Slant Six Distributors - Info and Questions

[BillGrissom]

I got a new electronic distributor for my 64 Valiant 225 cheap ($35 ebay), in a Mopar box PN 3755468, though metal tag is 3755467. It was missing internal plates, but had vacuum advance (VA). My guess is a dealer mechanic robbed the internals to fix another distributor since the tag was loose in the box and VA wasn't screwed tight.

I got a new coil plate for $4.50 at rockauto (bought 2, jump on that!), but needed the baseplate in photo below, so removed the one from my points distributor. The coil plate fit on it like a glove, so perhaps same PN (no stamp). The distributor bodies appear identical (points on left), though the shafts differ. The points base plate has 3 copper standoffs which the points plate slides on. That makes a needed electrical connection, un-needed with a coil pickup, so perhaps they simplified the baseplate design but otherwise kept the mechanical dimensions. I'll eventually get a baseplate for my points distributor to have ready as backup. If anyone has one lying around, PM with price.

The VA differs between my points and electronic distributors. I suspect my points distributor is a recent rebuild, since very clean and the output wire looks new. The points plate has a single hole for vacuum advance, on the outside, which the VA fits. There are no labels on the VA.

The coil plate has 2 holes for VA. One is hard to see, underneath the coil. The VA rod curves inward to fit the inside hole. The VA has stamps 375041 (?, hole thru the numbers) and 8.5R (rod), and paper tag 3282.

Another kink, I ordered a VA from rockauto (cheap), described as "improved mileage" (hasn't arrived). A working hypothesis is that the "bent VA" acting on the inner hole brings on advance sooner, which improves combustion at the risk of knock, which probably works better with the leaner mixtures of late 70's engines.

I don't think quick advance is a big risk, since many Jeep off-roaders connect their VA to direct manifold vacuum. They they say that avoids stumbles when they give it throttle to climb rocks (don't ask me why). Anyway, I plan to eventually add EFI, O2 sensors, and a knock sensor, so I won't fear pushing the advance limit.

A few other issues.

I know nothing about the adjustment slot on the base clamp of the distributor. Right now it is in the middle. I guess it allows positioning the VA to not bump the motor mount or fuel pump (went electric). I expect it won't need changing, since weren't all slants the same?

I read posts about "short tip" rotors causing misfire. My points distributor has a grey rotor with tip protruding ~1/8" past the plastic. I got the United Ignition Wire Tri-Pak tune up kit, PN 1-7600. It's blue rotor has a tip that barely protrudes past the plastic, so I thought "short tip problem", but when I installed both on the shaft, it actually protrudes slightly farther, so just has more plastic. I am waiting for SSD or another guy to say "UIW makes junk", but the blue cap has copper posts and is labelled "made in USA". I know nothing of them, I just liked the price on rockauto.

Instead of a Mopar igniter box, I installed a GM "8-pin" HEI module and coil. I'll post more once I get it running, and tell you the polarity that works for the pickup wires.

Please add any info you have on slant distributors. I am guessing I don't need a capacitor at the ignition coil since I don't think the GM system uses one, and I think noise was only a problem with AM radios, plus can add a filter at the radio if needed. I understand that the small capacitor inside the points distributor was just to extend the life of the points.

 

[Bill Dedman]

RE: "many Jeep off-roaders connect their VA to direct manifold vacuum."

The only thing worthwhile I know about any of this is, I have heard that there are two varieties of vacuum available from the fittings on most carburetion setups; manifold vacuum and venturi (also called "ported") vacuum, I believe.

What I have heard is, that ported (venturi) vacuum is a much weaker signal than manifold vacuum. What this means is, the diaphragms in distributors that are intended (OEM) for only "seeing" ported vacuum are possibly not robust enough to experience manifold vacuum without rupturing.

This may just be an old wives' tale... or, not.

It makes sense to me.

Maybe someone with real-world experience in this matter could shed some light on it....

I'd like to know the truth.

Anybody?????

 

[Johnny Dart]

I don't think quick advance is a big risk, since many Jeep off-roaders connect their VA to direct manifold vacuum. They they say that avoids stumbles when they give it throttle to climb rocks (don't ask me why). Anyway, I plan to eventually add EFI, O2 sensors, and a knock sensor, so I won't fear pushing the advance limit.

Do not hook your Vacuum Advance to your manifold. That is considered full time vacuum, which is for things like power brake boosters. You hook the VA to the ported side of your carb.

