I ran into a lot of the same issues you are currently running into. The popping issues I solved by replacing all of the rubber goods sealing the carbs. Mine were hard and I had numerous air leaks causing a lean issue. Put on the new rubber and synced. No more popping sounds.
Another error on my part was to tear down the carbs, clean them spotless, and put new parts in them as my carbs looked as dirty as your did. Here's my mistake: I failed to clean the fuel pump and I had to pull the tank out and use the three part system to clean it and coat it so I didn't have to worry about rust again in the future. Unfortunately for me, that meant I had to pull the carbs again and go through them one more time AFTER I cleaned the fuel pump and coated the tank as more rust particles had made their way back into the carbs.
If you haven't already, please take a look at the inside of the tank and see what it looks like as tearing the carbs apart, although not rocket science, is time consuming and a pain in the ***. Wish Ya the best!