Will Distrib vac advance swap 3AC and 4AC?

[sherlock]

Anyone know if the vacuum advance will swap between distributors on a 3AC and 4A? Going to bone yard and the really old tercels are getting hard to find.

[ARCHINSTL]

Butler Toy's site shows them as different, but anecdotal reports are probably better.
Tom M.

[Petros]

I bought a 4ac distributor from the local Pull-a-part (only $7.50) complete for use in my 3ac. They appear fully interchangeable, they look almost identical externally, my intention was to put the whole distributor in my 3ac in the hope that it might have a better advance curve to produce more power. If not I have a spare distributor.

I will check them side by side in the morning and post if I find any difference. Some 3a versions have one advance and one retard port, on the 3ac and I believe the 4ac there is one advance and two retard ports. On the "C" version, there are other conditions to retard the spark to reduce emissions, and power and fuel economy. Me, I would just disable any and all retard lines.

But if the difference is the number of retard ports is more than your car has, it will work fine, you just cap-off the extra port.

[4wdchico]

On the 3ac both diaphragms provide ignition advance when vac is applied. The sub diaphragm, the one closest to the dist. body provides extra advance when the HAC is above 3932' of elevation on FED certified t4wd's. See page ec-40 in the FSM, or pg 128 in he PDF, for details. Also the sub diaphragm is interrelated with the AAP (aux accel pump) system and when the engine coolant is cold the system has vac applied to improve cold driveability by having more ignition advance at idle and more accel fuel applied when the driver steps on the throttle. Toyota loved to make the vac emission system complex by having several functions served by one vac circuit. The FSM for the terc 4wd does not really get into this very well but I have experience from trouble shooting them on 22R carbed truck motors, the truck manuals are better.

[ARCHINSTL]

This is an interesting comment about the 22R motor. While I did not access the truck FSMs while searching for answers about which vac line(s) to hook to the dist with the Weber, I found a lot of info regarding same on 22R threads in various sites. Much more than on Tercel or Corolla sites.
In the end, as detailed in another thread, I hooked the Weber carb vac port to the front upper dist port, the manifold vac port to the lower dist port - and capped the rear upper port (that which hooks to the HAC present on Federal cars).
Car runs great.
Tom M.

[4wdchico]

This is an interesting comment about the 22R motor. While I did not access the truck FSMs while searching for answers about which vac line(s) to hook to the dist with the Weber, I found a lot of info regarding same on 22R threads in various sites. Much more than on Tercel or Corolla sites.
In the end, as detailed in another thread, I hooked the Weber carb vac port to the front upper dist port, the manifold vac port to the lower dist port - and capped the rear upper port (that which hooks to the HAC present on Federal cars).
Car runs great.
Tom M.

My only concern is that you may have too much total vac advance for hot weather driving. Ping may become an issue when the weather warms up You only installed the Weber since the weather turned cold, right?

So disconnecting the sub diaphragm from manifold vac when the weather is hot might be a good idea. After all, that sub dia. is only intended to come into play when the coolant is cold or at higher elevations and under those conditions extra ignition advance is a good idea. It would be fairly easy to use the TVSV to switch the sub diaphragm on and off according to engine temp. Page EC-22 in the FSM would give you the vac switching capabilities of the TVSV.

With that accomplished, I'd make sure that the vac port on the Weber is indeed ported vac (ported vac will show no vac at idle but will have vac once the throttle is opened). Then I'd just tune the engine, by ear: Keep turning up the total advance by turning the dist. until pinging can be heard on acceleration, then backing off until it goes away. Re-timing this way a couple of times a year to take into account both hot and cold weather driving would be a good idea.

[ARCHINSTL]

4wdchico - I did install the Weber when it was cold (OK-cool outside), but - I wonder about your post for a couple of reasons:
1. Cal and Can cars are set up like this;
2. According to the Fed sticker under the hood, the carb vac goes directly to the lower dist port and is not related to/governed by the TVSV. The only other OE thing this tube is connected to is the "P" port of the EGR modulator. And the manifold vac goes directly to the front upper dist port.
The HAC tube goes to the rear upper dist port; this HAC part is not on the Cal/Can cars, and I unhooked it a couple of years ago, as it only affects the sub-diaphragm above 3900 feet; this is on EC-40 of the FSM (also shows the manifold source to the dist).

I will have to check whether the Weber port is indeed ported vac; everything I've read states that it is (although anecdotal, of course).
I thought I read on some 22R sites that the manifold vac was there for advance at idle when vac was not available from the carb to advance (may be misusing terms here).
Tom M.

[4wdchico]

Tom,

Part of the problem is terminology, the AAP vac circuit and the HAC systems both control the sub dia. thru different hoses. That is why you have two hoses going to that dia. ( you are calling it the upper dia.) on Fed cars. But remember that the AAP/extra ignition advance function is only desired when cold.

The vac source for the main dist. advance dia. (you are calling this one the lower dia.) is a carb vac port indeed. As you noted, this vac system is also connected (with a t connector)to the p port of the egr modulator. Remember what I said about Toyota liking to have vac circuits serve several different functions? If you will note the chart on page EC-20, see where it shows that when the coolant is cold that the TVSV flows air from port J to port M. Port J on the TVSV goes to a clean air intake on the air horn of the carb, and it is a clean air, always atmospheric pressure, intake for the vac system. The TVSV turns off the EGR system when the coolant is cold by bleeding air into the vac circuit, thus negating the vac that causes the EGR to function. This also negates any vac from actuating the main dist. adv. dia. when the EGR system is not functioning because when the EGR valve is not open the EGR's vac modulator has air free flowing between ports O & P. I just verified this with a known good vac. mod. that I have.

So, to sum things up, Toyota intended for the EGR system and the main dist. advance dia. to only function when hot. The AAP/ sub dia. system was only intended to function when cold, with the exception of the sub dia being called into play when a Fed car is at higher elevations with a working HAC.

Keeping the EGR system working on your car could be as easy as putting a small fuel filter on the J port of the TVSV, to act as an intake air filter. Much less chance of having engine damaging ping in hot weather.