when you say "nothing"...please elaborate.
for example...
1. when key turn to START = you hear the anti back fire solenoid clicking or not?
2. when key turn to START = the engine rolls over but does not start?
3. when key turn to START = engine rolls over sometimes, but hesitates to roll over ...like heavy resistance?
4. when key turn to START = engine rolls over and starts, but then when key returns to RUN, engine stops ?
please explain the symptoms with a little more detail. Because it matters these details. "nothing" can mean alot of different things to alot of different people.
Here are some things wrt to the above list of symptoms.
1. if the anti back dire solenoid is not clicking, then either the circuit to it is borked or the device itself has failed. This can cause engine start problems...usually this means the engine may run for a moment then gets starved of fuel.
2. if the engine rolls over but does not start, we can assume the starting circuit (and safety switches) are operating normally...look for fuel or ignition related problems.
3. if it rolls over sometimes and sometimes it struggles to roll over...then we can again assume the starting circuit (and safety switches) are operating normally, but there is something causing resistance to turning engine over. This can be related to a poor battery state of charge or a bad connection, or a failing flywheel starter motor/solenoid....make sure battery is fully charged and check all power AND ground connections...look for any corrosion and see if you can bypass the starter solenoid to test ...it may have failed internally. Likewise, try the same with spark plugs removed. It's possible the engine has a problem and this creates alot of resistance to turn it over. If it turns over easily with sparks plugs removed...then, look at valves clearance.
4. if engine rolls over and starts but when key returns to run and engine stops means that there is a fuel/carb problem most likely. check make sure the choke is not stuck and the spring to it is not broken or loose...the choke valve must be able to rotate easily. Also, sometimes a failing ignition module (coil) can produce these symptoms....It can start and run momentarily, but then fail to produce an adequate spark when "excited" under load. Make sure the spark plugs and spark plugs wires are not damaged too.
report back what you mean by "nothing"....The list above may or may not change with recommendation based on the details of what you mean by "nothing"
hope this is helpful