they can be difficult that's for sure, I've had success using either a piece of coat hanger bent like a hook and applying pressure or also using a long shaft small bladed flat screwdriver again also applying pressure from the back side of the carb cover maybe try a bit of luberplate (white grease) along the edge of the rubbers as well.
Dielectric grease helps a lot. It still can be hard with brand new boots as they need to stretch a bit. It took me almost 3 hours getting my on! lol Quite a few times 'almost there' just to have it pop out on the other side or all three tilt and off. I found a 'system' eventually, using a few thin pieces of metal and a bunch of screw drivers and prying bars jammed so the carbs want slide back and back off, to keep them installed while working on the other side, but that's hard to describe and a long time ago. Eventually the boots expand a bit and it gets easier.
1982 Kawasaki KZ1300 A4
1981 Kawasaki KZ1300 A3
assembling engine, chassis & electrical from 79 - 89 parts
Here's what I wound up making the gasket out of...it's silicone weatherstripping that I found at Lowes. The adhesive on the back is surprisingly good and it didn't pull off even with all the pushing and pulling trying to get the airbox on.
I think it was bout $7 for a roll. I wound up pulling it apart and making the gasket out of one long piece. I just cut halfway through the outside of the strip where it needed to make a turn around a corner.
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