Vacuum Advance

I got into this subject on a J K ram distributor. There are two ways to advance timing; vacuum advance and mechanical advance. The vacuum advance is accomplished by a can attached to the outside of the distributor body. The mechanical is done by two spring loaded weights inside the distributor which move outwards with increasing RPM. For the J, the factory specified an initial timing of 15 degrees BTDC. Other Mopar engines in 1963 were 10 degrees which means the vacuum advance for the J is unique to the J.

Having the correct operating vacuum advance is important. You can expect the original diaphragms in the vacuum can are more or less shot. What to do if you need a new vacuum advance? You could try to get your old advance rebuilt or you could try a modern unit.

I bought a new one from Vans (TP-VACADV/BB/DPE). It is for the Mopar BB and Hemi late sixties dual point distributor. The problem is most vacuum advance units have more than 8 degrees and this new one is no exception.

The stop on the vacuum advance is that square flag you see on the arm. Usually there is a number stamped on the arm which tells the maximum advance. On the J this is 8. The new unit from Van's is stamped 11. This is way too much.

I measured the difference between this new advance and the one from the J. I figured out that I needed to stop the flag .040" sooner. I took .040" brass wire and I added it to the flag. I made sure it stays there with dab of weatherstrip adhesive. By adding this brass wire, I have changed the max advance on the new unit from 11 to 8 degrees.

If you were to look inside the vacuum advance unit, there is a large spring. It sits against the vacuum diaphragm and controls the initiation of vacuum advance. Sometimes you will find washers with the spring. This is like a pre-load. With washers, you increase the initial starting vacuum required to move it. The spring's stiffness, or spring constant, determines how quickly the advance increases with more vacuum, although this has no impact on total advance.

I noticed the J diaphragm and spring seemed stiffer than Van's. This means the J holds off vacuum advance until the carb is closed a bit more. I looked at the spring inside the old J vacuum advance and was surprised to find about 6 or 8 washers under the spring, which partially explains the delayed advance start. Being less stiff, it is likely Van's vacuum advance will start advancing sooner but I don't see this as a problem. The rate of vacuum advance for any engine (to improve gas mileage while cruising) has to be similar. From past experience, looking at graphs, especially old Studebakers, they published graphs of this across several engines, and all have the same shape versus vacuum. Mopar only publishes "Check points" on advance curves (vacuum and centrifugal), which can be frustrating for us, but acceptable for servicemen.
Given this, and that the Van's unit is brand new, I decided not to open it. Messing with it might tear the rubber diaphragm. Be gentle with the arm too; any twist can tear the diaphragm (like that idle stabilizer on long rams, most of which are broken because someone used one wrench to adust it).



Here is a quick look at the J mechanical advance. I measured the diameter of spring wires, the number of coils and length of slot.
The light spring: 8 turns .018" wire, overall length .55:, when relaxed in mount the spring coils touch.
The heavy spring: 5 turns .050" wire 1.35" long, big spaces between coils when relaxed.
The slot for the timing stop is .395" long. Limiting mechanical advance is important. Sometimes the slots was filled with braze or machined tiny block etc. Note only fill outward end.


Don't lose this critical spacer. The open cone goes down toward
engine. It sometimes sticks to cam bottom and rolls under bench.


Looking good, spacer is in its location.


Ready to advance. Note that light spring weight has a bronze bushing. This weight moves a lot.


The vacuum advance plate is mounted on a real ball bearing, not pins and sliding brass feet used on single point distributors. Make sure this bearing moves freely. Lubricate with 75 W.

Since I was deep into the distributor, I did other things.
I moved the point arm spring to the inside of the terminal flag held by screw head you leave in. This relocation increases the load on the points a bit. Then I moved the wires held by a nut on other side for no more dropped screws. I replaced the always frayed ground jumper (have to solder to do that made longer) and point to point to terminal jumpers with ultra flexible silicone hi temp wire, (Amazon red and black) terminals soldered on, cleaned up, oiled through grease fitting with GL 4 75 w synthetic ( no GL5, that is bad for brass).



Ultra flex silicone wire (Amazon)


New longer ground jumper folded up leg. No solder blobs allowed


Heat shrink just right length, prevents break off at attachment by grabbing outer insulation


Have to enlarge small #8 eyelet with reamer to fit over #10 stud . #10 eyelet is too big.
Even #8 crimp part is shortened about 1/2 cut off -- wire off inlet terminal post cannot stick up


On terminal, one ear of factory part removed so new terminal sits at 45 degrees.
Also need to file the Bakelite insulator to allow that


Beautiful!! Note long ground jumper loop, wires and point arrangement at mount flag,
brass washer under clamp screw so gap does not change as you tighten points
Although this picture shows a brass round head screw, the correct screw to use is a 6/32 hex head
so you can tighten them easily with a small wrench. I did not have those hex head screws
when I took this picture.