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I have a 1975 Kawasaki KZ 400 that poped through the right side carb and now is doing it through the left. I've checked the valves and they are good along with the compression and timeing doesn't seem th be an issue. I rebuilt the carbs about a month ago and it ran good for a short time. the first time i filled it up some hidden rust came off the tank and got into the carbs. Since then i have cleaned them. I hane also changed the rubber carb holders with some universal ones, they fit the carbs ok but the distance from carb to head has changed some.

I can't figure out what the problem is

It pops when idleing and decelerating so i thought it was the idle circut in the carb gut now i know they're clean and it still has issues. at first is was just the right side and now that is doing better but the left side has started. I was begining to believe it was a broken intake valve spring on the right side until the left started poping also

If it is sucking air around the intake would this cause the problem?

Sucking air around the intake would make it idle high for a while before idling down. Popping under deceleration and at idle indicates exhaust leak.

Or just an overall lean fueling condition. Check the carn/boot seal very closely for an air leak (spray some carb cleaner or other flamable substance around the boots at idle and see if the RPM's change) and if that checks out, try going up a size on the low speed jets.

I found out that the flange on the new boots is bigger than the original and there was a space on the botton that was not sealing. I siliconed the old boots and the problem is fixed. I just need to make an adapter plate for the new boots and they will work.