[ho428]

I haven't used an OEM distributor in years and on a build I'm working on I dug out my old distributor and noticed it has both vac line hook ups off the distributor.
The Pro systems 750 I have doesn't have any vac line hookups so I can't hook the line above the throttle plates, but I can hook the below plate line to a port in one of the intake runners.
Any issues hooking up a vac advance that way?
Should I just cap the above line port?

[Scott65]

That's interesting, I don't have an answer, but I'd like to know more about the theory behind that design...

[ho428]

I'm thinking there may not be a return spring in the vac canister and needs both hooked up to pull the plate back and forth. But I just don't know for sure.

[Scott65]

Yeah, but wouldn't that cause more advance at idle than at part throttle cruise, where vacuum advance was more typically desired? Or am I thinking about this wrong?

[steve25]

Pulling vacuum off of just one runner will only provide 1/8 of the vacuum that the vac can should see!
This will kind of act like ported vacuum at idle but at larger throttle openings will drop off the advance some 13% faster then it should by my math!

[Schurkey]

Originally Posted by steve25

Pulling vacuum off of just one runner will only provide 1/8 of the vacuum that the vac can should see!
This will kind of act like ported vacuum at idle but at larger throttle openings will drop off the advance some 13% faster then it should by my math!

Nope. You'll never notice the difference between a port on a runner and a port directly into the plenum.

Some power brake boosters--which use MUCH more vacuum than a vacuum advance--pull their vacuum supply from a runner.

[Kenth]

I have found using the vacuum port on #1 runner will give a weaker and fluctuating needle on the vacuum gauge, at least at idle speed.
Can´t imagine using it for anything other than intended, as the thermal vacuum valve on air-cleaner.

[dataway]

Looks like that is a retard port, used on 68s. Did not last long at all, by late 68 a service bulletin came out about modifying the system to remove that line, and looping it at the TVS. Was an emission thing that evidently didn't work well.

In most cases you are not supposed to cap the second port in the vacuum can ... that will keep the advance diaphragm from working properly.]

Bullentin number was 68-T-2 I think.

[ho428]

Yeah, this distributor came from a 68 350, it's been 40 years since I pulled that engine out and apart so I don't recall anything about how the vac lines were hooked up.
I have a mid 70's HEI distributor, but pretty big and ugly though to use in a 68. I also have a spare MSD box and Blaster coil but no distributor.
I like the looks of that FAST unit, price is good too, just have to find a vac port to use.

[Scott65]

Could the current vacuum cannister be replaced with a conventional one? Or is this distributor a one off deal?

[Formulajones]

Ford used the same deal for years. One advances and one retards. Generally (on the Fords anyway) they were run to a thermal switch on the intake manifold to control timing during cold starts and warm up. Without that I'm not so sure the system would function properly and probably cause you more headaches than it's worth if you just try to hook one to ported and one to manifold. I don't believe that's how it's designed to function.

On my SCJ I simply made it appear to be hooked up properly but only had the advance portion of the vacuum advance working, and it worked fine that way. So to answer the question of whether or not the canister has a vacuum return spring, I can say the Ford version I mess with apparently does because it functions fine with just advance working. Not sure about GM. If it were me, and not worried about original appearance, I'd be replacing the canister.

[dataway]

Distributor is the same as other years, just the vacuum can is different. A line ran from the retard line to the TVS, then to the carb. Retarded advance on deceleration I think. But caused some starting rough idle problems so by late 68 they did away with it, looped the two ports at the TVS, plugged the port on the carb and did something with the port on the can ... I know they didn't plug it, because that kept the diaphragm from moving on the advance side.

The serviced bulletin has been posted in the past in the 68 technical section I think.

[lust4speed]

That two sided vacuum can was also on my 67 HO (build date Feb '67). I was thinking the service bulletin came out earlier than what was mentioned. I know the engine was much happier with the second vacuum line disconnected.

[dataway]

Yep .. it did come out earlier as seen in the attached pic. 10/67. What is strange is that 68's were still coming off the line to probably June 68 that did not have the modification done at the factory.

Not sure if they ever changed it at the factory ... by 69 they were no longer using the two port vacuum can ... I think they were still using the five port TVS but it was looped at the factory.

[amcmike]

The only time it's going to make a difference is at idle, and perhaps the transition from closed throttle to open (slight higher spike, whether that translates to a higher vacuum advance might depend on the vacuum canister setup).

As Kenth mentioned, having it pull from one runner as opposed to the plenum, it might not give as clean a signal to the can. But, I'd say first set it up without, then attach it and see how the engine responds. You'll have to adjust the idle speed down with that much advance. In the end it will come down to what your static timing is set to, how much the vacuum can adds, and how much timing @ idle it will tolerate.

[Skip Fix]

When I ran NMCA Top Stock and they factored(class was wt/factored HP) our cam HP based on idle vacuum almost every one ran full vacuum. You figure at WOT no vacuum no advance. Not sure how much an issue for a road race car on pulling "cruise vacuum" as you are probably either on or off the throttle.

[ho428]

I ordered the FAST distributor, figure I'll give it a shot. Already have some 8.5MM wires and ends, I assume I can use the MSD blaster coil I have.

This build is a street 365, basically a spare parts build with left over race parts to put in my 68 Convertible, giving it a bit of an upgrade over the base OHC-6, 3 spd, peg leg w/drum brakes.

.060 over 350 block, zero decked, Speed Pro pistons on SD rods, balanced 350 crank, studded main. Old Melling pump I went through, 70's swap meet pan I added a baffle to. 3/4 windage tray, yeah I know but I got tired of seeing it on the shelf.
Ported 17 heads with 2.07-1.66 valves, milled to 75cc with screw in studs, real 350 head gaskets, around 9.5:1
1.5 Roller rockers, Comp solid flat tappet that's seen at least 3 engines, .510 int .531 exh, 240-248 @ .050, a double roller chain from the old race set up, crower solid lifters.
Summit fuel pump, port matched T2 with crossover blocked, Pro Systems 750, set of old Headman headers.
Other parts laying around for the swap are a factory bellhousing, Flywheel and Zoom clutch from decades ago, M-20 w/hurst comp plus and an 8.5 Posi with 3.55 gears in it, and 11" disc brakes all around. Picked up some V8 frame and motor mounts at a swap meet a few years back as well.
I even found a set of pulley's, water pump, one alt bracket and upper rad hose in the attic. It ain't hoarding if it's cool usable stuff.

Hopefully it'll be a fun little driver.

So far the only current parts to buy were the pistons, rings and this new distributor.
Like some long branch manifolds but the PYPES exhaust system alone will be expensive so I'll use the headers for awhile.

And before the OHC guys get all in a wad, the plan is to build up a Sprint 6 that I have some parts for and swap it back in once it gets done. The Base 6 engine has a blown head gasket, leaks oil and the 1 bbl carb is junk.

[ho428]

New distributor is in, have to make up some plug wires and hunt for a few missing brackets, she should be ready to go.

[KEN CROCIE]

I use those vac. controls for boost retard with turbo engines. If you blow into the normal vac adv. nipple , you get boost retard, up to 10 degrees. Never hook up the inboard nipple and leave it open to the air. That style of vac. adv. control can be replaced with a conventional vac. adv . unit-no problem

[Scott65]

Looks good. Report back on how it works.