I had this old lawnmower that would not run. I would pull the wire and it would turn over a couple times, but nothing would happen. No explosions would happen.

I thought based on some quick googling that this could either be a spark plug issue, or a carburator issue. The spark plug looked fine upon taking it out, but new ones were 5$ so I decided to replace that since it was easy. As expected it did not help.

Then I thought it must be the carburator. I blew some carburator cleaner which is mostly ethanol into the carburator, and then the machine would start. It would not run well though, and after letting it die out, it would not start at all until I put more carb cleaner into the carburator. I think this is because the engine uses more gas to get started than it does to keep running. I am guessing some gas was still getting into the carb from the fuel tank, but not enough to start the engine. The carb cleaner was giving it that boost it needed to start.

So I took apart the entire carburator, and sprayed it out using my compressor and the carb cleaner. I made sure to get every single small hole using my wire brush. Then I drained the gas tank, and cleaned that out using the compressed air and carb cleaner, as well as the fuel lines.

After putting the carburator back onto the machine, it still would not start… If I sprayed carb cleaner into the engine, it would start though. Upon looking at the engine, I noticed that the choke was not engaging upon starting the engine. The engine had one of those fancy auto chokes, so it should have engaged upon starting. This was my own problem since I had installed the mechanism for engaging the choke wrongly. After fixing that, it started just fine with no problems.

Now the engine starts fine, and it runs fine with no problems.

My theory as to what happened was that the gas had decomposed, and the ethanol had caused it to gum up the engine. This is why I use ethanol free gas with my own small engines. It is also a good idea to empty the entire engine of gas whenever I am done with it so stale gas does not sit there and decompose, but that is annoying and not practical at least for me.