As far as tuning potential goes...... there is none! They were built for economy and nothing else. Way underpowered for a car of that size/weight.

You would spend twice the money of a full engine conversion to gain any extra power.

Your first mistake is trying to do anything with a Pierburg carb. They are complete cack!!

Stick a Weber on, they are a far superior carb to Pierburg, far easier worked with and will let the engine breathe easier which can only be a good thing.