All the usual suggestions for problem engines....

check for air leaks, tighten up all nuts and bolts, new fuel, clean out carb, set HSN LSN to instruction sheet innitial settings, hotter plug, colder plug, different brand plug etc etc

I'm not sure that checking the valve settings wouldn't atleast allow you to rule them out ?

Be methodical and try one thing at a time....

[edit] sorry - take her out and do all this on a test bench to remove a few variables from the equation....

good luck!