I'd rule out the vents by removing and blowing them out to confirm they are clear. Next, take your fuel filter off and put fuel through it the wrong way (dumping fuel into a waste container). Confirm it flows well.
If still an issue, pop the bowl off and see if everything is clean and the float still works.
Check out the pilot jet while your in there confirming it's clean and clear. Don't go reaming it out with something, but confirm no build-up or blockage.
I can't explain the foam/bubbles because the carb isn't pressurized.
Really dumb question, but when were the valves last checked?