to see that the cable properly operates the throttle. The cable can be slipping in the clamp just enough to cause the throttle to malfunction.
• Move the throttle control to its open, or "start" position. Or push the throttle toward the front of the engine. If the cable is slipping in the clamp, tighten the clamp screw with a screwdriver after you push the cable forward toward the engine until the throttle stops. The throttle is now in a fully open position. Set the handle control lever on 'fast" or "start' and tighten the clamp.
• If the engine starts, let it run several minutes, then pull the control lever to "stop." If the engine slows but does not stop, loosen the cable clamp and pull the cable just a tad toward the control lever until the engine stops. Then tighten the clamp.
• Go through this sequence until the mower stops and starts on command of the control lever.
Fuel Problems. Does the engine have fresh fuel in the fuel tank? An empty tank and dirty gas are two big troublemakers for engines that won't start.
• Remove the gas in the tank and dispose of it in a safe place, even if you think the fuel is good.
• Refill the tank. If you are just starting the mowing season, start with new gasoline. Last year's gas can be weak.
Spark Plug Wire. Is the spark plug wire attached tightly to the plug? Spark plug wires, through engine vibration, can become loose.
• Pull the wire off the plug. On the end of the wire you'll notice a thin metal "socket" which goes over the plug terminal.
• Lightly crimp the metal socket with pliers. e Replace the socket on the plug terminal.
Kill Switch. Is the kill switch or spark plug grounding device turned off?
• If the switch is off, check the kill switch wire to see if it is loose or faulty.
• Older mowers have a metal flap that is pressed over the spark plug to stop the engine. Check to see if the flap is off the plug.
Fuel Tank Cap. Is the vent on the cap working properly?
• Remove the cap on the fuel tank.
• Check the pinhole in the cap to make sure it is open. If not, use a paper clip or piece of wire to open this port.
• Try starting the engine. Still won't start? Leave the cap off the tank and try cranking the engine. If the engine starts, the cap is faulty. This sometimes happens on brand new engines. Replace the cap. Do not run the engine without the cap on the fuel tank or try substituting the cap with a plug, such as a piece of cloth.
Engine Choke. Is the choke operating properly? It should be closed while attempting to start the engine.
• Make sure the choke is not stuck or incorrectly adjusted.
•Some engines have a soft rubber primer. The primer must be pushed firmly four or five times when the engine is cold to fill the carburetor with fuel. Do not press the primer if the engine is hot, as you may flood the carburetor.
Flooded Engine. Is the engine flooded?
• Put the throttle control in the "stop" position.
• If you have a manual choke, open it wide.
• Pull the starter rope (or crank the engine) at least 6 times to help clear excess fuel from the system.
• Then put the control lever on "start" and crank the engine normally.
• If it still won't start, remove the spark plug and dry it off. Return the spark plug and try again.
• If that fails, turn the needle valve or mixture screw /, turn counterclockwise, and crank the engine. If no luck, turn this screw another quarter turn. Try cranking. If it won't start, turn the screw clockwise 1 full turn, and try cranking the engine.

The following checks are more detailed than the previous ones. We suggest that you put the lawn mower on a work table and look more closely for troubles.
Lack of Electricity. Is the engine getting electricity? You need a helper to determine this.
• Remove the spark plug wire from the spark plug.
• Have a helper pull the starter cord while you hold the wire's metal plug lead about 1/4 inch from the top of the spark plug. You should see a tiny spark between the plug lead and the plug.
• If you see a spark, go to Step 4. If you don't see a spark, or see an intermittent spark go to Step 6.
• Remove the spark plug and check it. If the spark plug is fouled with deposits it may not be sparking. Replace it with a new plug. Normal deposits are tan or light gray. Other deposits can indicate a variety of engine problems. Check with your local lawn mower or small engine maintenance store. If you are at all in doubt about the spark plug, replace it with a new plug. They are not expensive.
• If the plug looks fine, put it back, ground the plug wire on the engine away from the spark plug, and crank the engine. Then remove the spark plug. It should smell like gasoline and may be damp. If not, the engine is not getting fuel. Check to see that the fuel lines are clear.
• Carefully examine the spark plug wire from the metal end to the coil. Bend the wire in your fingers and look for broken and cracked insulation. These cracks are sometimes difficult to see, so look closely as you twist the wire back and forth.
• If the wire is broken, replace it. You can have a professional replace the wire, or you may be able to replace the coil in which the wire is secured. Coils are not expensive.
Cutting Blade. Is the cutting blade loose?
• Empty the fuel tank.
• Remove the spark plug wire from the plug and tuck it away from the spark plug.
• Tip the mower over so the hole where you add oil to the engine faces upward.
• Try to wobble the blade. Some lawn mowers can't be started if the cutting blade is wobbly or there is no cutting blade attached.
• If the blade is loose, tighten the blade bolts with a wrench. If the bolts can't be tightened, have a pro make the necessary repair. Do not operate the mower until the blade is properly tightened and secure on its mounting.
Air Filter. Is the air filter clean?
• Remove the cap holding the air filter on top of the carburetor with a screwdriver. Remove the filter.
• If it is made of paper and dirty, replace it.
• If the filter is foam-like a kitchen sponge-wash it in dishwasher detergent and water, then rinse out all detergent and dry the filter.
• Soak the filter with clean engine oil, squeeze out the excess oil, and replace the filter.

