The easiest way to gauge how tight is by the gap between the cup washers on either side of the rubber. The gap in your picture looks about right which must be the same on all the studs to ensure it’s straight. If the rubber is older, I’d suggest replacing them all along with the larger gaskets between the carb and manifold which look unusually thick on your picture for a Crossflow.
Air leaks will certainly cause popping, although that’s also part of the Crossflow experience!
These for the carb to manifold https://www.burtonpower.com/sandwich-plates-misab-40-45dcoe-x-2-set-of-4-webmisab.html
http://www.usa7s.net/vb/blob:http://www.usa7s.net/d9e7b36a-96be-4a89-8cee-4c88dfbb2641
these for the nut washers https://www.burtonpower.com/carb-mounting-kit-dcoe-carbs-x-2-webcmk.html
http://www.usa7s.net/vb/blob:http://www.usa7s.net/fd83baa5-f2e1-44ba-bee3-06c9425b4dd9
You should be able to purchase from Redline or Pegasus in the US.