MODERN GAS GOES BAD QUICKLY.
It is DIFFERENT than it was 20 years (or less!) ago. It breaks down quickly, and can leave a host of problems, including corrosion inside fuel systems (due to methanol)... hardened/crumbling rubber seals, and numerous jet clogging issues.
Many years ago when I worked in Suzuki shops, many guys would come in in the Springtime and say the same thing:
"Uhhh, my bike won't start, and I can't understand, because it worked FINE last fall!"
yep. They'd sign the work-order for a "tune up" and the first thing we'd do is drain the float bowls. 85% of the time: PRESTO! Ran like a champ.
But now the gas is far worse. I just inherited a Kawasaki 1100 that had been sitting for two years; what I expected would be a simple carb douche-out turned into a nightmare... full rebuild of the petcock with new seals...full gas-tank rinse with muriatic acid... repalcement of fuel-rail seals between the individual carbs....and THREE attempts at cleaning out all the green powder and Mung in the various fuel passages.
Okay.. SO.
If you have gas in the airbox, it most likely means that your petcock FAILED somehow, (or you F-d Up and left it on Prime), the fuel leaked into the flaot bowls... filled THEM up... ran over into the intake manifolds and into the comb. chambers... then seeped past the rings and slowly-but-surely filled up the crankcase. When you went to start it, there was so much hydraulic pressure from too much fluid (gas and oil) in the crankcase that it pumped out the breather tube up top.. and down into the airbox.
Bad news, but not terrible:
First, make sure your petcock is sealing properly... put a tube or hose on the end of the petcock spigot and turn it to "Reserve". NOTHING SHOULD DRIP OUT. If it does, then that means you'll need a petcock rebuild kit (seals.. pretty cheap on Ebay).
Next, take your sparkplugs out and hit the starter motor. If you're lucky, you won't get a spray of wet gas come shooting up. If you do, make sure the petcock doesn't leak (again), and turn the motor over enough times that you're sure it's mostly free of wet gas, which could cause HYDRAULIC LOCK and bend a con rod.
Now you'll have to CLEAN the oil out of EVERYTHING.
This means fully draining everything in the oil system, including the oil cooler and filter. You can't take a chance on running with any fuel left in the lubricating system, as it will dilute the oil and damage the bearings, including the Mains. You SHOULD run some cheap (supermarket) oil in the bike for ten minutes, after it's all been cleaned out, and then dump THAT, (and Yes, i did all that with the Kawasaki).
THEN you'll need to clean out the carbs completely... that means EVERY MOTHERF***ING ORIFICE YOU CAN FIND. remove all jets, spray every hole, big or small (don't forget the tiny holes in the carb bores where the butterfly comes down, AND the little jets in the edge of the bellmouths) with good carb cleaner, and blow it all out with high pressure air. Don't forget to clean the little rubber-tipped float needles and their seats, (for that use a Q-tip with a tiny shred of steel wool on the end).
DO IT AGAIN.
If the bike has been sitting, especially outside, there most likely is JUNK in there... and what sometimes happens is that MOLD actually grows in there. (Look for greenish powder coating).
All that done properly, you shouldn't have any more problems.
If it still won't run on all four evenly, then you've still got blockage in the jets somewhere.