Jet valves are very small extra intake valves that feed a nozzle in the combustion chamber. They are fed a lean mix from the carb thru the intake mani to the skinny passages under the mani flange, and from there to the passage under the jet valve head. The intake rocker has an ear with a lash adjustment screw that opens the jet valve at the same time as the intake valve. The nozzle in the combustion chamber sends the jet valve very lean charge into the incoming main intake air/fuel mixture, and makes the whole mess swirl around the chamber. This is supposed to make for cleaner combustion (higher probability of firing a leaner mix). The skinny little jet valves will eventually break off the valve head, letting the stem, spring, and retainer tour the valve train. In short, the answer to the question is that a jet vavle is a problem waiting to happen. Caravan 2.6 heads minus the jet valve assy. is what we get from clearwater, and you usually get manual lash adjusters with them, unless you order it with hydraulics, since that's what caravans came with....