Vacuum advance and ignition questions

[irowiki]

Surprise, I think my vacuum advance isn't hooked up which may or may not explain some of my bad idling and stalling when coming to a stop when cold. Unless I'm chasing the wrong demon here!

There's three ports on the vacuum thing on the distributor, there's a bottom port with a tube going to my carb, and two ports above that, currently not connected to anything. I found the diagram for "Federal Vehicles". The "bottom" seems to go to the carb and the "EGR Vacuum modulator" (in my case, there is no EGR vacuum modulator anymore)

Of the other two, I think the driver's side tube goes to the HAC and the passenger side one goes to a check valve and I assume to the engine. I do not have a HAC anymore. I assume for the short term I'd need to plug the HAC side, get a check valve and figure out where it is supposed to attach to on the engine? Any ideas?

Actually in my car it is flipped so the top goes to the carb, driver side to the engine and passenger side to the "HAC". I'll try and get a picture later.


Also, I bought a tachometer, connected it to the signal wire on the ignition, and it doesn't seem to be accurate. It shows 1200 RPM For my idle, and if I stomp on the gas, it only goes up to 3000-4000 RPM. I revved up to 25 MPH in 1st gear and it only showed about 4000-4500 RPM but the engine sounded much louder, in the 5000-6000 range. Is the tachometer bad or the wrong kind, is my timing off, or am I actually not redlining it when approaching 30 mph in 1st and am idling at 1200 rpm?

(For reference, the owner's manual says the max downshift speed into 1st is 29 mph so I assumed that is near redline)

Also from reading the manual, the HAC kicks in at +3900 feet, I live at about 5000 feet. Can the HAC be reinstalled if I found all the pieces or is it not worth it?

[Petros]

the HAC is a simple diaphrame device that leans the fuel mixture by creating an air "leak" under the carb and advancing the timing when you are above 3900 ft (air pressure). It should help your economy and reduce your emissions if it is there, and likely improve the high altitude drivablity.

You should make sure your vac advance diaphragm is good since it will not likely help much of the vac advance is not working. for lower elevation driving it will make no difference, the early tercel4wd did not even come with them.

I do not think any of the vac lines should be going to your EGR, it should be going into the tube manifold along side the valve cover, and from there plug into the one of the ports in that thermal vacuum device at the coolant out let at the front of the head (where the upper radiator hose attaches). Most of the Tercels running around out there do not have functioning vac advance diaphragms, they give up long before now. Most never even know they are not working, you can get by without a vac advance at all, they help improve part load fuel economy when you are at steady cruising speed. Many cars before about 1970 did not have them at all, the car will run fine without it if there is no vacuum leak.

It sounds like your vac routing is messed up, first thing is follow the vac diagram and get it routed correctly, look for and fix any leaks, plug all ports and vac lines. do this and it should run better. Just make sure if you have any missing vac operated equipment that all of the vac lines are capped off, that is likely where your problem is.

[irowiki]

To be honest I think the only vacuum operated thing that is still working is the brakes! Everything was either disconnected or removed, there's quite a few empty ports (like the thermo valve on one of the radiator inlets on the engine) and electrical connections.

I would love to put the HAC back but I'm not even sure where to start.

[Petros]

I likely have all of the parts you need to get everything back in working order. I know I have an extra HAC, the fuel mixture controller thingy that goes next to the coolant overflow tank, most of the vac lines and hoses, etc. I have no plans for it, I can put it in a box and send it to you, cheap.

[dlb]

Surprise, I think my vacuum advance isn't hooked up which may or may not explain some of my bad idling and stalling when coming to a stop when cold. Unless I'm chasing the wrong demon here!

i do not think your vac advance will help much with a rough idle or stalling when coming to a stop when cold. as petros mentioned, the engine can run just fine without the vac advance. the only area where you might notice a working vac advance unit is during highway cruising (slightly better power) and when you fuel up, since correctly advanced timing will yield better mileage. i think your idle and cold running issues are likely due to your carb settings, ignition timing, and the various disabled components of the carb itself. however, that doesn't mean you should not bother with the vac advance.

