Backfiring through exhaust at idle?

[Kiletravis09]

So I just completed a carb rebuild on my 86 gl1200A. I kept all the OEM parts on the carbs, did an ultrasonic cleaning, removed pilot screw and replaced all rubber seals and gaskets including the tiny o-ring on the pilot screw. I reassembled the carbs put them back in the bike and bing bam boom....she fired right up.

This bike had not been started in 10 years so before I fired her up I lubricated the cylinders, drained and cleaned the fuel tank, replaced all fuel lines, installed new fuel filter, and inspected and replaced all vacuum lines as needed.

This bike runs very well at the stock 2 full turns out in the pilot screw and it’s a blast to ride. The only problem I have is that at idle it backfires out of both exhaust pipes and I dosnt seem to want to settle at idle. It kinda goes up and down as if it’s trying to regulate itself. Also when I test drove it earlier this evening when I pulled in the clutch to come to a stop it took a while for the RPMs to come back down to idle. When I put my hands behind each exhaust pipe it back fires out of both exhaust pipes at the same time.
It’s not a super loud bang just kinda like a hot burst of air.

So far I have checked and inspected the spark plugs, check ohms on both primary and secondary coil lines and they checked out perfect, checked compression, and made sure I didn’t forget to plug any vacuum lines back up.

I also synced the carbs.

I sprayed starter fluid around the intake boots and other areas that could leak vacuum. Got nothing. It’s sealed up like a dolphins blow hole.

Iv done some research and I have figured out that it might be the air cutoff valve? But I thought that only engaged under deceleration? Could that give me problems at idle?

Also maybe I just missed something in the idle circuit? Also what’s the non removable brass tube in between the main jet and pilot jet? It that just a vent? I didn’t really focus on cleaning that.
I really don’t want to remove the carbs again but I fell like this time it won’t take me near as long.

Please help me figure this out so I can really enjoy this bike. It was a barn find that only has 11,000 miles on it. My wife and I are ready to take a nice trip on this thing.