The big screw on the side of the c125 is the idle mixture.
Clockwise=richen.
The power valve under the fuel inlet is for top-end adjustment.

Impco's don't work well with worn-out engines, not enough vaccumn...

Impco has a kit availible, to increase the fuel pressure of the vapourizer. It works well, I've used them when converting LPG to CNG. P/N on the bag is ACI.74 600623.

Hope that helps...