Learn to Fix your Lawn Mower: Fix Majority of No-Start Lawn Mower Issues Pertaining to Fuel, Yourself!

In most instances where a lawn mower fails to start, the issue is related to "fuel delivery" problems. For more information, see the previous post titled: Why Won't My Push Mower Start? Solution: Mind your Gas! As highlighted in this post, two-thirds of all service requests for small engine equipment stem from fuel-related issues. It's recommended to familiarize yourself with both preventive measures and solutions to handle these problems on your own. The following article will help you do just that. Continue reading to learn how to fix your lawn mower.

Begin by inspecting the fuel tank. Is there fuel inside? If the fuel is over a month old, consider replacing it by safely draining and refilling with fresh fuel. Dispose of the old fuel through your local hazardous waste facilities. Pro tip: If you need to tilt the mower to drain the fuel, also perform an oil drain to prevent tilting the unit incorrectly and contaminating the air filter with oil or gas. For guidance on properly tilting a lawn mower, visit this post: How to Properly Tilt Your Lawn Mower On Its Side: Your Probably Doing It Wrong!

Lets assume that the gas was old or its age unknown, and you had to tilt the mower to drain out the gas. You heeded the advice above and drained out the oil as well, either by removing the drain plug or tilting the mower to drain oil out of the dipstick tube. The following unit is not equipped with an oil drain plug, instead you simply drain the oil by tipping the unit, or siphoning out the old oil using an appropriate fluid extractor. It is advisible to drain oil first, then remove the old gas.

First step and probably most important! Check for clean fresh gas!

Now that your unit is free of fluids (those being oil and gas) it is safe to tilt the machine however way necessary and without concern of contamination.

Now we can begin the carburetor service. Most likely if the machine had gas stored in the tank for an extended period of time, gas was also sitting inside the carburetor. The problem is, when gas is left in any small engine for extended periods of time, it tends to form a varnish, and block necessary ports and holes required to properly start, and run the machine.

There are several carburetor styles for various lawn mower, brands, configurations etc. Follow these general tips before you attempt to tear apart your unit's carburetor. The examples in this article pertain to Briggs and Stratton engines, however the general troubleshooting procedures and methods can be generally applied to all walk-behind lawn mowers.

1: Before disassembling any linkages or springs connected to your carburetor, take a picture of the correct setup of your machine for future reference during reassembly! During each step of disassembly ensure you take pictures of gasket and O-Ring locations, especially if your unfamiliar. This can prevent improper reassembly and lots of headaches! Be gentle handling O-Rings, gaskets, etc. If in good condition, these items can be reused.

2: Determine what style carburetor your unit is equipped with. Look for a primer bulb or auto choke linkage running to a thermostat near the muffler.

3: Is your carburetor connected to the gas tank above it or below? If below, you have a gravity-fed bowl style carburetor (it may be equipped with a primer bulb). It will have a length of fuel line, running from the bottom of the fuel tank to the inlet port of the carburetor. If the carburetor is situated above, it will most likely have a primer bulb and will be mounted directly onto the gas tank. There will not be any fuel line present.

Pro tip: If your unit is indeed equipped with a primer bulb, and when pressed does not make any discernible noises as if moving gas and or air, you can be assured the carburetor needs cleaning! Or inspect the bulb itself for tears. It should return to a rounded shape after being pressed, not remain flattened and pressed in, after released.

For bowl style carburetors, located below the fuel tank: After removing the bottom bowl nut, a good practice is to now inspect your fuel source (tank, fuel lines or in-tank fuel filters, if so equipped) for potential obstructions. Be sure to not lose that bowl nut! What can happen is blockages can form in the tank inlet screen filter, or the fuel line may collapse internally blocking and starving the carburetor of gasoline, and causing a no-start. To check for these, with the bowl nut removed from the carburetor, add a small amount of gasoline to the empty tank. Have a small container to catch the gasoline as it travels through the tank, through the fuel line and into the area of the carburetor, where the bowl would be (but has been removed for this test). Does gasoline flow easily to this area? Is it a slow trickle or is no flow present? If no gas flows, then you have an obstruction at either the small inlet screen in the fuel tank or in the fuel line itself. Replace bad fuel line and clean the fuel tank.

Within the metal bowl (or plastic bowl on newer models) is a plastic device called a float. Inspect this float, and if you see gas inside of it, it must be replaced. If gas gets inside it will prevent the float from properly raising and dropping under its own weight as it should. You will be able to lightly press it upwards and it should create a seal when its needle valve meets the seat, located within the carburetor body. It will drop down on its own when released. To test these, again put in a small amount of gasoline into the fuel tank and when you see gasoline coming out of the fuel line, gently press up on the float. Does it completely prevent gas from flowing into the area where the bowl would be? If so the needle and seat are functioning as they should. If a small amount of gas gets by the needle valve, while gently being held up, then you must look to clean the needle valve itself and its seat, and inspect the needle valve if it is rubber tipped for pitting or deformation. Any small pieces of debris will prevent the conical tipped needle valve, from properly seating in the seat of the carburetor body. It should look like a uniform conical point, not pitted or rounded at its tip or irregularly shaped. If it is, it must be replaced.

