I'm not following exactly what you're saying about the check valve, a picture might be better. You just need to replicate the feed it had on the bike.

Having a guess here, is the feed to the carbs a dead end from the pressure reg? If it's flooding after shut down either the pressure is climbing when the flow stops or the needle valves are not sealing properly when the floats are up. Stick a pressure gauge in the fuel line to the carb and watch what happens when you shut it off (low pressure range). You'll soon see if it goes up, holds steady, or gradually drops off. If it's a dead end line it should hold some pressure after shut down. What ever that pressure is is what the needle and seat can hold. If it goes up briefly then your pressure reg's the problem.

Also, 1psi is almost 1 meter of head (like having the tank 1 meter above the carbs) you can probably drop it some more. The formula is P = row*g*h or roughly P = 7*h kPa with h in meters for petrol 1psi=6.89kPa.

Have a cuppa and mull it over, you'll figure it out.