Vacuum Advance

[celentanogj]

There is plenty of material concerning to vacuum advance or not vacuum
advance; I decided to go back to a stock type vacuum advance distributor with the Pertronoix Ignitor II system: I'm currently using the MSD Street Fire box, mechanical distributor and blaster II coil-I just do not like the setup, from the reading others like the way the MSD works and others think it's junk. I'm hoping the new distributor with the Ignitor II works out if not back to points.

[DanEC]

As long as the distributor has not been modified to greatly limit the amount of centrifugal advance so that a large amount of initial advance can be dialed in (like 18 or more) - the vacuum advance should work out fine. Be sure it's hooked up to a full time vacuum port on the carb (one pulling vacuum from below the throttle blades). The only drawback I've experienced with vacuum advance units is sometimes the engine head design, compression and cam will not tolerate a vacuum can that pulls in a lot of advance and you can get a light trailer hitching action (light missing) on the highway in high gear from too much total advance. In this case you may have to find a vacuum can with less advance or cobble up a limiter to restrict the amount of vacuum advance in the distributor.

[celentanogj]

Thanks for the advice. The 428 CJ is just a little more aggressive than stock and the vacuum canister has the adjustment screw, I'm going to do some testing to dial it in and use the vacuum timing port off the carburetor if it needs more than it will be direct off the manifold. As a friend told me the most work you want to do on this car is put gas in it.

[rads42]

Any follow up to this? I'm curious how This worked out for you.

[KevinW]

I connected my vacuum advance up to the Timed Spark port as shown here (page 5, Fig. 6).

https://documents.holley.com/199r10487-1rev6.pdf

Why should I connect to the full vacuum port when Holley is telling me to connect to this other port?

[DanEC]

Most likely because that is where auto manufacturer's wanted a port for connection to retard low speed timing to increase engine heat and decrease emissions at the expense of performance, fuel economy and lower cooling temperatures. I doubt any of that (poor low speed performance, high cooling temps, poor fuel economy) appeals to you which is why everyone interested in a performance engine connects their vacuum advance to a full time port originating below the throttle blades. Since back in the 60s Holley typically had one port above the throttle blades and one (ported) below and since the full time vacuum port was often required for the choke pull off vac can, it became common to cut that hose to the choke pull off, put a "T" in it and connect it to the distributor vac can also. Then cap the useless ported outlet.

[KevinW]

Thanks Dan - that cuts to the chase.
And I do have absolutely terrible mpg (single figures).
I'll give it a go connecting direct to full vac port.

[RET_COP]

I run ported. I have a 14* advance bushing in my MSD and run 19* initial and 33* full. Good size solid roller Pro System carb. Steady idle at 1100 no overly rich smell and cruises nice. I guess every combination is different.
DanEC did say, "
"As long as the distributor has not been modified to greatly limit the amount of centrifugal advance so that a large amount of initial advance can be dialed in (like 18 or more)" this is probably in my case.