I recently installed the muffler and noticed it smelling a bit rich. I figure I would turn in the mix screw clockwise to lean it out; but I`m getting tired of my improvised tool of a flat blade bit chocked into a 1/4 inch open end . I`m thinking about an ez just mix screw if there's room around that vac line bracket.
For those that don't know what a ez just mix screw is. It's a replacement mix screw that has a tuning knob. I`m thinking about making my own like this thread:
Out of the old mix screw. At this point I would take it to a "real" mechanic and pay the $$, but reputable mechanic seems to be a dying art around these parts, much less old tech. like carberators. Around here everyone with a beer in their hand fancies themselves a mechanic or you`re paying out the wazoo for questionable work. the manual process seems like a piece of cake with a visible mix screw. Hidden away, Sometimes, I`m not even sure I`m turning it.
That will not fix your problem. What makes you think it is rich? how is it behaving?
The idle mixture is set at the factory on all North American Tercels, and than sealed with a soft metal plug. Unless your seal was removed, there is no "mix screw" that I am aware of. In either case, it will not affect fuel economy much anyway, as Paul points out, that only affect idle mixture. unless you have a weber conversion, there is no mixture screw. Even with the weber conversion, idle mixture only affects idle, and easy to adjust, no tool required. The mixture in both the factory carb and the weber is adjusted by changing out the fuel or air jets with different size jets.
the idle adjustment is easy, no reason to waste money on a mechanic who was not even born when these cars were made, usually no experiance with carburetors in general. There is nothing to be intimidated about adjusting your own carb, it takes a flat screwdriver and just follow directions in the service manual.
it may not be the mixture adjustment at all, it does not just go out of adjustment for no reason (unless you or the previous owner messed around with it). But even if it was tampered with, getting all of the different adjustments correct only takes a few minutes (it takes longer to read about how to do it than actually doing it).
I suggest that it more likely is something else is wrong causing to run rich (if indeed it is running rich). You have to verify that it is functioning properly before you try and adjust it anyway. Do this:
First, study the vacuum diagram from the servcie manul (or from this website) for your model, and trace each of the vac lines from each end (I actually run my finger on them), and verify they routed correct, both ends are connected, and there are no cracks or holes in any of them. This might seem like a complex task, but it only takes about 10 to 12 min to do, and critially important. Do not assume the last guy that messed with our car knew what he was doing, even if he was a "real" mechanic (most do not know what they are doing with carburated engines).
Than check the Auxilarly Accleration Pump (AAP) by pulling the vac line for it, and see if it is full of fuel or dry. if it has fuel in it, that is your problem, cap off both ends of it, (it will drive okay once warm, it is only for cold accelarations). If you can replace the diaphragm than you can connect back up the vac line.
Than there are a number of idle adjustments, cold and warm, that can be done by following the service manual procedure. there are three important ones, the main idle screw on the bell-crank (next to where the throttle cable attaches to the carb), the cold fast idle adjustment next to the main idle screw, and the device on the other end of the throttle shaft that delays the closing of the throttle plate with a vacuum diaphragm when you let off the throttle (it keeps the throttle from slamming shut when you let off the gas, preventing a spike in emissions). Also check that your throttle cable is not adjusted too tight, so it allows the throttle plate to come all the way back on to its stop.
If this does not solve the problem, than there are a number of other things to check/fix, and perhaps a carb rebuild would be in order. but this procedure solves over 90 percent of throttle/carb issues on the 3ac engine.
I`m running the stock carb. I ripped my carb apart and rebuilt it with a rebuild kit after dipping it in chem-dip and pulled the mix screw to dip it. Checked the vac. line routing. replaced any suspect lines. I think it's idling rich based on the smell of gas coming out of the tail pipe when it's idling. all 4 cylinders are firing. the Idle oscillates between 1,000 and 1,500. Idle mix screw anti tamper plug was removed when I got it.
I've tried shoving screw drivers through that vac line bracket obstructing the mix screw; it's a pain if not impossible. I don't know how the manual expects you to make the adjustments with the air filter installed. I`m pretty sure something isn't right enough to run the carb tuning procedure seeing how the idle is oscillating, and I can never get the speed screw to reduce the speed to the proper FSM table level without the car dying. The speed screw quit adjusting the RPM when I turn it.
I`m pretty sure I got the mix screw turned 1/4 turn in using a piece of rod I beat into a flat blade then bent into a L shape to make idle mix adjustments and I smell no gas idling. If my name was Toyota I would have made the idle screw a protruding thumb screw with a flat blade option. I understand this makes the anti tamper plug impossible. possibly no room to protrude from the bracket. Difficult at best to stick a screw driver through everything attached to that bracket.
i admit that it's a pain to access the idle mixture screw but it has been very do-able for me. i just shine a light in there so i can see the flat of the screw and make sure i'm turning it.
I don't have the stock idle screw in place, its a pan head screw, I suspect shenanigans, I got it set to 800 as per EPA but it sometimes fluctuates as low as 550.
It takes more fossil fuel resources to produce new vehicles than it would take to maintain and fuel any 70's or 80's vehicle for a million miles.
Seems I may have found the root cause of the idle foul, Had a TVSV hose that was blown out going to some diaphragm on the passenger front side of the carburetor.
It takes more fossil fuel resources to produce new vehicles than it would take to maintain and fuel any 70's or 80's vehicle for a million miles.
. I`m thinking about an ez just mix screw if there's room around that vac line bracket. 
