Why Won't That Lawnmower Start?

Startup of an engine basically requires two things, fuel and a spark. If you aren't getting a spark at the plug or fuel isn't getting through to the engine, the machine will never startup. It is essential however that a proper startup procedure is followed when your lawnmower won't start.

IMPORTANT!!!

When working on a mower, remember that gasoline/petrol is inflammable. Ensure that there are no sources of ignition such as naked flames, stoves, sparks, cigarettes or other hot objects in the vicinity when you are checking the fuel tank or draining the float bowl. Adequate ventilation will prevent build up of vapor. Don't leave rags moistened with gas lying about as they could possibly ignite.

Before working on the underside of an engine, ensure the controls are set to off and that the spark plug is removed and the engine has cooled down. There is the possibility that if there is vapor in the cylinder, a hot engine could give a "kick" if the blade is turned during removal or for cleaning the deck and although the engine may not run, this would be enough to chop off fingers or a hand!

If the mower is placed on it's side for cleaning or removing the blade, keep the carburettor high side up. Otherwise engine oil will run through the breather pipe into the carburettor, fouling it and possibly destroying a paper air filter if fitted.

• Use new fuel, old fuel can make starting difficult.
• The throttle should be set in the full revs position. Often a mower will have a position for starting. This turns on the choke. Position the control at this setting when starting a cold engine.
• If the engine has a primer button, press it in about 5 times.
• Pull the starter rope out until you feel resistance. Allow it to return gently and then pull the cord out sharply. This should be repeated until the engine starts but not more than about 4 times.
• If the engine hasn't started at this stage, push the priming button another few times and try to start the engine again by pulling the cord several times. If it doesn't start, it may have flooded so leave it for a while for fuel to evaporate.
• Once the engine has started, switch the throttle from the start to run position.

• Don't use old gasoline which can cause difficult starting
• Is there enough gas in the tank?
• Has the spark plug lead become disconnected?
• Has the vent in the fuel tank cap become blocked?

• Socket Wrench Set - Inch sizes for American engines or metric for European or Japanese engines. You don't need to spend a fortune on a set as you are not going to be using them everyday, but don't buy rubbish either as you can use them for working on other equipment. The alternative is to buy a set of combination wrenches, open at one end and box at the other end. These are also known as ring spanners in the UK. You shouldn't need anything greater than 3/4 inch or 19mm

• Plug spanner - For removing spark plugs

• Screwdrivers - Flat head and Philips, big and small sizes

The cable feeding high voltage to the spark plug should be checked first. This must be attached tightly and not loose. Sometimes the cable can pull out of the rubber boot which pushes onto the spark plug. Using a spark plug wrench, remove the plug. Place it in contact with the engine block to ground it. Pull the starter cord and check there is a spark. You may need to hold the plug tight against the engine somehow as it may bounce around when you pull the starter cord. If you don't get a spark, there might be a crack in the plug insulation or it could be dirty and need to be cleaned with an old tooth brush and some gas. Allow it to dry and try for a spark again. If you are still not successful, try another plug and check again. There is no harm having a spare plug for this purpose or use a plug out of a car. Don't actually use this plug in the engine unless it is the correct type as it could cause piston damage if it is too long.

If you still can't produce a spark, there could be a problem at the kill switch. This shuts of the engine when the mower is turned off. You may have to remove the cowl from the top of the engine to check this.

Inspect the wiring on the engine. Damaged insulation can short voltage to the chassis. All connections should also be checked to ensure they are not loose. Spades can pull off and screws holding ring crimps can work their way loose from vibration.

• The air filter must be clean for optimum engine operation. Foam elements can be washed in hot water and detergent to remove oil and dirt. They can then be squeezed dry in a cloth. Re-oil with about a teaspoon of oil distributed evenly throughout the foam. It is possible to clean a paper filter by blowing from the inside with compressed air. These type of filters however should ideally be replaced.
• A kinked or blocked fuel line can restrict fuel flow to the engine.
• Next check the float bowl. The float and bowl work like a toilet cistern, keeping the fuel at a constant level so that it can be sucked up into the carburettor. Firstly, if a gas tap is fitted, turn this off. Loosen the screw or bolt holding the bowl and hold a container underneath to catch any fuel. Fully remove the bowl. If no tap is fitted on the tank, the fuel should flow freely, otherwise turn on the tap and check for good flow. Gently press the float upwards and ensure it cuts off the flow. If it doesn't, debris may be blocking the needle valve. Also the float may be punctured and not able to float properly. Both of these conditions will cause flooding of the carburettor and possibly fuel flowing out of the air filter. If no fuel flows out of the needle valve when the float bowl was removed or the fuel tap turned on, there could be two causes. Firstly the the fuel filter might be blocked. This may be inline or in the tank. The second cause is a blocked valve letting fuel into the bowl. This is less likely as the fuel filter should remove dirt from the fuel. In any case the valve is likely to be a brass insert screwed into the carburettor which can be removed. Finally remove any debris in the filter bowl.
• If fuel flows freely and you have a spark, there might be a dirty jet in the carburettor and the latter will have to be removed. It is usually pushed onto the engine and held with a clip or held on with a flange arrangement and screws. Once you have taken it off, the carburettor can be cleaned with a proprietary spray or compressed air. Don't use wires to poke at the jets as the soft brass surfaces can be damaged.
• If the fuel tank is lower than the carburettor, there is likely to be a diaphragm fuel pump which can become cracked,punctured or inflexible and need to be replaced.

• Check the throttle cable is actuating the throttle linkages. These can sometimes get seized or obstructed by grass or small twigs.
• Ensure that the governor linkage isn't keeping the throttle closed. With the air filter removed and using a torch you can determine the position of the throttle and choke valves. These are plates which open or close in the intake. The outermost plate is the choke which fully obstructs the airway when on. The throttle plate will create least obstruction on full throttle and the governor linkage shouldn't be closing it before the engine starts.
• The carburettor should make a good seal with the engine by being screwed or clamped on tightly and any gaskets shouldn't be worn.