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My Lawnmower Won’t Start: A Troubleshooting Guide – Part 2

February 1, 2010

Part 1, My Lawn Mower Won’t Start: A Troubleshooting Guide

You have already done the ignition systems check outlined in article #1 and still your lawnmower won’t start. So what else could be the problem?

In order for an engine to run it needs ignition, fuel, and compression. So now we will check the fuel system.

A Word of Caution
Some larger, late model small gas engines utilize fuel injection. but fuel injection engines are quite complex and should be referred to professionally trained small engine technicians if the fuel injection system is suspected of being faulty. Fuel injection systems require specialized tools and training to properly diagnose problems and are best left to skilled, trained professionals.

The Obvious: Is Your Lawnmower Out of Gas?

First things first. Obviously you have ensured that the engine has ample fuel, but is it fresh. Small engines are very low compression engines, approximately 7 or 7.5 to 1 compression ratio. With this in mind at such low compression ratios, the fresher the fuel is, the easier to ignite. One of the most common problems in small engines is stale fuel.

Diaphragm Style Carburetor

On a smaller horsepower lawnmower engine with a diaphragm style carburetor. You check the level of fuel in the tank and make sure it is fresh. If it the gas smells like normal gas then the fuel is fine but if it has a different smell then it would be best to replace.

After cranking the engine a time or two, remove the spark plug and check to see if it is damp on the electrode. If not, then the carburetor is not providing fuel.

Remove the carburetor/fuel tank assembly from the engine taking care to pay attention to the linkages. While you are this far into it, go ahead and remove the engine blower housing (recoil assembly housing) and check the intake manifold pipe. Make sure it is intact and assembled tightly to the intake port of the engine. Some late model engines use a plastic intake manifold pipe which is prone to cracking.

Commonly, you remove the carburetor from the tank, clean the carb mounting area, and fuel pool on top of the tank. Replace the diaphragm or diaphragm/gasket assembly. Reinstall the carburetor/fuel tank assembly on the engine making sure to reinstall the linkages correctly. Crank the engine and on smaller horsepower units this should be all you need to do to get the engine running.

Float Style Carburetor

On small gas engines utilizing a float bowl style carburetor, the fuel system consists of the fuel cap, fuel tank, fuel line(s), fuel filter, carburetor and possibly a fuel pump.

The Fuel Cap

First check the fuel tank cap. Is it venting correctly? The fuel tank must be able to vent (take in outside air) in order for the fuel to flow. A quick check is to remove the fuel cap while listening closely. If you hear an intake of air when the fuel cap is loosened, then the cap is not venting correctly.

First Fuel Line Connection

Next go to the first connection of the fuel line (usually at the filter). Take a small container and remove the fuel line holding the open end over the container. Does fuel flow? If you have adequate fuel flow at this point, you can assume the fuel tank, fuel supply, fuel cap and first length of fuel line is o.k. If fuel flow is restricted, then your problem is in one of these components.

The Fuel Filter

Now is the time to replace the fuel filter. Then reinstall the second length of fuel line (usually between the filter and fuel pump or carburetor). Again using the container, check fuel flow at the end of the second length of fuel line. If flow is adequate, you can assume the fuel system is good through the end of the second length of fuel line.

The Fuel Pump

If you have a fuel pump, plug the fuel line into the fuel pump and take the fuel line loose at the carburetor. Crank the engine and check fuel flow at the end of this length of fuel line. If there is no fuel flow, the problem lies with the fuel pump.

You need to determine if it is a vacuum operated (impulse) or mechanical fuel pump. Vacuum operated (impulse) fuel pumps are the most common. Check the vacuum line between the fuel pump and the vacuum nipple of the engine. Common problems are cracked vacuum impulse lines caused by the heat of the engine. Replace the short length of vacuum line and check the fuel flow again. If flow is present and adequate, the vacuum line was bad. If there is still no fuel flow, likely the fuel pump diaphragms are bad and you need to replace the fuel pump.

On mechanical fuel pumps the test is the same, but there is no vacuum line. There is a plunger arm that operates the pump. After checking the fuel pump over, if no fuel flow is present, replace the fuel pump. Pump rebuild kits are generally not available, so the only option is to replace the entire unit.

The Carburetor

Once you have established that fuel flow is present (and ample) and the engine still won’t start. Then the problem lies with the float style carburetor. You can remove the fuel bowl from the bottom of the carburetor with the carburetor still installed on the engine and check for fresh fuel, dirt, etc. If there is good fuel present, but the engine still won’t run, its time to remove the carb from the engine. Remove the carb, remove the fuel bowl and check the float for damage or sticking. Then check the float valve (inlet needle valve) for damage or sticking.

If both the float and float valve are o.k. the problem is more than likely foreign material in the jets of the carb. Proper cleaning requires disassembling the carb, cleaning with spray carburetor cleaner and reassembly. It is advisable to go ahead and purchase a carburetor repair kit for the engine model and after cleaning the carburetor, reassemble it with new parts. Reinstall the rebuilt carb on the engine and test run.

At this point since you have checked the ignition system and fuel system, your engine should be up and running. If you still find that your lawnmower won’t start, the only thing left is the compression of the engine. We will deal with that in article #3.

More My Lawn Mower Won’t Start in part 3.

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