This is a powerjet RZ carb
And this is where the jet itself is located
The basic principle of operation is:
The remainder of the carb is conventional, and the pilot, cutaway, needle & needle jet and main jet function as normal.
There is a tube from the float bowl to an area where vacuum will exist when the throttle is wide open. In this tube there is a jet, about the size of the pilot jet in the carb, that meters an extra amount of fuel in addition to the main jet at WOT.
This allows you to use a slightly leaner than normal main jet so that the transition (when excellerating) from needle to main mixture is not overly rich, but the correct amount of fuel is delivered for long periods of WOT.
When these first were developed circa 1972? I was racing a TD3 powered superkart and fitted these as an aftermarket kit that was available at the time (they are still available about $30 a carb).
We found that by adjusting the length/diameter of the hose we could delay the time it took for the power jet to become operational (by a second or two) this gave us an advantage (in a kart) at places like entry to a long sweeper, where the slightly leaner mixture would give a crisper response and better excelleration ....enough to pull alongside a compeditor for the run through the sweeper.....and just before detonation would set in, the powerjet richened it up......
Ahhhhh memories....... I won't tell about the times I got it wrong though.....
Graham