Backfire through vaccum line?

Got it guys! Took off the cover and check my valve lash today, while they're all in spec three aren't perfect so i swapped some shims and got all my exhausts at .15 and intakes at .10.
The problem was something else though! Don't know if it was the po or what, but my cams were timed wrong. Both were off a tooth in the opposite direction. Couple bolts later and a little turn on my cams and BOOM! No more carb backfire. Also got rid of a little hesitation she was having in the 1k-3k rpm range in 1st and 2nd gear that i was attributing to slight carb tuning issues. Inexperience takes its toll i guess, but now i know!
Z1 driver, I've started leaving it a little below the feedneck and havent had the issue with gas coming out again. Guess i was just a little overzealous with the fuel. You figure id have been fine with leaving it a little low anyways with that 5 gallon tank and all. :silly:

Thank you for all of your input, i probably wouldn't have got back under the valve covers this season if it wasn't for you guys! You'll make a kz rider out of me yet!