If you haven't found any leaks in you fuel lines from the tank to the carb, then the bubbles could indicate an improper priming procedure. Real quick run me through your steps of priming the fuel system before you've tried to start your truggie.
What I do.
I take a longer piece of new/clean/chemical residue free fuel tubing, and prime with that, All I do is swap out the tubing from the pipe to the tank, then blow into the clean tubing until I see no more bubbles, after I'm primed I just put the used tubing back in place. This is how I primed all my nitro engine before I found this huge plastic syringe that kinda looks like the ones a pharmacist includes with cough syrups for babies, only its fricken huge! I got it at a liquidation store.
Remember to mark the end of your clean contaminate free tubing, I color the end that plugs into the pipe with a black permanent marker, so I don't put that end in my mouth.