POP pressure is Bull chit. The metering spring is just one of the components that 'ramps' in the fuel from the low speed circuit.
You may be able to fix the idle by simply shimming up the metering spring. If you can't find a few small washers to put under the spring, then make up some shims from what ever you have available (tin can, aluminium) and try them. If they start to fix the idle problem, then maybe you can find a stronger spring from one of your other carbs to stick under the metering lever.
The 'ramp' of the low speed circuit is determined by the size of the metering orifice and the spring rate. The spring rate is determined by the thickness of the spring and the pitch. Together, the regulate how fast the low speed circuit comes in.
The other remaining 'ramp' tuning is the pre-load on the spring. A higher pre-load delays the ramping in of the low speed circuit. Since you have a problem with the idle, it may be that a higher seat pressure (more pre-load on the seat) will lean out the idle. A higher seat pressure works similar to dropping the float level in a float carb. It takes more 'suction' to start the fuel flowing.
Try shimming the spring to start with and then go from there.
