Your weeping Mikuni cards, they are supposed to be gravity fed on a chopper, so..... Get a chopper fuel tank & duct tape it to the roof, I thought about bonnet mounted chopper gas tank, but I think the cops might frown on that...

The Serious Answer:
• You need to bleed the pressure out after you turn it off.
• Some short bits of 1/4" & 5/16" pipe, preferably cad plated like Volvo use.
• 1st, a piece of 5/16", 2" long, half way along, drill a small hole, braze or silver solder a 1" long piece of 1/4" or 5/16" pipe to it, stick that on the hose feeding your fuel pump.
• 2nd, a piece of 1/4" and/or 5/16" pipe & make a second T-piece out of & stick that between your regulator & carburettors, the hole you drill in the side of that should be as small as possible.
• Connect the bleed T-piece & the pump pick up T-piece together with the appropriate sized hose.

This will solve the Mikuni problem.

The SU's: Take them off & see if the throttle blades are the right way around & you can't see daylight around them when you hold the piston up, that's the only real source of air/fuel mixture that can make it idle fast smoothly. Other sources of air after that will make it pop & backfire. Some wick-in Loctite should be applied to the throttle blade screws AFTER you are sure they are Hunky Dory/Ridgee Didge!

Cheers,
Paul.