Learn how to repair a lawn mower that will not start and doesn’t run

The first thing we need to understand, there are three critical things a 4-cycle gas-powered lawn mower engine must have happening at the right time, and in the proper amounts, before a OHV gas engine will start and run without fail. Good clean, fresh fuel, good compression, and the ignition-providing spark across the gap of the spark plug make-up the critical components internal combustion engine must have before it will ever start and run properly.

So let us take a closer look at each of the 3 critical areas, and learn how easy it is to fix gas-powered lawn mower engine problems, and save time and money, we would otherwise spend on costly repair bills.

First things first, let us take a good look at fuel system problems. The gasoline we buy today, blended for use in modern automobiles and their non-vented fuel systems are pretty much a perishable product, much like the milk we use as fuel for our bodies. Oh yeah gas goes bad, think about it for a moment, exactly what happens to milk after it is left setting in the fridge for several weeks, it goes bad right.

Well today’s gas we buy goes bad while setting in the fuel tank of our lawn mowers engine, just like the milk in the fridge. Imagine for a moment, pouring a tall glass of sweet milk, raising your glass expecting a tasty refreshing mouth full of sweet milk, and as your reward, a big surprising mouthful of chunky disgusting cottage cheese like sour milk invades your pallet.

Forgive me but I really do feel the need to ask, how much spitting and sputtering would you do? Now let me ask you, how do you think your 4-cycle OHV lawn mower engine will react to a tank full of bad gas? I dare say you remember the sour milk analogy; bad gas will have the same effect on a lawn mower.

More to the point, when it smells bad, it is. The easy solution, only use fresh fuel, less than thirty days old is best. As added insurance, it is a good idea to get into the habit of using a good fuel stabilizer. One that will help reduce fuel system problems while extending the storage life of the gas in your gas can and other outdoor power equipment you have around the house.

Let us face the facts; we sometimes forget to run our gasoline engines completely out of gas before we put them away, in that case, fuel stabilizers are a good thing, which will reduce fuel system problems.

FYI, when using alcohol blended fuels in your gas-powered lawn and garden tools. Be advised alcohol over time draws moisture into the fuel system. “Water” in the fuel is a bad thing and will destroy the enter workings of a carburetor.

Next, let us discuss compression. Before our lawn mower engine will start and run properly, it must first pull clean fresh air through the air filter element, carburetor and into the engine cylinder. This air-cleaner is located inside the lawn mower engines air-cleaner assembly cover. It is extremely important this filter element is clean, in addition, in most cases dry. However, there are lawn mowers engines equipped with air-cleaners that use a light film of oil. The oil makes the filter more effective at filtering small particles of dust and dirt from the air intake in extremely dusty conditions, like late summer, fall, and when bagging leaves. Your lawn mowers owner’s manual will have helpful information on what filter your lawn mower should use when, and how to clean, or replace dirty air filters.

After passing thru the air filter, the clean fresh air mixes with atomized fuel creating a volatile fuel mixture in the carburetor, before entering the heart of the internal combustion engines, combustion chamber where the lawn mower’s engine makes its power.

The combustion chamber consists of three basic components, the cylinder, cylinder head, and a piston that compresses the fuel mixture after it is inside the cylinder. As we discuss the internal combustion engine cylinder, it is important we understand the two different types engines used on our lawn mowers.

There is the 2-cycle engine like those used on the older Lawn Boy mowers, made obsolete by today’s CARB standards. Then there is the widely used, much-improved 4-cycle OHV engine used on today’s new mowers that is, more efficient, and run cleaner than their predecessors did. For that reason, let us explore the inner workings of today’s 4-cycle OHV lawn mower engine. OHV 4-cycle engine

For many of us, the acronym OHV holds little or no meaning. In that event, please allow me to help you better understand its meaning. OHV in simple terms, The OHV internal combustion engine design manufactured using the Over Head Valve design.Just like the motor in our car, the cylinder head is located at the top of a 4-cycle engine cylinder.

The cylinder head of an OHV lawn mower engine contains two separate engine valves. First there is the intake valve, second is the exhaust valve. When everything works right, which is to say, both the intake and exhaust valves are adjusted and seating properly. The piston traveling down the cylinder with the intake valve open and exhaust valve closed pulls the air fuel mixture from the carburetor into the combustion chamber.

This is the intake, stroke one of the 4-cycle. As the piston reaches the bottom of the intake stroke (BDC), with the piston at the bottom of the cylinder the intake valve closes, the exhaust valve stays closed, and the piston starts moving towards the cylinder head, starting stroke 2 of the 4-cycle, the compression stroke.

If for any reason, such as not adjusted properly, or something keeping the valves from closing completely, the piston will not compress the fuel and air mixture that is critical for the engine to start and run.

Checking the engine compression with a compression tester will help to determine if there is a problem with a lawn mower engine has valves, head gasket, piston, or piston rings.

With the piston at the top of the compression stroke, (TDC), stroke 2 of the 4-cycles is complete, and the air fuel mixture is ready for ignition. When all is as it should be, the OHV engine ignition system sends a 40,000 – 60,000 or so VAC surge of high voltage electricity, required to jump across the gap of the properly adjusted engine spark plug. Igniting the compressed air fuel mixture getting that puppy started and generating optimum power, the power stroke, stroke 3 of the 4-cycle for those of us still counting. Check your lawn mower owner’s manual or better yet an aftermarket repair manual, for the proper setting of the spark plug gap.

