Tuning help on '79 LTD 1000 build

Hi Friends. I have a '79 LTD 1000 that has been my project bike for several years. I think I am close but would appreciate some advice for the last little piece of tuning. Here is the setup.
Bored and installed a MTC 1075 kit with high compression 10:1 pistons
Vance and Hines super sport exhaust
Pods
Stock carbs...Mikuni vm26ss rejetted from 102.5 to 115 mains, 15 to 20 pilots, needles on 4th notch, mixture screws at 1.25 turns
Dyna S ignition with green coils
Timing exactly right, compression at about 160 psi, and I only run the best gas
It starts, warms up and runs fantastic. Here there are two problems...
When I shut down the throttle it backfires every time. Usually it sounds like loud popcorn, but sometimes it is worse, like a gunshot.
Second problem... plugs are showing rich. Mostly clean but black with a little soot.
Since the problem happens between the closed- 1/8 throttle position I would think it is the pilot circuit but 20 should be a good pilot jet. Backfire usually is a timing or lean issue but I am 100% about the timing and the plugs are showing rich not lean.

I am very interested in your ideas.

When was the last time the valves were adjusted?
Steve

Sooty plugs seems like rich on 0-1/8 throttle or ignition misfire...but backfiring can also be a lean condition. That 1.25 turns out...have you tried other settings?

When I turn a bike off I flick the killswitch off, then on again once stopped and then turn off and remove key. When you turn the ignition on or off even without the motor turning you always get a spark, on later models you can see the electronic tacho needle jumps. You have unburnt fuel in the pipe igniting.

That and the sooty plugs makes me think too rich, using pods sometimes calls for a different setting to the FSM spec.

Maybe try tweaking that idle mix screw and see if it makes any difference, Also check the dyna ATU is snapping back sharply - ie not sticking on advance like some of the factory ATU's do.

Are you getting good carb balance at idle?

Most are at .004. Cylinder 3 exhaust is a .006. #4 is at .008.

Carb balance is excellent. I'll verify that the Dyna advance function today.
I'm thinking your idea about the mixture screws might have merit. It requires that the engine cool off before each adjustment because of their location, but I am a patient man. I'll post results.
BTW, I live in South Florida so this is a great time of year to work on getting this right.

#1. I'd push those needles back down into the jet one notch and check and readjust the float levels to their correct settings.
#2 Make sure the collector has the proper baffle and correct amount of packing.

FIXED!!!! The culprit was...me I guess. The camshaft gasket was the wrong model and was missing the piece the went around the emissions ports. I attached a picture. When I went to pods I plugged the ports under where the reed valves were located. Because the gasket was for a non-emissions model it was missing the spot where runners went between the exhaust ports and reed valves on the cam cover. It had four exhausts leaks between the cam cover and head. You wouldn't think that would be enough to cause the issue I was having but once I pulled the cam cover and made little gaskets to block those ports the problem was fixed. It runs great!!!

By the way, Thanks everybody for your suggestions. You had me thinking in new ways. It is amazing how the smallest exhaust leak could create issues.

I took it out for a 25 mile ride tonight. Awesome!!!

That's a good lookin ride. Glad you found a simple problem.