The 28s have a single stage accelerator pump. This means that there is a diaphragm that is actuated by the throttle. It provides a single squirt of gas when the throttle is opened... how far is required depends on the adjustment. The gas is necessary to overcome what had become progressively leaner jetting through the late 70s due to EPA regulations and became necessary to keep a lean spot just off idle. The lean spot can show up as balkiness when accelerating and backfiring when the throttle is almost chopped. IF the VM28 w/pump are adjusted and tuned properly, they can provide great performance. As far as "good or bad", that question is hard to answer as ALL carburetors have strong points and weak points and whether a partiular set is good or bad depends a lot on your riding habits and priorities. ALL VM series carburetors work well when tuned and jetted properly for the application they are intended. If you want all out performance, some are better suited while others are better suited for low end performance and economy. If you use a set intended for performance and your goal is performance at lower speeds and mid-range and economy, you will be disappoint and the converse is also true.
