[bandito2]

Fuel, spark, air... that is what it takes to make it run. If it ran fine up until it didn't, then messing with the valves was probably unnecessary. Many times it is a fuel delivery issue; that could be a blocked, clogged, pinched or kinked fuel line, clogged fuel filter, defective fuel pump (or blown fuse), closed or plugged fuel petcock, some problem in the float bowl, clogged carburetor jet, possible engine flooding (Too much fuel) or simply something as unexpected as water in the tank or a batch of really, really bad gas.

Spark may be weak or inhibited by bad or weak battery, weak or faulty ignition coil, bad spark plug, faulty spark plug wire or connector, defective stator, defective voltage regulator/rectifier, faulty ECU, loose connections, faulty connections causing grounding.

The air situation most often applies to problems with keeping the engine running or very rough starting; that is, some engine firing does occur, but the proper combination of fuel/air is not happening well enough to start and stay running.

Start with checking to see if it is getting fuel to the carb.
Then check to see if there is spark and that it is strong enough make it fire.

It's not often that both fuel and spark fail at the same time. But it could with a grounding or fuse issue that would prevent the fuel pump AND ignition from working.

There is more I'm sure, but start with the basics of going from most likely and least complicated or invasive before going on to the more complicated and technical stuff. Being methodical and thorough should get you where you want to be in your search for the source and solution to the problem.