Is there too much gas?
Are you using the choke each time you try? If you are, try with out.
Or you could go to my favorite next step. Let it sit a while give it a shot of starting fluid and see if you can get it started.
Well a known compression number will give you a good idea of whats goin' on. anything under a 120 lbs and there is a problem somewhere, checking the valve clearance should be done once a yr on older units