My lawnmower would not start, so this is what I did to troubleshoot & fix. Keep in mind that once the starter is cranking an engine needs three things to run: air, fuel, and spark. If you have those three things it will run (barring internal mechanical problems); if any are missing it won't start. And more than one thing can go wrong at once.

Problem: Won't start, nothing happens when I turn the key
1. Charge the battery

2. I connected a jumper cable to the positive battery terminal and touched the other end to the bolt on the starter where the starter cable is connected. The starter spun but the engine didn't "kick".

That told me the battery was charged and the starter worked, so I needed to look for a problem in the wiring, to get the starter spinning when I turned the key. I figured it was most likely the key switch or the starter solenoid.

3. I took out the key switch. This mower uses a generic switch held onto the dash panel. You spin a round plastic-covered nut off the front of the switch, then pull the switch out the back side of the dash panel. I pulled the connector off the back of the switch and looked at the contacts in the connector and on the back of the switch. The terminal that is connected to the battery (red wire in this case, IIRC) was pretty corroded so I took a fine file and removed a lot of the rust, so both sides of the connection were shiny.

(I could have stopped here, had I know what was wrong.)

I plugged the switch back into the harness and tried again - nothing.

Since this process took me four days I decided to put the battery charger back on to make sure it was topped off. The mower has been out in the rain all week with the wires hanging out, so that could have discharged the battery. When I put the charger on it the voltage read 7V, so the battery was once again part of the problem. After a few minutes the charger read "bAd bAt" meaning it won't charge & should be replaced.

4. I pulled my truck over and connected jumper cables between the batteries. I turned the key and PRESTO the engine cranked. It still didn't start, though.

5. I took off the air filter and sprayed some Berryman B-12 carburetor cleaner in the intake.

No start

6. I disconnected the spark plug cable and took out the spark plug (with a spark plug socket). The spark plug looked good and was very clean. I didn't gap it but it looked like it was around .050 inches. I probably should have closed it up about half way but I didn't.

I plugged the spark plug back into the spark plug cable and clipped a test lead from the negative lead (the ouside L-shaped lead on the end of the spark plug) to a spot on the engine that looked well-grounded. You don't strictly have to use the test lead, but it makes it easier, in my opinion.

I cranked the engine again and watched the spark plug. I could see a spark jump across the gap in the spark plug, so I eliminated that as a problem. I put the spark plug back in the engine. Be sure to put a little oil on the spark plug threads, especially with an aluminum engine block.

At this point I was pretty sure the engine had electricity to crank the starter and the spark plug was sparking. As long as the carburetor was connected to the engine it would have air (I guess the choke could have been closed because of a problem with the linkage) and the carburetor cleaner should have worked as fuel, so it should have started. Maybe I missed something...

I went back to the intake and sprayed more carburetor cleaner in it. This time I held the button down far enough to release some pressure out of the can, and it sprayed in past the throttle blades pretty well. I cranked the engine again and this time it fired! It started and it ran and kept running. If it had died in a second or two I would have looked at the fuel system for blockage, starting with the fuel filter, or possibly bad gas.

So in the end I had dirty connections at the key switch and a bad battery.

I also have three flats, one of which popped off the bead. I'll deal with that in the morning.

My last post on this: http://sharpcraft.com/content/node/102