If you still can't get your mower started, and you own one with condenser and points (electronic ignitions do not have these parts), chances are that the condenser and/or points need replacement. You can buy replacement parts in many home centers and garden outlets. Some kits have instructions on replacing the parts. If not, follow these directions:
Removing Breaker Points:
• Gather the tools you'll need, including a wrench set, hammer, flywheel holder or belt, pliers, screwdriver, flywheel puller, feeler gauge, and the replacement parts.
• Disconnect spark plug wire and ground it to the engine away from the spark plug.
• Remove the engine cover.
• Remove the screen that covers the flywheel.
• Remove the clutch assembly (if unit has one) and the flywheel nut. Use a belt or flywheel holder to keep the flywheel in place while unscrewing the nut.
• With the flywheel nut off you will see the flywheel key in a slot between the flywheel and the crankshaft. If the key is bent, broken, or worn, replace it.
• Remove the flywheel using a flywheel puller, which can be rented or bought. Do not pry up on the flywheel as it is pot metal and will easily break. Lift off the flywheel. Under it you will see a small metal covering. Unscrew the covering to expose the points and the condenser.

Check the points. They need to be replaced if they don't open when the crankshaft is turned.
Note: First check that the points locking screw is not loose, are pitted, or have a metallic residue. (The residue can be sanded down to allow for temporary use in an emergency.)
• Remove the condenser clamp, condenser, and breaker points, noting where electrical system wires attach. As the screw is removed from the moving point, the spring and point will lift off.
Installing Breaker Points. There are two types of point systems. In one, the fixed breaker point is part of the condenser (Fig. 3). In the other, both breaker points are separate from the condenser (Fig.4). The following instructions explain how to install and adjust the points in each system.

Single Condenser/Breaker Point
• Lightly clamp the new condenser in place.
• Slip the ignition coil and stop switch wires into the hole between the condenser and breaker point. A spring holds the wires under tension. There is a plastic tool in the kit to hold back the spring tension while the wires are threaded into the hole.
• Install the new moving point, remembering to reattach the other electrical system wires. Fasten the spring to the arm of the point and the spring post.
• Move the condenser forward so it touches the tip of the moving point. Turn the crankshaft via the blade until the moving point opens to its widest gap.
• With the points set at the widest gap, insert a 0.020 in. (0.5 mm) feeler gauge between them and adjust the condenser until the points just touch the sides of the feeler gauge. Tighten the clamp on the condenser.
Reassemble the engine.

Separate Condenser and Breaker Points:
• Clamp the new condenser in place.
• Install the breaker points, remembering to reattach the electrical system wires. Fasten the spring to the arm of the point and the spring post.
• Turn the crankshaft via the blade until the moving point opens to its widest gap.
• With the moving point set at the widest point, slightly loosen the stationary breaker point mounting screw. Insert a screwdriver into the pry slot of the breaker point bracket to adjust the point gap. Slide a 0.020 in. (0.5 mm) feeler gauge between the two points. Both should just touch the sides of the feeler gauge. Tighten the stationary breaker point mounting screw.
• Reassemble the engine.

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Updated: 02/2018   copyright 2012 U-Repair.com