there are three things your are looking for, spark, fuel and air. if you don't have even one of those things, it wont fire. you already have spark, good. now you need to check for fuel. check the jets and make sure they are not clogged. clean your carb, and make sure your fuel line isn't clogged either. if you have old gas, something is probably gummed up from the gas evaporating and leaving gummy residue. if you do have old gas (or its yellow), drain it, and put new gas in. put as high octane as you can find in it. runs better with higher octane. low octane makes it hard to start. also adjust your air/fuel mixture screw to 2 1/2 turns from soft bottom (barely touching the bottom). check for compression. should have at least 130 psi. check your kickstand safety switch, and your clutch safety switch. check your static timing.