Thanks for sending my your question. there are many causes why a motor like yours does not start.
I will give you some of them and they maybe you will at least have something to go on.
1. The engine may have lost its spark. If this is the case, it may be an electronic ignition that has gone bad and needs to be replaced. It also could be a spark plug or a safety switch that has stopped working or is not being pushed in all the way.
2. The carburetor may not be giving enough gas for it to start and run. This would mean that the entire fuel system should be cleaned and the carburetor rebuilt. You also could have bad gas or water in the gas that can cause this problem.
3. The valves may need to be adjusted. This should be done about every 2 years. If they are not adjusted correctly, it throws things out of time.
4. There is a possibility that the flywheel key might be sheared. This can cause the ignition to be out of time.
Before I would attempt to do anything, I would get a good spark tester and test the spark. If you do not have spark, then I would remove the engine cover and find the electronic ignition coil. (That is the part that the spark plug wire connects to) There is a small spade clip on this coil that has a small wire connected to it. This is the shut-off wire that connects all of the safety system and the ignition shut-off to the coil. REMOVE this small wire from the coil and then turn the engine over with the starter while checking for spark. If you then have spark, you know that it is not the ignition that is bad, it is one of the safety switches or wires. Now check all of the safety switches and see what you find.
After you have spark. And the engine cover is back on and everything is assembled, remove the air filter. "You can take about a tablespoon full of fuel and put it into the carburetor and then try to start the engine. If it fires or tries to start, then you know that the problem is fuel related. It most likely is a dirty carburetor that will have to be cleaned and rebuilt with a complete carburetor kit. Make sure that you also clean the fuel line and the gas tank and get rid of all the old fuel. Also check the screen inside the gas tank to make sure that it is clear. Also check to make sure that the fuel pump is working. You can take the fuel line off of the carburetor and place it into a coffee can and turn the engine over with the starter. If gas PUMPS out of the fuel line, then the pump is probably good. I will give you my comprehensive fuel and carburetor answer at the end of this post.
The valve clearances must be checked about every 2 years. There should be between .004" and .005" between the valve stems and the rocker arms when the valves are completely closed.
The flywheel key could be sheared and needs to be checked. You would have to remove the engine cover and then the flywheel to check to make sure that the key is perfect. If it is even the slightest bit sheared or offset, the ignition timing will be off. CAUTION !!! Do NOT use a standard wheel puller to remove this flywheel or you can break the expensive flywheel. There is a special tool designed to do this.
Here is my comprehensive carburetor and fuel answer that may give you some ideas.
As engines sit or get older, fuel that is left in the carburetor can turn to gum and varnish and cause this and other problems. Also, any gasoline that was left in a gas can for a period of more than 30 days must be discarded because it also has begun to turn to varnish.
Today's gasoline's contain MTBE and alcohol. (Ethanol) They turn to "Junk and garbage" very quickly. Alcohol is partially water (H2O). And they call it "Oxygenated fuels! It is the oxygen that breaks down the organic compounds in the fuel and turns the gas to "Garbage" (Gum and varnish) The fuels we had just a few years ago had no alcohol in it and would store for longer periods of time before going stale... and fuel stabilizers do almost nothing to prevent the fuel from going bad with the changes in today's fuels.
Do not buy gas from the "Discount" Stations. The discount stations get a reduced price on gas because they may be buying fuel that is nearly 30 days old already. You may be getting fuel that's nearly stale right from the pump when buying from a discount station. Purchase your fuel from the well-known stations such as Shell, BP, Sonoco, Phillips 66 etc.
More than 70% of all of our repairs in our lawn mower business are due to this same issue. You most likely have dirt, gum, varnish...etc in your carburetor plugging up the small passageways and jets in the carburetor.
The carburetor must be removed from the engine. Clean all parts with carburetor cleaner and blow out all the small holes and passageways with compressed air. Remove all of the non-metallic parts since the carburetor cleaner will cause them to be disfigured decompose and plug the carburetor as time goes on.
Wash the carburetor cleaner off of the metal parts by washing them in warm, soapy water then rinsing with clean water. Dry them by blowing it off with compressed air. Make sure that all the passageways are blown dry before reassembling.
Reassemble using a NEW carburetor rebuild kit.
DO NOT TRY TO REASSEMBLE WITHOUT USING A COMPLTE CARBURETOR KIT! You will just end up having to do the job again.
ALWAYS clean the fuel tank and replace the fuel line when doing this repair or you may have to do it all over again. The inside of the fuel line disintegrates over time and these small pieces of rubber will plug up the carburetor too. Dirt and water from a dirty fuel tank will also plug up the carburetor. Find the Model, type and serial or code numbers off of the engine and take them to your local dealer to get the carburetor repair kit.
If you do the work yourself, take pictures or at least make a drawing of where all the linkages, gaskets, and component parts go. Correct reassembly is critical.
If the carburetor still doesn't work correctly, you may have to take it to someone who has an "Ultra-sonic" cleaning machine. This machine uses carb cleaner and ultrasonic vibrations to get the very small passageways clean when traditional methods fail.
If you don't feel comfortable with this kind of repair I would suggest sending it to a reputable shop.
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