Are there any performance characteristics that would indicate this needs to be done? I am getting a little pop/backfire through the carb when cold, never used to. Seems fine once warmed up. Thanks in advance for any help.

 

if it seems to run warmer than usual the valves might need adj also if they have not been done since it was new they for sure need to be ckd once they are done its usally along time before they nwill need it again

 

I just checked my valves after about 20 hours use, they were in tolerance..... What a hassle getting to them.
I would venture to guess it is more carb or spark. What does the plug look like?
Do you run it often? If the gas is "stale" (Gas loses about 1 octane level per month). It could be related to that.....
The valves will cause the engine to run hot (like the other guy mentioned) or be real noisy. This engine, however, is noisy be design...