Trouble with gpz900 rpms

Gettings all. I have a 1985 Gpz900.  It's got air pods, rejetted, and what appears to be an aftermarket exhaust made by kawasaki.  I bought it from a friend for a great deal.  He let me drive the bike about a year ago and it ran great for the most part.  It pulled hard up to redline.  Well, he has 3 bikes and this one just sits.   He started it once a  week and lets it warm up, then drives it around the block.  Other then that, it hasn't seen much except 88 miles in the last year.  He also had the carbs professionally rebuilt by a local, reputable motorcycle shop 2 months ago.  However now, the bike runs way worse.  Everything seems normal when starting the bike.  The bike idles ok with the choke.  There is a loud popping type of sound coming from the intake/carb side.  If i put my hands on the air filters or carbs, i can feel the popping easily and its pretty strong.  It revs great when sitting.  So i take it for a ride and its got a ton of torque at low rpms.  However, somewhere around 5500 rpms, it just stops accelerating. Its really bad, like a rev limiter.  If i am really easy on the throttle i can get it close to 6000 rpms.  There is no missfire from the electrical system that i can identify.  I took the carbs apart and everything looks very clean and new.  they are adjusted 2.5 turns out, with a 140 main jet.  Not sure if its normal, but i can hear a whine sound from the gas tank... I open the filler cap and it makes a sound like the gas tank is pressurized.

So my questions are, what is the popping from the intake side?  And why do i have great power untill 5000-6000rpms and can't go above 6k rpms? any suggestions or ideas?  My thinking is that its a lean condition, but i don't understand why as nothing has changed since it ran great last year.  Maybe when the carbs were rebuilt the wrong size jets were installed? I tried putting tape on the airpods to reduce flow, but it made no change at all. Any help will be greatly appreciated. Thank you in advance.

The popping out of the carbs is a sign of a leak valve/valve seat ring by no valve clearance. Check/adjust the valve clearance first. If possible do a before and after compression test. 
The valves are shim less adjustable with a screw and a lock nut.
Don´t ask how do i know it.

Thank you for responding.   It only pops below 2500 rpms... Does that make a difference?  I would really hate to have to adjust the valves on this bike.  I did another test run with my gas tank lid open and i got zero hesitation at 6k rpms.  Could my gas tank not be venting properly? and what is an easy fix for this?

The gas tank cap vent could be clogged by dirt/rust/dust.
A vacuum will build up and cause a lean fuel/air mix, (popping/back firing)
Disassembling the tank cap parts and give them a good clean.
A valve clearance check/adjustment is a must do to prevent more damage and will optimize the engine performance!!!
It´s no rocket science, trust me.