Having to use the primer bulb usually signifies a lean condition....some things to check are:
Are you allowing it ample warm up?
Is your vent on the gas cap turned "open"?
Do you have a leak/pinhole in your fuel line?
Do you have a pinched fuel line?
Is your fuel/oil mix at the proper ratio?
Fresh gas/clean plug?
Dirty carb/fuel deposits?
Also, even though it won't cause a lean condition, make sure your intake/air filter foam isn't blocked...
Start with the simple things first...bet it's one of these.
Let us know!