Jeep CJ/Scrambler 1971-1986 Repair Guide

Vacuum Throttle Modulating System (VTM)


This system is designed to reduce the level of hydrocarbon emission during rapid throttle closure at high speed. It is used on some 49 state and all California models, with a V8 engine.

The system consists of a deceleration valve located at the right front of the intake manifold, and a throttle modulating diaphragm located at the carburetor base. The valve and the diaphragm are connected by a vacuum hose and the valve is connected to direct manifold vacuum. During deceleration, manifold vacuum acts to delay, slightly, the closing of the throttle plate.

To adjust
  1. Run the engine to normal operating temperature and set the idle speed to specification. Shut off the engine.
  3. Position the throttle lever against the curb idle adjusting screw.
  5. Measure the clearance between the throttle modulating diaphragm plunger and the throttle lever. A clearance of 1 / 16 in. (1.6mm) should exist.
  7. Adjust the clearance, if necessary, by loosening the jam nut and turning the diaphragm assembly.