Even after doing the carb vent T-mod, sometimes one or the other vent line gets kinked or plugged with dirt. I've had both. I've also had similar issues due to a broken baffle within my muffler, which created a randomly occuring backpressure issue. There are about a hundred other things I'd check at this level of simplicity....and I'm no mechanic.
I wondered, for example: are you thinking stuck intake or exhaust valve, or stuck petcock vacuum feed? The latter is not uncommon, the first an unlikely cause for your troubles, IMHO.