exactly...you can not check for air leaks if the engine is not running.
it could be a blown intake gasket causing an extrmely lean condition.
sure, gas is getting into the float bowl, but are you sure that it's getting into the engine?
A plugged up main jet?
is the choke on?
i know, silly things, but sometimes the silly stuff is what gets you.
it won;t run so you want to dive in and go to the most difficult things.
check the simple stuff first.
even if gas spills out of the top of your tank, that doesn't mean that the cap is vented.
that means that the gas cap seal is dry and possibly rotted.
take the cap off & try it.
you're sure that you have spark at tdc?
how could you see the piston AND watch for spark at the same time?
this is tough to do, and you can't really do it slowly to make sure the piston is at tdc RIGHT when the plug fires.
if it won't run on starting fluid, then you have a timing issue or a MAJOR air leak.
if it runs on starting fluid, you have a fuel issue.