As far as distributors, I would have bought a 73 dist from Rock Auto.
Instead of piecing a dist together, you could of had a bolt on part.
I got a rebuilt A1-Cardone 73 /6 dist, from Kragen. Thing was cheap and works well.
Mine has 17degs built in. Would like it to come in a little quicker, so some spring changes might happen down the road,

 

[BillGrissom]

As far as distributors, I would have bought a 73 dist from Rock Auto.
Instead of piecing a dist together, you could of had a bolt on part.

Thanks. That is good advice for others. I checked rockauto and a 74 slant electronic dist (rebuilt) is now $36 bare or $53 w/ vacuum advance. The "high mileage" vacuum advance I got was ~$6 I recall (gone now). Rock is like that, grab a deal when you see it. I might have forgotten to check Rock, being sucked into ebay. I didn't notice the new distributor was missing the "points plate", thinking that was part of the pickup plate.

Bill D., no risk rupturing the diaphragm with full vacuum. I already applied that with a hand pump. I don't know the pros/cons of applying straight manifold vacuum to the advance. It sure isn't what Chrysler intended. I just mentioned that many Jeep owners do that, probably on their AMC 6 cyl motors. Seems like that would increase base timing (which you trim out) and retard timing at full throttle (good?).

I installed the distributor yesterday, after jacking the engine up and flipping the new motor mount to clear (note: take more photos before removal). It looks like the phasing is a bit off, being perfectly aligned at full vacuum advance. That is eyeball, I will check with engine running, shining a timing light on a cap w/ hole. Richard Ehrenberg sells a custom "phasing fix" reluctor on ebay w/ multiple slots ($$).

 

[Bill Dedman]

Bill D., no risk rupturing the diaphragm with full vacuum. I already applied that with a hand pump. I don't know the pros/cons of applying straight manifold vacuum to the advance. It sure isn't what Chrysler intended. I just mentioned that many Jeep owners do that, probably on their AMC 6 cyl motors. Seems like that would increase base timing (which you trim out) and retard timing at full throttle (good?).
quote]


How much vacuum did you apply to it? Repeated exposure (such as might be experienced in the real world) to 20 inches might put a sensitive diaphragm (ported vauum style) at risk, I don't know.

Thanks for the information!!!

 

[Dartman1969]

I am confused :tard:
If you hook your vacuum advance to manifold vacuum you have full advance at idle. If you are going to do that why would you bother with a vacuum advance unit at all? :banghead: Wouldn't you just advance your initial timing?
~Michael

 

[Bill Dedman]

I am confused :tard:
If you hook your vacuum advance to manifold vacuum you have full advance at idle. If you are going to do that why would you bother with a vacuum advance unit at all? :banghead: Wouldn't you just advance your initial timing?
~Michael

I think that if you just advance your initial timing to whatever the vacuum advance value is (say, 15 crankshaft degrees, for instance) then, if you leave the vacuum cannister hooked up, you will have the intiial (15 degrees) PLUS the vacuum advance (another 15 degrees) which is too much total advance.

But, hooking it up to manifold vacuum means you lose all that advance when you open the throttle, just when you need it...

Not a good plan, it seems to me.

 

[BillGrissom]

The "improved mileage" VA arrived. It looks identical to the one on the electronic distributor, with the same "8.5R" mark on the rod.

I have my VA connected to the correct carburetor port. I suggest searching the Jeep forums for claimed pros/cons of connecting to manifold vacuum. I think Bill Dedman explains the behavior correctly - your base advance at idle would decrease as the throttle opens. I don't know if that would be good or bad. I think you need more advance at low manifold pressure, so it sounds like it would be the correct direction. Off road driving has unique demands in high throttle at low rpm when climbing rocks.

Eventually, I plan to let a Holley Commander 950 control my timing, so the mechanical and vacuum advance in the distributor will be "merely interesting" since I will add to that, with a knock sensor having final authority.

