Drivability mystery problem - ignition timing

General discussion about our beloved Tercel 4WD cars
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Petros
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Re: Drivability mystery problem

Post by Petros »

if idle speed shoots up after the throttle is blipped I would guess that sounds like it is idling too rich. if it was lean the speed tends to drop off when you blip it. those carbs have a idle richness mixture screw at the base of the carb (even with the throttle plate level). with it idling, turn it in ward a bit at a time, and than outward. usually where it runs smoothest is the best setting: screw inward until it idles rough, than count how many turns as you back it out, where it again starts to run rough. than place the screw setting half way between.

if the idle mixture adjustment does not help (it controls the mixture at idle, and just above it), it might be the low speed jets in the carb are too large. there are websites for the weber than gives you a way to trial and error the jet size. You will need a jet kit that gives you a varity of low speed, high speed, and air emulsion jets to experiment with. I do not think they cost much. it is easy to change out the jets, it has a cover on top of the carb you pull off and exposes all the jets for fast changing. you do not have to dismantle the carb, or even remove it from the car.

good luck.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
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eightyfive4WD
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Re: Drivability mystery problem

Post by eightyfive4WD »

Thank you for the Weber tuning tips, Petros. That will be next on my list, after confirming proper ignition timing.

NWMO, thanks for the description of the timing light hookup. I was going to check this task off my list today, however after referring to the FSM, I notice that my distributor's vacuum advance seems to be set up differently than the vacuum advance in the FSM figure. I only have 2 ports on my vacuum advance, and I see 3 in the FSM. Does anyone have suggestions of how I should proceed? My guess is that I should just remove & plug the single side port vacuum line on my vacuum advance, since I don't have 2 ports there like in the FSM. I'm unfamiliar with this system though, so I wanted to confirm that here before proceeding.
Image
Image
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Here's a few photos showing my current vacuum routing. I know that it's really not ideal to have spliced together bits of vacuum line like I have here - I'll get that replaced in the near future. If anyone has any comments about my vacuum routing, by all means let me know. I am not positive that this is the best way to have it set up for the Weber, this is just how I did it. Up until this backfiring issue it's seemed to work okay.
Can anyone confirm my process before I proceed with plugging the single hose routed to the side of the vacuum advance in order to check timing?
1985 Toyota Tercel DLX 4WD 5 spd wagon (manual swapped, daily driver)
2007 Toyota Tacoma TRD 4WD access cab long bed (road trip/adventure-mobile)
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dlb
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Re: Drivability mystery problem

Post by dlb »

I'm not clear on how you have your vac hoses connected -- are you using the T to connect both vacuum advance ports on the distributor to the one vacuum port on the weber? If so, that is not correct.

The lower distributor advance port should be connected to the weber's vacuum port, while the upper distributor advance should be connected to manifold vacuum (the ports circled in purple here)

weber1.JPG
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eightyfive4WD
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Re: Drivability mystery problem

Post by eightyfive4WD »

dlb,

I'm sorry, I tried to make my pictures & illustrations as clear as I could - yes, currently I have the single weber vac hose going to a tee, which connects to both advance ports on the distributor.

It sounds like now is the perfect time to get some new vacuum hose and redo my routing.
I'll connect the weber's vacuum port to the lower distributor advance port, and I'll get another vacuum line routed from the manifold vacuum (purple port in your photo) to the upper distributor advance port.

So as far as the FSM's timing procedure, I will disconnect & plug the single upper distributor advance hose, and proceed to check my timing.

Image

After reviewing another post of yours, dlb, it seems that since I had my vacuum lines routed improperly, this could entirely be explanatory of my strange sputtering & backfire.
To be certain, I will still re-route my vacuum lines properly and get my timing checked.
I got 10' of new vac hose from O'Reilly and I'll give this another shot next chance I get.
1985 Toyota Tercel DLX 4WD 5 spd wagon (manual swapped, daily driver)
2007 Toyota Tacoma TRD 4WD access cab long bed (road trip/adventure-mobile)
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dlb
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Re: Drivability mystery problem

Post by dlb »

Yup, you are correct on how to set the timing. Fingers crossed this all helps with your problem.
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Nordical25
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Re: Drivability mystery problem

Post by Nordical25 »

Is your car US-spec. car and is there altitude compensation system? If yes, then there should be 3 ports.

There are at least 3 kinds of vacuum advance units:
1 port vacuum unit: 1 line to carb.
2 port vacuum unit: 1 line to carb and 1 line to TVSV due to emissions (TVSV at manifold). There could also be a variant, which has a line to carb and to height compensation system.
3 port vacuum init: 1 line to car, 1 line to TVSV and 1 line to altitude compensation system due to emissions and driving in higher altitudes.

Availability of vacuum units is getting challenging so it could be that it has had a different unit from original. Then you need to figure what are the two ports for. One is from carb to vacuum unit but second one could be to manifold or to height compensation system if you have one.

Is there a spare part number written in your vacuum advance? It is usually in the top of it if it is a original Toyota part.
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eightyfive4WD
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Re: Drivability mystery problem

Post by eightyfive4WD »

Nordical25,

The car is US-spec, & I do not have the high altitude system.
Yesterday I was not aware of the 3 different vacuum advance units, but I now understand that my Tercel is equipped with the 2 port vacuum unit.

This could entirely be different than what was standard on my Tercel; I am not sure. I believe my Tercel came equipped with the federal emission equipped 3AC from the factory, but I actually have a 3A-SCV (swirl control valve) engine in the Tercel. Previous owner didn't know much about it but my understanding is that the 1st or 2nd owner swapped this JDM engine into the Tercel (I am the 4th owner of mine). I am not currently utilizing the swirl control valve vacuum device on the back of the valve cover; I don't know how it's intended to be routed, especially with my Weber 32/36 setup. So, I've just bypassed it for now.

