First thing to remember, is with a correct idle spring you should be able to drop the clutch at idle in first gear and you car should start moving strong without stalling the motor.
A too strong idle spring will be fully extended below certain RPM, lets say 1000rpm, so below 1000rpm there's no more governing from the spring and any load applied on the motor will make it die (the spring does not add anymore throttle to compensate) It's also true for a too soft spring that anything higher than a certain RPM, lets say 1000rpm again, the soft spring is fully compressed, so the RPM will start to raise and hang there, or until the middle spring start to govern at its rpm range.
You need to find the right ''strongness'' of your idle spring, you can also extend your spring to give it more headroom to govern.
As of your rough idle and bad smelling, it's due too a not enough aggressive cam plate (too soft injection) for what a VW TDI engine need.