The next step would be to remove the carburetor from the intake manifold of the lawn mower, they are usually held on with two bolts. Next using carburetor cleaner and or compressed air, gently blow into each hole of the carburetor until air and or carburetor cleaner visibly flows through it.

Pro Tip: Ensure that when using compressed air and carburetor cleaner that you wear safety glasses and wear appropriate nitrile gloves to prevent unnecessary skin exposure to gasoline. Never blow onto yourself when using compressed air.

Next inspect the carburetor bowl nut. Within it are small holes or (jets) as well. You want to insure that light can pass through these small holes. They can run up through the bowl nut, and across its width. This insures that gasoline and air mixtures can move freely throughout the carburetor. When old, stale gas sits in units for prolonged periods, these small holes get obstructed and block this necessary, flow of fuel. Hold the cleaned bowl nut up to a light source. Does light pass through these small holes? If so, they are sufficiently clean.

Next after referring to your pictures you took prior to disassembly, reinstall the carburetor in the reverse of disassembly, ensuring that all bolts are started by hand first, before using a tool to tighten, to prevent cross-threading, and reinstall all necessary O-Rings and gaskets. Tighten to a reasonable torque, for this application gently, hand-tightened, using a 1/4 inch ratchet is more than sufficient.

If reassembling a metal bowl style carburetor, ensure you've properly reinstalled the bowl O-Ring, where the bowl meets the carburetor body. If you add gas and it begins to leak, do not try to remedy the leak by further tightening the bowl nut. Instead, ensure that the O-ring is properly seated and in good condition itself. Overtightening the bowl can deform it and cause it to not seat properly on the carburetor body further causing a leak. Usually, leaks of this nature can indicate a problem with the needle and seat not sealing within the carburetor. Or, the float has become full of gas itself.

Pro Tip: If your lawn mower is equipped with a plastic carburetor, the main jet is not located on a lower bowl nut. Instead, the main jet it resides as another plastic piece inside the carburetor, and it needs to be gently pried out using a small flat head screwdriver. This plastic main jet will have at least two small ports in it that would require cleaning using the compressed air, or carburetor cleaner method, outlined above.

The above carburetor style is situated above the gas tank; there is no fuel line present, instead a red primer bulb, when pressed, is used to suck up gasoline through a pick up tube within the carburetor.

Follow the above instructions for carburetor tear down, however there is no needle valve and seat, or carburetor bowl to contend with. Instead there are screws that are threaded directly on top of the gas tank.

Before you can access this carburetor, you must remove the air filter housing and in doing so, inspect this foam-style air filter for cleanliness. If dirty, soak in dish soap, and air dry before reassembly. Ensure you saturate in clean engine oil and hand squeeze out excess before reassembly. Refer to this post on the importance of air filters here: How's Your Engine Been Breathing Lately? Air Filters and Engine Maintenance Tips for Lawnmowers to Live By!

This style carburetor relies on a diaphragm and gasket, ensure you take pictures before disassembly if you are unfamiliar with their order and orientation. If they are stiff and brittle it is best to replace these. Same as with all carburetor cleaning procedures, using compressed air and carburetor cleaner, ensure air and carburetor cleaner passes thorough all holes. This style carburetor is equipped with an emulsion (or pickup tube) ensure its tube screen is cleared and unobstructed. After reassembly, gently tighten all the screws holding down the carburetor in an alternating pattern to ensure even tightness. Do not over-tighten. Ensure gasket and diaphragm lines up with screw holes to prevent accidental tearing during screw down.

After servicing your carburetor, ensure that if you did drain the oil as suggested, to fill it with the appropriate weight oil as outlined in your engine's service manual. If you do not have access to this, it is advisable to use SAE 30 weight oil for summertime use equipment such as lawn mowers. It is desinged for the higher operating temterature of small engines. However, 10W-30 and multigrade full-synthetic 5W-30 are also acceptable alternatives, for most modern small engines. Again, consult your manufacturer's engine service manual for specific details.

Always inspect the fuel system for leaks before starting, and pinpoint and remedy the source of any gas leaks.

Now, with all your carburetor ports and jets sparkling clean and the fuel tank clear, you're all set! If your fuel tank is equipped with an inlet screen, make sure it's clear too. These checks are your ticket to avoiding most fuel-delivery related no-start issues. By taking these steps, you're not only preventing problems but also saving yourself numerous trips to the small engine repair shop. Awesome job!

Never the Norm!