Backfire through vaccum line?

Hey there guys. Just finished up my first build and took her on a few rides, and not super often, but sometimes I get some like backfire through my vaccum line (only have one to the petcock off carb 2, the rest are capped). She's an '82 KZ1100, pods, rejetted 142.5 and 40 i think it was (Did the carbs a while ago so I'd have to look for sure). 4 into 1 exhaust (yes its sealed very well) and a 1:1 shorty muffler. Valves should be good, they were shimmed to with in spec, checked when I painted the engine. Not really a problem unless I really get on the throttle into high rpm range with a fresh full tank it'll force some fuel out the top when it does. Just curious if anyone knows what could be causing this? I looked around and couldn't find that same problem from anyone on here. Any advice is always welcome!

Maybe the valve-lash needs adjustment, and/or the engine is getting too much fuel.
That's my $.02

If fuel is being forced out of the filler cap, then it is very likely that the tank vent is blocked. You will need to take the cap apart to check the vent.

When you fill the tank, do you stop filling at the bottom of the filler neck, or do you try to get the fuel level all the way up to the cap? There needs to be a little room for the fuel to expand as the sun and engine heat it up.

daveo, I'll double check the valve lash, though i do know i need to re-tune the carbs. initially tuned them in not knowing i was only on 3 cyl because dyna mini coils need a dwell control and that's what i had on at first and burnt up a coil. shes on all 4 now with proper coils though and I admittedly haven't done a proper tune.

TexasKZ, I doubt I've got my tank vent blocked, but I will check it easy as it is to do. I usually stop at the bottom of the filler neck. It has only came out the top directly after a fill up and only when I've hit high speed and upper RPM range. I'm betting I get that blow back from the vacuum line and it makes a bubble big enough to push some fuel out the top (probably through the vent itself) since its so full. Hasn't happened once my tank gets to 80%. So I'm fairly sure that the root cause of the fuel leakage is the blow off from the vacuum line. Just need to check my valve lash I guess, but I'm 99% sure it's on point and if daveo is right then I may just need a proper tune. here's to hoping!

Stock BS34 main jet spec is 120.  The stock engine having pods and aftermarket exhaust the main jet size recommendation is like two sizes up, and you are eight.
You may want to post a pic of your spark plugs with a request for KZ guru comment on color.

So far as I know, the only way to get positive pressure in the vacuum line is from a significant backfire in that carb. Are you getting big backfires as you ride?
Also, the vacuum line dies not have access to the fuel tank unless the petcock diaphragm is torn or missing. Even then, the pressure required to force fuel out of a properly sealed and vented cap would easily blow the little vacuum hose off its little fitting.

 It looks like you need a valve job to get a proper valve to valve seat ring sealing. A carbon build up on the valve/valve seat is a PITA
I don´t know the engine history/milage but an insufficient intake valve sealing can cause a back firing into the inlet ports/carbs.
A compression/leak down test tell you the truth about your engine.

Unfortunately I probably wont be able to pull the valve covers and check my lash till this weekend. I'm going to richen my mixture a little today cause my plugs were a bit on the light side and see if that helps. Maybe I just got a little full on that fill up when it happened. I haven't filled it as much since and havent had the problem but I'm pretty sure I heard a little carb backfire still (my muffler screams quite a bit) I don't have SIGNIFICANT backfire through the carbs, only regular backfire I have is a little pop and crackle on decel. makes sense that it wouldnt go through the petcock now that you say it Texas KZ, but i know its all good cause I rebuilt the petcock also. I know when I did my initial tune I was getting some backfire through that vacuum line every so often with no tank on and the line just chilling. daveo I'm pretty sure I got the right jet sizes cause when I rebuilt the carbs and rejetted I found an article from a guy who did the same setup, I just forgot what they are cause that was last fall and haven't messed with them since.

Needless to say I'll be going over my valve lash this weekend. Seems like the likely culprit, but I might as well pull a carb bowl and check my sizes while I'm at it and make sure I didn't FUBAR. I'll keep you guys posted and let you know what comes of it.

btw looks like you've had some fun with that Scirocco!! my valves we're pretty clean when I pulled the vavle covers over the winter, and I checked the lash at the time, but the PO had adjusted them and all seemed copacetic but hey maybe I fudged the check, second look couldn't hurt.

A little popping and crackling on deceleration is perfectly normal, and is actually afterfiring; that is, firing in the exhaust, not the intake. Stock exhaust systems typically muffle it enough that it is hard to hear. 
BTW, when running the engine with the tank off, that vacuum line should be plugged, or you will have a wickedly lean mixture in that cylinder.

Yeah I knew the crackle was normal.
Did not, however, know that I should have that vacuum line plugged! Thank you! should tune just fine if I've turned  the mixture screw out the same as the other carbs though, correct?

I usually stop at the bottom of the filler neck. It has only came out the top directly after a fill up and only when I've hit high speed and upper RPM range.

not sure about your bike but if I fill to the bottom of the filler neck on my Z1, I will leak fuel from the tank as the gas expands in the heat. I would fill a little less gas and see what happens. My Z1 doesn't care about rpms. It could be on yours the upper rpms are raising the front end enough to allow the gas to flow out of the vent.

My solution was to not fill so full. A backfire though the carb will not affect how gas in the tank acts with a vacuum hose attached or not on the petcock. If it did you would be sucking gas through your vacuum hose. Might just be a Coincidence ...