I think you're getting way too far ahead, worrying about valves. There are simple diagnostic steps you should go through before tearing into stuff.
You've already done the first: check for spark. So if the spark is fine, the next thing to check is fuel - is any getting into the cylinder? Try starting the engine, allowing it to turn over several times, then remove the spark plug and inspect it. If it's dry, and doesn't smell strongly of gasoline, then maybe fuel isn't getting through the carb. At this point I would try starting fluid, or you can add a half teaspoon of gasoline directly to the cylinder through the spark plug hole; reinstall the plug and try starting again. If it starts, then dies, you have a fuel-supply problem - gasoline isn't getting from the carb to the cylinder.
There are three or four possible reasons for that: Gas tank cap vent plugged; fuel line plugged; fuel filter plugged; carburetor dirty. Find the fuel line coming from the gas tank, and pull it off the carburetor nipple. If gas doesn't flow out, then it's the fuel line or fuel filter. If it drips unsteadily, maybe it's the gas tank cap vent. If gas flows out steadily, the problem is in the carb, so the next step would be taking that off and disassembling it for a thorough cleaning. Only if that fails would I start looking at the intake valve.
It's very common for a small bit of gunk to get into the float valve or the main jet on these carburetors and stop the flow of fuel. Even a very tiny particle can do it.
If fuel is getting to the cylinder but it still won't start, then maybe the gas is bad (water in it) or maybe the plug isn't sparking after all. Or something is blocking the air intake, so the gas can't get mixed with air to ignite; but usually if that's the case it will at least cough before dying.
Last edited by Albionwood; 07-19-2008 at 12:29 AM.