When you roll the throttle back you cut back on the amount of air entering the venturi.
The fuel entering the venturi will not respond as quickly and for a brief time you will have richened the mixture.
You will find references to the "Roll Off" tuning on many web sites. I grabbed this one just to show you I am not making this up.
The following "Roll-Off Method" is an easy method to check the jetting of the main circuit:
With the engine fully warmed up, cruise at 3500 RPM in third or fourth (allows a little more time to evaluate the test response). Apply full throttle and allow engine to reach 5000 RPM (time enough to settle-in and for accelerator squirt to dissipate). Quickly Roll-Off the throttle to a 80% throttle open position. This will momentarily richen the fuel mixture. If the engine responds by gaining power and accelerating then this indicates a lean jetting configuration. If the engine goes flat, stutters or runs rough then the mixture is rich. Remember that the fuel delivery on the main circuit is the summation of fuel supplied by the main jet, the emulsion tube and the main air correction jet. The Roll-Off test provides an indication of jetting during a particular region of operation.
The Roll-Off test provides an indication of jetting during a particular region of operation.
This is the most important thing to come to grips with tuning any variable venturi carburetor. The effect reflects ONLY the fuel circuit the carburetor is using at the time of the test. Basically there are 4 combinations of fuel delivery circuits and all must be tuned separately. Idle, 0-1/4 throttle, 1/4 to 3/4 throttle and 3/4 to Full throttle.
Idle = pilot air screw and pilot jet; 0-1/4 tapering off pilot circuit, and on to the slide cutaway and needle jet size; 1/4 to 3/4 tapering off needle jet and slide cutaway, and then onto needle taper position; 3/4 to Full tapering off the tapered portion of the needle and on to the main jet.