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Lawn Mower Won’t Start

If you have landed on this webpage with hopes of finding out why your gasoline powered push lawn mower won’t start, you are certainly closer to finding an answer! After a long cold winter or extended period of time, like many of you, I have found myself pulling on the lawn mower start chord so many times, I often felt like I was leading an exercise class. Due to my limited budget for lawn equipment repair, I was forced to figure my small engine issues out alone. With dozens of engine repairs under my belt, I have now set out to document my lessons learned in a lawn mower DIY mini-series of articles so that you can get back out on the lawn cutting grass. Below, I have identified the main knowledge areas required to diagnose a malfunctioning engine.

It should be known that small engines are not beyond the realm of the DIY’er or weekend warrior, so on to the troubleshooting! Let’s determine why your lawn mower won’t start:

•· Have you checked whether your piece of equipment has gasoline in it?
•· Does the engine have a sufficient amount of oil in it?
•· Is the air filter clean?
•· Is the spark plug lead properly fastened to the spark plug?
•· Did you give the pull chord of the lawn mower at least 10 – 15 good strong pulls?
•· Have you pushed the primer bulb several times if your piece of equipment is equipped with one?
•· Has the mower been tipped upside down? (If yes, you might have flooded the combustion chamber with oil)
•· Have you confirmed there are no cracks in the engine block?
•· Can you confirm that there has not been any water intrusion into the engine?
•· Can you confirm that the pull chord is operating freely, and that the engine rotates smoothly in the off position?

Once you have determined that none of the above issues are preventing your engine from starting, you will need to move on to the next set of possible scenarios. Lawn mower engines, like all engines, have three particular requirements in order to start: fuel delivery, ignition, and compression. If your engine has an issue with any of the aforementioned critical items, your engine will not start, or will perform very erratically, which could eventually lead to engine destruction.

Ignition Verification

The ignition system as a general rule, is responsible for producing electrical impulses or “sparks” which ignite an air/fuel mixture that is directed to the combustion chamber of your small engine. In order to verify that your ignition system is functioning as designed, follow the below directions:

1. Remove spark plug lead from spark plug
2. Remove spark plug with a wrench or ratchet (ensure that you do not damage the ceramic portion of the spark plug!)
3. Reconnect the lead wire of the spark plug to the spark plug now that it is unscrewed from the engine.
4. Orient yourself over the lawnmower in such a way that you can pull on the starter chord while also holding the spark plug electrode against a part of the engine that is metallic, and which has not been covered with paint or debris.
5. Give the start chord on the engine a few slow and steady pulls, and keep your sights on the spark plug electrode gap (on the tip of the plug)
6. Verify that as the start chord is pulled, a small blue spark arcs over the spark plug gap.
7. If a spark exists, then you can rule out ignition as the immediate cause for you no-start issue. On to the next step!

Compression Verification

Now that you have confirmed that your small engine is getting a spark, In situ grinding crankpins  it is time to verify that all of the internal moving parts are operating in such a way that sufficient compression is being created in the combustion chamber of the engine. Put simply, compression in developed in the combustion chamber from the up and down action of the piston, rod and crankshaft. As the piston moves towards the spark plug, compression increases in the chamber until the point of air / fuel ignition, while conversely, when the piston moves away from the spark plug on its down-stroke, a negative pressure or vacuum is created in the combustion chamber, which draws in more of the air/fuel mixture for the next power stroke.


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