There's three ports on the vacuum thing on the distributor, there's a bottom port with a tube going to my carb, and two ports above that, currently not connected to anything. I found the diagram for "Federal Vehicles". The "bottom" seems to go to the carb and the "EGR Vacuum modulator" (in my case, there is no EGR vacuum modulator anymore)

Of the other two, I think the driver's side tube goes to the HAC and the passenger side one goes to a check valve and I assume to the engine. I do not have a HAC anymore. I assume for the short term I'd need to plug the HAC side, get a check valve and figure out where it is supposed to attach to on the engine? Any ideas?

Actually in my car it is flipped so the top goes to the carb, driver side to the engine and passenger side to the "HAC". I'll try and get a picture later.

first, you should test the vac advance ports. put a hose on the lower port and suck. you should hear it 'pop' as the diaphragm flexes, and if you plug the end of the hose with your tongue, the diaphragm should stay flexed until you remove your tongue and release the vacuum. do the same for one of the upper ports but plug the other upper port—they share a diaphragm so if one is not blocked, you will just suck air through it. if a port doesn't hold vacuum, just plug the line coming off the carb or replace the vac advance unit. they can still be bought new.

the lower port is connected to the EGR vac modulator but it is not controlled by it. both the lower advance port and the EGR vac modulator are actually connected to and controlled by ported vacuum from the carb.

regarding routing, the upper port with a brass restriction on it (might be hard to see in place—use a small camera to take a pic of each port and compare) goes to the HAC. if you don't have HAC, just plug this port for now. the other upper port goes to manifold vacuum, and the lower port goes to ported vac on the carb. use this colour-coded diagram to help find where each thing is. the HAC line is grey, the manifold vac line is green, and the ported vac line is red.

http://tercel4wd.com/download/file.php? ... &mode=view

however, i'm not sure if your car ever had an HAC. the vac advance unit may not be original. i know a canadian member here has a first gen terc and its emission controls are quite different from our second gens. i also did a google image search for "81 tercel vacuum diagram" and found this diagram that shows an ECU and an 02 sensor. look on the underside of your hood, if you're lucky there will still be similar a sticker with the vac routing and that will tell you what should go where. here are both images i found.

Also, I bought a tachometer, connected it to the signal wire on the ignition, and it doesn't seem to be accurate. It shows 1200 RPM For my idle, and if I stomp on the gas, it only goes up to 3000-4000 RPM. I revved up to 25 MPH in 1st gear and it only showed about 4000-4500 RPM but the engine sounded much louder, in the 5000-6000 range. Is the tachometer bad or the wrong kind, is my timing off, or am I actually not redlining it when approaching 30 mph in 1st and am idling at 1200 rpm?

(For reference, the owner's manual says the max downshift speed into 1st is 29 mph so I assumed that is near redline)

those rpm don't sound too far off to me. if it's true though, your idle is way too high. should be down around 850 rpm.

Also from reading the manual, the HAC kicks in at +3900 feet, I live at about 5000 feet. Can the HAC be reinstalled if I found all the pieces or is it not worth it?

yes, the HAC can definitely be added. you just need to tie it in to the vacuum routing correctly. follow the diagram i already posted here. members who live up high have reported poor mileage without it so i would get it.

[irowiki]

I'm back! Real Life (tm) has been nuts, so I've finally had a chance to poke at the Tercel some more.

Car idles around 1200-1400 RPM with the hose from the carb hooked up to the dizzy vacuum advance as follows:



The timing shows to be past the 20 BTDC when said hose is connected. That can't be right!

If I disconnect and cap the hose, idle goes to 800-900 (bounces around) and timing goes to about 5 degrees BTDC. What do you think, fellow 3-AC gurus? I'm thinking either it is connected wrong or the vacuum advance is shot in some way.

I replaced the dizzy cap, rotor and spark plug wires this weekend. The cap and rotor were very worn. Now she pulls much better from a stop.

