MGB: O/D Restrictor

Perusing the Moss catalogue, they list an o/d vacuum restrictor p/n 373-855. What does that do and what hooks up to it?
Does that mean all o/d cars are supposed to use manifold vacuum?
TIA

Chris, they were only used on three-synchro ODs if I recall correctly. They used a vacuum switch to prevent OD switching under heavy throttle. I'm not putting one on the '67 that I'm converting to OD, since I won't be hitting the switch when my foot's on the floor, and I consider it to be just one more goofy little electrical doodad that can and will screw up!

Ken

I wouldn't use it either. My OD can be switched in or out under full accel or decell with no problems. My car is a '78 and the OD is an LH type out of an '80.