How to adjust the Spark plug -gap

Ok, some of us are scratching their head wondering what and where the spark plug gap is. That is ok; first thing, let us look at where a spark plug is located. A spark plug is the thing the spark plug wire that is roughly the same size as a #2 pencil connects to in the cylinder head.

The spark plug gap is on the opposite end of the spark plug where the wire snaps onto the plug. At the end of the threaded portion of the spark plug is a narrow piece of metal bent into the shape of a capital L. That covers the electrode located in the center of the spark plug. The high-voltage generated by the ignition module, travels through the center of the spark plug to the end of the electrode, where the spark jumps across the air-gap between the electrode and the L-shaped piece of metal connected to the bottom of the spark plug that serves as the ground completing the circuit.

The air-gap between the electrode and the L-shaped piece of metal is the plug gap. To adjust the gap, simply bend the L-shaped piece of metal with a spark plug adjusting tool, to make the gap wider or narrower, and check the gap with a spark plug gauge to ensure a proper setting.

Oh, by the way, the spark plug gap will not come pre-adjusted to every lawn mower engines air-gap specification from the factory, so do yourself a favor. Be sure you check the air-gap setting before installing a new spark plug. Your lawn mower will start and run better, with increased efficiency for a long time saving you money and several headaches when you do.

Unfortunately, many homeowners and renters store their lawn mower without first winterizing. Leaving untreated fuel in the gas tank and carburetor float bowl is a sure-fire recipe for fuel system problems. Stale gas will gum-up and clog jets in the carburetor quick fast and in a hurry.

The fix, drain the bad gas out of the fuel tank, fuel lines, filter and carburetor float bowl. Remove the carburetor from the engine disassemble and clean the pilot and needle jet passages, float and float bowl.

There are way too many different engines used on lawn mowers today to cover them all in this article. For instructions that are more specific please feel free to refer to a Haynes small engine repair manual that covers your mower engine for step-by-step instruction.

Short cut that more often than not works to get a mower engine to start, and run, remove the float bowl nut from the bottom of the carburetor float bowl, make sure the float bowl stays attached, any bad fuel in the float bowl will drain from the bowl when you unscrew the nut (counter clockwise).

Clean the small holes in the float bowl nut with a good carburetor cleaner, take proper precautions when using carburetor cleaners, they will damage painted surfaces, plastic parts, and are harmful to your body!

With the bad fuel drained, float bowl nut clean, reassemble and add fresh fuel to the tank and hopefully your lawn mower engine will start, if not, you will need to remove and completely clean the carburetor and fuel system.

Is the mower out of gas?

Did you hit, or run over something with the mower in the grass?

Oh yeah, people do run out of gas, put their mower away in the garage or garden shed with an empty fuel tank and forget about it until next time. Only to pull on the starter rope until they either break the recoil starter rope, or further frustrate and exhaust themselves. Do yourself a favor, check the fuel tank and make sure it is not empty.

Hitting something in the grass will cause mechanical problems in a lawn mower engine. Something as seemingly harmless as hitting a baseball in the grass, exposed tree root or some other foreign object will do several bad things that will in turn. Kill the engine straight away or keep it from starting after shutting the mower off, damage the blade, bend the crankshaft or shear the flywheel-key all of which are expensive to have repaired at a mower repair shop. Many repair shops will tell you, buying a new mower is a better investment.

The most common thing that happens in cases like these, when the mower blade hits a small foreign object or tree root the flywheel key, by the way an inexpensive part, gets either partially or completely sheared. Throwing the engine ignition out of time the mower may stop running, backfire, run poorly or not start after shutting the engine off.

Tip, when the starter rope handle jerks back, the engine is most likely, either out of time, or the engine cylinder is flooded with raw gas or oil from the crankcase, remove the spark plug and pull the rope to clear the gas and oil from the cylinder.

The fix, First remove the spark plug from the engine, with the plug removed, place the mower on its side with the carburetor / air-cleaner cover up. Then check the mower blade for damages, is it bent, chipped or has gouges in the cutting edges.

Rotate the mower blade in a clockwise rotation, while watching the engine crankshaft. You are looking to see if the crankshaft wobbles side to side while it spins. This is to ensure the crankshaft is not bent. When it wobbles from side to side the shaft is bent, and it is time you buy a new mower.

Otherwise, replace the damaged flywheel key and you will be back in business. With the mower back on its wheels, it is time to fix the damaged flywheel key. Remove the blower-housing, top covering the flywheel. Pull the flywheel off the top of the crankshaft and replace the sheared flywheel key. Sounds easy right, it really is but here again. Each engine has its own specific steps where a good Haynes small engine repair manual helps guide you through the simple step-by-step procedures.

This is important; when you need parts for your lawn mower engine. You will absolutely need the model, type and serial number off the engine and not the body of the lawn mower. To ensure getting the proper engine repair parts you need. Only when replacing the mower blade, wheels, cables and such will you need the model and serial numbers off the lawn mower body.

I hope this article really helps you see you too can easily repair a lawn mower. When it will not start, and doesn’t run. After all, there is nothing more bothersome than needing to cut the grass and the mower not working. Eliminate the frustrations associated with dealing with the mower repair shop, their inflated in season repair costs, and time wasted spent on waiting weeks on someone else to fix a lawn mower. Make the needed repairs yourself, and reward yourself with the time and money you save.

Best of luck and enjoy gardening. Mike