Beginning with an illustration from the prior page, note that the vacuum pipe routing in that illustration is completely wrong. Item 1 is drawn upside down. - The pipe running to the carburetor needs to be on the bottom of the fuel separator bulb, while the pipe running to the distributor needs to be on top of the bulb.

Part way through production, while trying to diagnose and fix a piston burning problem, the factory deleted the vacuum advance function. They also recommended that earlier cars be retrofit with non-vacuum distributor. Right at end of Twin Cam engine production they finally figured out what was causing the problem (and then knew how to fix it), nothing to do with the ignition system. But since the engine was being "abandoned", they never bothered to change specifications and parts back to original style with vacuum. As such, many Twin Cams have non-vacuum distributors.

The vacuum advance function is advantageous for better throttle response from lower engine speed, and better fuel economy while cruising at partial throttle. Fully prepared race cars do not idle slowly, and seldom run at partial throttle, either full on the throttle of hard on the brakes, and not much in between. As such, if you want the car primarily for racing, then race cars don't have much use for a vacuum distributor. But if you want to drive it on the street, then there are advantages to the vacuum advance function.

If you want to install (or re-install) the vacuum distributor, then we need to be concerned with routing for the vacuum pipe, which is very poorly documented in factory literature. So let's start with a collection of pictures. The first two pictures show the fuel separator bulb attached to front of the timing cover, which can be functional but is not original. The third picture below (not a very good picture) shows correct location for the fuel separator bulb on the rear side of the engine front plate. The bottom pipe runs back to the carburetor venturi vacuum port. The top pipe runs between the front plate and cylinder head (below the cam covers), across to other side of the engine, to emerge close to the distributor vacuum unit.

With more time I hope to post more pictures and better documentation of the attachment clip on left side of engine, and routing of both ends of the pipe. Stay tuned.