Over the summer I helped my friend Bill get his Ford 8N running again. He used it to clear the snow on his driveway over the winter, but then couldn't get it going again in the spring.
If an engine runs but has issues, there are some straightforward ways of troubleshooting. But when it won't start at all, there are many possible causes. You hit the starter, it cranks, but doesn't fire. Out of gas, stuck carb float, blown head gasket, bad ignition coil, burned points, etc. What could it be?
Here are the main issues that we had to fix to get Bill's tractor going again.
Elbow fuel filter clogged. Unscrew the intake elbow from the carburetor and examine the filter. It should be clean and empty. Bill's was solidly packed with gunk. We put it in a vice and aimed a propane torch at it, then blew out the ash. There are supposed to be 3 filters on the 8N, but only one was operational, and the tank was dirty. So we drained the tank most of the way, dropped in a length of chain, and shook it around. Drain, rinse, repeat.
Distributor rotor broken. We had 3 rotors break on us. If you're getting no spark, this is a good thing to check. 2 of them were because the new distributor cap was a bad fit. We ended up buying a rotor + cap together from Napa, and they worked well.
Battery backwards. Ford 8N, and many older 6-volt engines are positive ground, with negative leading to the starter. Someone convinced Bill otherwise about a month ago, and the machine has been cranking backwards ever since. It was compressing fresh air and trying to ignite it, then blowing it back out the carburetor. *sigh*
Some troubleshooting techniques:
A little carb cleaner sprayed in the air intake or directly in to a cylinder will tell you a lot (it acts like starter fluid). If the tractor still doesn't fire, then the problem isn't fuel related.
A spark plug overgapped (1/4" or more) is a great testing tool. Pull a spark plug wire off one of the plugs, connect it to the overgapped plug, grounded it to the metal. If you still get a spark, then electrical issues are probably not the problem.
You don't need much compression to get started. Compression testers are useful for diagnosing engine health, but they are only useful when the engine is hot. To get started, you just need to make sure you have *any* compression. Pull all 4 spark plugs, and put your thumb over each hole in turn. If you feel good pressure, that's enough for now.