The D-jet system uses a Manifold Pressure Sensor (MPS) as its primary indication of engine load. The MPS is a silvery hand-grenade-looking thing with a single vacuum hose fitting and a four-wire electrical connection.
The L-jet system uses an Air Flow Meter (AFM) as the primary indication of engine load. The AFM is a piece which bolts onto the air filter box, and has a single five-wire (I think) electrical connector on it. The AFM has a quarter-circle "bump-out" on one side.
For more info on the D-jet system than you ever wanted to know, check
http://www.914fan.net/djet.html
I'm not sure of a similar site with L-jet info.
The 79 bus did originally come with L-jet.
--DD
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Pelican Parts 914 Tech Support
A few pics of my car:
http://www.pelicanparts.com/gallery/Dave_Darling