Rough start and high idle

[Tercedes]

Hey guys. I just happened to stumble on an 84 Tercel 4WD and bought it cause it was cheep and I was in the market. Turns out this is a great car! I'm selling my Jeep Cherokee for something less costly and was pleasantly surprised to find this little beasty. Anywho...
So I've done all the basic maintenance like plugs, wires, air and fuel filter, fluids, distributor and cap, and whatnot. But I cannot seem to get the thing to idle under 2600rpm. Sometimes it wants to get down to a lower, more sensible speed (2000), but when it does it get's really rough and either revs back up or dies out. When I first start it it comes right up, but I have to gas it to keep the rpms high or else it dies out. If it dies out after that first start I have to take off the air filter and prop open the air intake on the carburetor in order to start it again. And until it it's thoroughly warmed up it runs really rough, almost as if it's missing. And sometimes it will diesel when I go to turn it off. I get about 26mph on the highway and the power seems a bit week but not bad considering I'm driving it over two mountain passes over 10,000 feet high.
I'm not new to basic car maintenance and troubleshooting, but I am rather dumb when it comes to carburetors. I've gleaned over the other threads in search for similar issues, but haven't yet seen anything that I can directly relate to.
Any suggestions?

[Highlander]

I'd say check the AAP (Auxiliary Accelerator Pump) at the front of the carb- its connected to the TVSV (the vacuum dingy right by the thermostat) and sits at the passenger's front corner of the carb. If the diaphragm is broken, you'll have fuel in the vacuum line, and it'll just keep sucking fuel into the intake manifold. Check out the FSM at the top of the forum here and take a look at the carb sub-section in the Fuel System section (pg FU-4-How appropriate!) to find the AAP. An easy way to check it out is to pull the vac line and plug both stubs and see how it runs then.
Good Luck, the carbs on these can be a real PITA due to all of the vacuum lines. BTW, it's a good idea to replace all of the vacuum lines from time to time. Also double check that all of the lines are connected properly- there's a color coded vacuum line chart in the repair guides section.
26 mpg on the highway is low- I get 28, with studded snows on, going up and down 7600' of elevation every day with no highway miles whatever.

[dlb]

sounds like it could be either a few big vacuum leaks and/or your idle settings are way off.

first, pull off the air filter housing and make sure that ALL of the little rubber hoses are going where they need to. there should be a sticker on the underside of your hood telling you what should go where, use that. then make sure that none of the hoses are cracked or broken. the air filter housing should have a few hoses attached to it itself, don't forget these when putting it back on.

next, use the factory service manual, aka FSM, to set your idle and fast idle speed. this is done by adjusting two screws on the carb. the link to the FSM is at the top of every page here, or use the PDF version:

http://www.tercel4wd.com/fsm/1985T4WD.pdf

go to the fuel system section to find the carb. there you will find all the info there for setting the idle and fast idle speed.

after all that is done, get a timing light and check the ignition timing on it. should be at 5* BTDC but you can advance it to 10* BTDC for slight power gain and better fuel economy.

[Lil Beast]

Look for vac leaks first but your problem sounds a bit more than just that! Check your timing!
I had the AAP problem but my MPG went to 10! One way to check the AAP would be to unplug the vac line connected to it plug it and start the car, or just siphon the line from AAP, if gas leak from the AAP it is faulty replace it! I don't know if it's available outside of a carb kit(I mean just the diaphragm)! I created this test when rebuilding these carb cause it start to get fairly common when the car as sitted for over a year! When the AAP is ruptured the vac line will suck gas thru it straight into intake, when idling for 2 min it should pop a thick stinky black smoke out of the exhaust!
But your problem sounds like a bad float adjustement! Try to adjust your mixture and idle! I'll use the lean best method! Once you've checked vac lines and timing! You want to reset the mixture screw by screwing it all the way in without any force you may break the needle! Than back it out 2 1/2 turns,adjust the idle to 700rpm, play with mixture screw until the rpm raise and then burps, set where it idles high but good, readjust idle, do like so until when you turn the mixture screw there is no changes! That should be set! If can't get it right at all, without engine dying at the 700 rpm! Your float would be the cause bad adjustment or worn out and won't stay adjust more than 5 minutes kinda!

[Petros]

sounds like vacuum leak as stated above, or several. As DLB suggested get the correct vac routing diagram for your car (the one in the FSM at the top of this page might be the same as your year car), if these were connected incorrectly it could act this was as well. When you run the car with the air cleaner off of it make sure the large vac hose that goes from under the carb to the air cleaner is blocked off (you should verify this is connected properly to the valve on the bottom of the air cleaner BTW). Than you can run it and inspect and listen for the vac leak, if not obvious spray all the lines with carb cleaner, when the engine slows down you found the leak. remember to also check the large vac line that goes to the power brake booster, that will create a big leak if it is loose or disconnected.

Also, there are three different adjustments on the carb linkage that affects idle speed, the fast idle adjustment on the throttle bell crank, the main idle adjustment (only affects idle speed when warmed up, if the fast idle is properly set), and the throttle dash pot on the side of the carburetor opposite the throttle cable. This adjustment prevents the throttle plate from slamming shut if you let off the pedal quicly, the adjustment screw should be backed off the linkage when at idle and only contact the linkage momentarily when you release throttle quickly to slow the last 5 degrees or so of throttle travel.

Good look.

[smokerx]

I had the same problem as OP. I sprayed carb. cleaner all over the engine and found a few minor vac leaks from loosening lines I fixed, which was not the problem. Next I checked the throttle dash pot screw that Petros mentioned. Sure enough it was screwed in. Adjusted it according to Petros post and the idle tuned down to normal again. I'm glad I didn't waste any time tracing vac lines which was going to be my last resort.

Maybe somebody mistook this as the idle screw. Maybe somebody used it to tune proper Idle which changed after extensive work I dunno.