Some more pictures of the carb and the various shot emissions systems:

https://plus.google.com/photos/10201297 ... 0369039105

[dlb]

i can't tell which port on the carb you have that vac advance hooked up to. i also looked at your vac diagram from under the hood and there are a few devices i'm not familiar with. i don't know where they direct vacuum to at what times, so i'm at a loss at this point. i can tell you that your outer vac advance diaphragm is working properly though. that is the correct amount of advance it should give when vacuum is applied to it. that diaphragm is the one that should be hooked up to ported vacuum so that it advances the timing at hwy cruising. it should not have vacuum applied to it at any other time or else you will be running way to advanced. the port on the inner vac advance can (the one with the bigger hole) should be attached to manifold vacuum. this is the one that advances timing at idle and hwy cruising. it only advances about 8* so with stock timing of 5* BTDC the engine should idle at about 13* BTDC with it hooked up. if you do get your vac advance stuff sorted out, you will have to lower your timing. it is normal that once vac is applied and the timing advanced at idle, the revs will come up.

however, looking at some of your other pics (particularly of the modified choke and throttle return spring) reinforces my opinion that you should either go to a wrecker and get another carb that has not been mutilated along with all the emission stuff you're missing and rebuild that before putting it on, or get a weber. i'm not a huge fan of webers but in this case, it would greatly simplify your situation. no more guessing games and sleuthing, plus more power. i think you can get them for under $200 in the US. you would need the 32/36 DGEV plus the adapter kit.

good luck man.

[irowiki]

Whatever port it is connected to on the carb it is definitely getting vacuum all the time.

The spring on the choke was actually my doing, the choke would not close all the way by itself

I am in the process of obtaining a stock carb with all the choke stuff intact (but it'll need rebuilding).

Is manifold vacuum the bolt with the hole in the middle to the left of the circle with the symbol on it (above and slightly to the right of the hose clamp) here?

[dlb]

yes, that is normally where the manifold vacuum ports are on the 3a. on all the 3a's i have seen there is usually a black plastic thing with 1, 2, or 3 ports sticking out the top, depending on what emission package was on the car. you can see if in the top right of the photo in this post:

https://tercel4wd.com/forums/viewtopic.php?f=7& ... nce#p45450

you can find one of those at a wrecker from any similar year of toyota and put it in. also, you should plug the hole in that bolt while your engine is idling to see if it is a vacuum leak too. looks like it might be. it would explain the rough idle.

[irowiki]

Interesting, so I want to get rid of the ported vacuum, as per the post there, and just go with manifold vacuum? So cap off the other two?

Unfortunately no wreckers around here have any Toyotas from the 80's Do you know the technical name or number of the part so I can search it?

[keith]

Believe it or not, it is called a filter. Should show it in the service manual found here. Only the three port version is available anymore, you just cap off the unneeded ones.

[dlb]

Interesting, so I want to get rid of the ported vacuum, as per the post there, and just go with manifold vacuum?

no, you can disregard the other info in that thread. it took a long time to figure out the vac advance routing so there is some misinformation in those older threads. basically, it is

inner port of vac advance w/ big hole --> gas filter (1, 2, or 3 pronged thing on intake manifold. plug the unused ports on it.)
inner port of vac advance w/ small hole --> HAC or plug it
outer port of vac advance --> ported vacuum on the carb body

i compared the vac diagrams for your carb and for ours, and it looks like the ports match up so you should probably be able to hook the outer vac advance directly to the port i marked with a sombrero here.



i also marked the gas filter with the fez hat. sorry for the wack indicators, i'm at work and these computers don't have MS paint or anything. i'm at the mercy of photobucket. be sure to test the inner vac advance can to make sure it holds vacuum though: plug the port with the small hole and suck on the other port. if you can suck air, it's no good.

[irowiki]

Oh, so it is literally a "gas filter", I was confused and thinking fuel filter, and then thinking "What?".

Having a hard time finding a gas filter but I'll ask Petros if he has a spare, since I'm getting the carb from him. I'm going to be hooking the TVSV back up too.

So since my outer port on the vac advance seems to have vacuum all the time, it is probably connected to the wrong port on the carb, and even better, I bet the proper port is plugged or busted.


All in all though, y'all are awesome and I'm learning more about this car every time I take a crack at it, so thanks!

[dlb]

no prob man, i'm learning along the way too. happy to learn, happy to help. good call on getting a new carb.

yeah, it sounds like your outer port is getting vacuum at idle--not necessarily all the time, but that diaphragm should be not advancing at idle anyway so it's definitely the wrong port. try the one i marked in the above pic, if it's still serviceable.

[irowiki]

So on the diagram, are those ports on the carb assuming you're looking at the carb from the top?