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have a push lawnmow,that want start.change the spark plug,put in fresh gas.but witt not start.

How long has it been sitting with old gas in it?

Here are the steps I go through to find why a mower won’t start:

 1. Check for spark.  A new spark plug does not guarantee that there is      ignition. Remove the spark plug and place so that the side of the plug is in contact with the engine block. Have someone squeeze the safety stop handle and pull the starter cord. If there is no spark, the plug is either defective or there is a problem in the magneto.

 2. If there is spark, replace the plug, remove the air cleaner and squirt some carburetor cleaner or even pour a little bit of gasoline into the carburetor.  IF the engine starts but doesn't continue running, you have a fuel problem. While you have the air cleaner off and you can see into the carburetor, make certain that the choke is closing.  On my lawnmower, the engine is choked by pushing the throttle control all the way open.  Sometimes this cable gets out of adjustment and the choke won't close.  It is then impossible to cold-start the engine.

   3. If all this fails, go drink a beer.  You really didn't want to mow the yard that badly anyway.

could be due to old gas/not stabilized which can lead to ethanol damage of carburetor…

Your local hardware store will probably stock spray cans of starter fluid. Squirt some into the air cleaner or muffler or any other opening you can find and try to start the motor several times. If it starts and continues to run, your problem is solved at least for this week. If it starts then dies, in 10 or 20 seconds, you probably have a plugged or disconnected gas line or the fuel pump has failed or the carburetor has failed. You can certainly try to fix it yourself if you are of a mind to, but the fuel handling hardware for small engines is quite complex and I find it to be hard to understand. Maybe time for a trip to the shop.

If the engine refuses to fire, probably the spark plug isn’t firing because the spark plug or cable or coil has failed. But it could be a problem with the flywheel having sheered it’s locating pin. The flywheel position controls the spark timing. The locating pins are deliberately made of soft metal so that if the engine runs into a rock or pipe, the inexpensive pin sheers instead of something more expensive. Many hardware stores stock replacement pins.

Pour a tiny amount (1/2 oz) of gas on/in air filter and see how it reacts to that.

Some old mowers will not start with a half full gas tank. If there is no spark, change the points.

What is the condition of the fuel in the tank? Is it old? Does it have fuel stabilizer in it?

I am willing to bet it needs a carburator rebuild, which can usually be done for less than $50 if you find a good lawnmower mechanic.

Oh, I thought you meant YOU. When I think of push mowers, I think of those oldies with no motor…

I used to have one of those, well, a new one, but same basic design. Didn’t like how it cut, so I bought a gas powered mower. Gave the reel mower to a relative that can’t really afford to buy gas right now.

One additional thing to check…the air filter. Mine was pretty bad. They get clogged quickly on lawn mowers.