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Looking at the vehicles you have listed,
On the VW a few things since its pretty old, some may cost a bit.
A newer high flow 3 way cat, as close to the engine as practical.
a wide band O2 sensor with a display (it could even be a multimeter, and translate the voltage to AFR yourself)
Use the wideband display to adjust the carb to stoc.
The other one
wide band O2 sensor, so you can at least see what is going on.
See what you might be able to do as far as programing the ECU.
For either one, make sure they are tuned properly and maintained.
The Cats require NOx and HC to work properly, but to clean up the NOx the most slightly rich will reduce the NOx at the expense of higher HC emissions.
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