What causes a carb to pop every once in awhile. Sometimes when I gas on it but not all the time

 

I'm going to guess that this is when you give it more throttle. The more you give it, the more it tends to do it. ???

If it spits back through the intake, 99% of the time it's because it's lean. The other 1% it's because of a valve train problem.

Someone will come on and post something about "check your 'timing'". Typical n000b response. Ignore them.