Adjusting the vacuum advance on a ZX distributor

[Walter Moore]

I have been tinkering with a 80ZX distributor for my 240Z recently. I bought a used one and took it apart. The advance plate (Breaker plate in the FSM) was broken, and I repaired it as I suggested in an earlier thread, but I was not happy with the operation afterward, so I ordered a new breaker plate and vacuum advance from a local Nissan dealer. I had trouble getting the "correct" parts, and ended up with these:

Vacuum Advance: 22301-P7901

Breaker Plate: 22136-H9501

Having heard all of the horror stories about too much advance I decided to measure the vacuum advance of my new setup. To do this I first measured as best I could the radius upon which the vacuum advance mechanism operates, and discovered that it is apparently 27mm. Since the distributor advance angle is equal to the arc sin of the linear movement of the vacuum advance pivot divided by the radius, I was able to determine that the two parts listed above, mounted in a D6K8-02 distributor result in a vacuum advance movement of approximately 0.234 inches, which calculates to 12.5 degrees at the distributor, or 25 degrees at the crank. Clearly this is excessive.

By comparison, the original breaker plate allows only 0.157 inches of movement by the vacuum advance, which works out to be 17 degrees of spark advance (at the crank).

The FSM lists the vacuum advance of my ZX distributor at 18 degrees, which is likely true when the NLA original advance actuator is installed. But with the parts I have available it will only produce 17 degrees.

Anyway, to make an already too long story shorter, I shortened the vacuum advance slot in the new breaker plate with a rivet and J-B weld. Then I used a Dremmel tool with a chain saw sharpening bit to lengthen the slot back out to the point where it allows the advance to move approximately 0.157 inches as measured by my optical-dial caliper measuring method.

(See attached picture.)

Unhappily the actual total length of the slot appears to be irrelevant because the vacuum advance actuator does not push the breaker plate all the way to the far end of the slot.

This weekend I installed the rebuilt distributor and confirmed with a dial-back timing light and my handy air-vac, that vacuum advance does indeed produce 17 degrees of advance.

So now I have this setup:

Initial timing: 10 degrees BTDC

Centrifugal advance: 17 degrees

Vacuum advance: 17 degrees

Apparently my points distributor, which I recently discovered had a broken vacuum advance, was in worse shape than I thought. The new distributor improves engine performance across the entire operating range. It finally idles smoothly, and it pulls much better in 5th gear.

You just don't realize how much better a car runs with a solid distributor.

The attached pictures show some of the things I have discussed above, and the spread sheet shows approximate timing advance angles produced by the vacuum advance movement listed. Be forewarned however that I have ONLY verified the advance angle produced by 0.157 inches of displacement. While the other displacements listed SHOULD produce the advance angle shown, I make NO GUARANTEE of the accuracy of the data.

Vacuum advance distance.xls