I don't have part numbers handy but the distributor itself is OEM, and so I imagine that the vacuum advance on the bottom is OEM also. Since I have the 2 port vacuum unit, rather than the 3 port vacuum unit pictured in dlb's post above, I believe I'm good to go with running the weber's vacuum port to the lower distributor advance port and the manifold vacuum to the upper distributor advance port.
Last edited by eightyfive4WD on Mon Feb 28, 2022 1:20 pm, edited 1 time in total.
1985 Toyota Tercel DLX 4WD 5 spd wagon (manual swapped, daily driver)
2007 Toyota Tacoma TRD 4WD access cab long bed (road trip/adventure-mobile)
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Nordical25
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Re: Drivability mystery problem

Post by Nordical25 »

Then this sounds to me for a kind of a custom build so you need to figure out best working combination by trying and learning since you have JDM engine with Weber carb.

I think that it is a good idea to try running the weber's vacuum port to the lower distributor advance port and the manifold vacuum to the upper distributor advance port in separate 2 vacuum lines. You can also test that you have a functioning vacuum unit similarly to "yet another car question"-thread.

I can take pictures of 1 port vacuum system if 2 separate vacuum lines update makes no difference. This system is the most simple one since our Tercels do not have EGR valves, catalytic converters or other emission stuff. Just let me know if you need details. But it is worth of trying 2 lines first since your distributor has 2 ports.
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eightyfive4WD
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Re: Drivability mystery problem

Post by eightyfive4WD »

I really would love to have the swirl control valve vacuum hooked up properly! I can try and do more homework but so far I haven't had success in locating documentation to assist me.

I had just a smidge of time earlier today, so I replaced the old hodgepodge of vac lines with nice new lines, routed accordingly as described above here. I took the car for a short drive around the block, it seems that there is less hesitation and backfiring, but it is still there under medium acceleration.
I was quite short on time today so no timing check yet but I will set timing to 10deg BTDC next chance I get!
1985 Toyota Tercel DLX 4WD 5 spd wagon (manual swapped, daily driver)
2007 Toyota Tacoma TRD 4WD access cab long bed (road trip/adventure-mobile)
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Re: Drivability mystery problem

Post by BaileySims »

If you are not running that scv, I think you remove the valve cover and disable it. I swapped in the same engine when my original one was far beyond repair That setup didn't work on mine either.

I had the intake manifold off and there are littled blades in each port that sit in a position and would assume would cause some drivability issues if it's not working.

I can't remember which was it was, but you just detatch the linkage from under the valve cover and make sure that the blades are not sitting in the wrong direction.
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eightyfive4WD
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Re: Drivability mystery problem

Post by eightyfive4WD »

Very interesting. To be honest, I don't remember seeing any apparatus like that next to the valves when I replaced my valve cover. Maybe a previous owner removed that stuff? It's worth looking under my valve cover again though so after I confirm proper ignition timing this is something else that I can check.
1985 Toyota Tercel DLX 4WD 5 spd wagon (manual swapped, daily driver)
2007 Toyota Tacoma TRD 4WD access cab long bed (road trip/adventure-mobile)
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Nordical25
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Re: Drivability mystery problem

Post by Nordical25 »

I think it is worth of checking timing and that swirl control system first and then do more.

Has your car been always like that or did the issues start suddenly?
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eightyfive4WD
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Re: Drivability mystery problem

Post by eightyfive4WD »

Agreed. Timing is next on my list, then I'll look into the SCV stuff under the valve cover & double check the carb fuel mixture.

The hesitation and backfire/sputtering that I am now experiencing started only recently. The improperly routed vacuum lines that I just replaced earlier today, those have been set up incorrectly like that for several thousand miles of driving. My Tercel has been a reliable daily driver for several solid months to me, and then out of nowhere this issue started.
1985 Toyota Tercel DLX 4WD 5 spd wagon (manual swapped, daily driver)
2007 Toyota Tacoma TRD 4WD access cab long bed (road trip/adventure-mobile)
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Petros
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Re: Drivability mystery problem

Post by Petros »

you can cap off the distributor vac lines and drive with any vac lines to the distributor. it will run fine without them. One of them advances the spark when at part throttle (such as a fwy cruise condition) and it only improves the fuel economy slightly. it will run fine without it as a daily driver. the other is for emissions controls, it is connected to the primitive ECU and as I recall retards the spark at full throttle or something similar, to reduce NOx production at full throttle, the third one as equipped on some cars (mountain states only), is part of the altitude compensation, advances the spark when the fuel mixture is leaned for high elevation. it too can run without it connected when below 4000 ft elevation (or something similar). you no longer have most of the emissions systems, nor any of the old fuel system, so the only one that is useful is the vac advance for part throttle conditions. it needs to be connected to the correct vac port and the correct port on the vac advance. if the diaphragm is rupture or does not work, just cap it off because it will than just act like a vac leak. it will harm nothing to drive with the vac lines for the distributor capped off.

Do not blindly route the vac lines without knowing where the are going. your hack of connecting the vac ports to the same line is a mistake. just cap off both ends of all vac lines to the distributor for now. if you want to rig the one so it will work, study how the distributor vac advance works in the Factory Service Manual, and route the vac line from the correct location.

You can set the timing with the vac lines capped off as they should be anyway. set it at 10 deg BTDC, you can go for 12 or more but if you get ping or per-ignition, back it off. the slightly advance timing will deliver better economy and a bit more power, and reduce exhaust gas temperatures, so it preservers the exhaust valves.

Good luck.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)
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Nordical25
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Re: Drivability mystery problem

Post by Nordical25 »

And one question when adjusting ignition, where is this red line connected into:
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