 

[moper]

In terms of the vacuum advance being plugged into ported or manifold...
The advance provides extra advance when vacuum is supplied to it. But you have to remember when vacuum signals change.
Ported vacuum has it's source directly above the trottle plate(s). So at idle, there is no vacuum applied and no additional timing. As soon as the throttle is opened, there IS a signal at the port, and vacuum advance comes in just when you need it because the engine typically has to overcome the drop in airflow at the moment the throrrle opens. The accelerator pump squirts a little, and the timing goes up. It stays up as long as there is vacuum present. But, at full throttle, there is almost no vacuum present in the intake, so the vacuum advance is not functioning at wide open throttle.
Manifold vacuum has a signal so long as the intake has vacuum. Which is all the time, except at full throttle. The wheelers want more timing because they typically do not use much throttle when crawling... They want the max timing to keep the torque high.
On either setup, the vacuum dashpot itself is adjustable. You cannot adjust the total advance it gives, but you can preload the diaphram and control the level of vacuum that is required to fully advance, and more imortant, the level that has to be reached before it beings to advance.

My '65 has the factory points, one very light advance spring and the factory light spring, the contrifical limited slightly, and I run a lot of initial. Vacuum is ported just like the factory wanted and I prefer. It's fairly responsive for what it is, and the gas mileage is quite good.

 

[KitCarlson]

Looking at your picture, the two vacuum advance units seem to differ in the way the link mechanism takes a jog at the connect point. Not sure about that.

If you have a phase problem, the first thing to check might be the polarity on the sensor coil wires.

Years ago I limited the vacuum advance travel, by adding a stop plate on the linkage tab, going into the dashpot.

 

[BillGrissom]

If you have a phase problem, the first thing to check might be the polarity on the sensor coil wires.

I haven't checked the phasing with the engine running yet. My comment was from an eyeball estimate. I will try both polarities and let people know which works, with photos of my HEI setup (new post). Engine not running because working on cooling.

When testing the HEI by spinning the distributor my hand, I did notice a potential problem. Sometimes the spark would self-oscillate at high frequency (1000 Hz?) when I stopped with a reluctor tooth aligned on the coil (with either polarity). I had the coil output connected to a spark plug (grounded body). I suspect the spark current caused a false pickup in the coil wires, causing positive feedback oscillation. I had alligator leads in the setup which didn't help. Perhaps the best way to avoid such cross-talk is to twist the pickup wires going to the HEI module and try to cross near any spark wires at 90 degrees. Shielded cable would help, but twisting the leads is most important. In the GM system, the HEI module is right beside the pickup coil (short wires are good), but also right under the coil (bad). Perhaps they have a shield plate. Maybe the feedback is intentional to give a multi-spark effect (joke). It did give a very bright spark in testing.

I also ran across a connector problem. I bought 2 Wells pickup coils (cheap). One had an "appliance dual bullet" connector (found on washers, etc) and the other had the correct Mopar dual bullet. The photo shows the difference. The coil connector (left) is the same as the appliance type (middle), which is incompatible with the Mopar one (right). The Mopar bullet has a female which is recessed in the housing and the male side protrudes farther to reach it. If you try to connect to an appliance type, you may have an erratic connection at the appliance male to Mopar female connection. Nothing like an erratic distributor pickup to drive you nuts. I lucked out by installing the pickup with the Mopar connector in my distributor. I found the problem when I tried to use an appliance connector and found it wouldn't connect snugly. I looked at my spare pickup and thought "great, it matches, the first one must be mis-shapen". Then I dug into my junkyard wire harnesses and found 3 connectors, all the Mopar type w/ the longer male (call them "excited bullet"), one on a 318 e-distributor, one on a 318 harness, and one on a slant harness. If your pickup doesn't connect firmly to your factory harness, you may have gotten a Wells pickup with the incorrect connector, as I did.

 

[KitCarlson]

I assume you have connected a wires to ground the distributor body and also HEI module. If you do not have the distributor grounded, that might cause oscillations, and possible failure.

 

[BillGrissom]

I assume you have connected a wires to ground the distributor body and also HEI module. If you do not have the distributor grounded, that might cause oscillations, and possible failure.

The HEI is grounded with a dedicated wire to a mounting bolt. I don't think it will work without that.
In the checkout tests, the distributor wasn't grounded, just held in my hand. I can't imagine it needs to be grounded since the magnetic pickup appears to be just two floating wires. I will verify with a DMM.

 

[KitCarlson]

You are correct, I had a brain fart thinking about the new $45 ignition I just installed. In your situation the HEI is not mounted on distributor. There may be some shielding to lessen interference from open spark if grounded.

When working with pertronix or HEI type distributors they need a ground. If the ground fails, the module can fail due to high voltage transients. This can happen if ground is broken when discharge happens. BTDT. Since the distributor needs to be loose to adjust timing, I add a ground wire to insure a good ground.