Hey all
Quick question, been working on this one awhile.

'89 Mazda b2200 pickup 2WD, carb, auto, starts real rough when cold. It'll fire, but wont keep an idle unless gas pedal is pushed a bit. Starting it with gas pedal pressed gets it running, but pedal gas has to be always applied. As soon as its shifted to gear, the idle drops even lower, usually making it just stall. If you feather the gas pedal just hard enough, you can 'lurch' it out of stalling, and it'll take off. Once the engine is warmed (4-8 mins of running) its totally fine, it'll idle like a champ and shifts to gear no problem. I'm thinking it could be vacuum leak related? Ive searched for one, no luck. It has new Caps plugs and rotor, new filters and oil. Have played with idle speed screw, which doesn't effect the cold idle speed. I'm stumped. ANy help greatly appreciated!
Thanks

 

I also thought it could be a vacuum leak. Now I think it has to do with the carburetor. I have to dig deep into my memory about those.
There should be a "cold running mode" built in. It's called a choke.
In the VW there was an electrical heating element that closed the choke when cold and the heat element opened the choke as it warmed up.
Whether it is electrical or manual have a look at the choke.
Next thing would be fuel pressure. A clogged fuel filter may not provide enough flow or the fuel pump might be weak or the fuel pick up might be half ways clogged.

I hope you kept track of the original idle air position and idle speed positions. Changing the idle air mixture affects the fuel air ratio over a significant portion of the throttle position range. Going wrong could damage the engine.

 

Thanks for the response
I've observed the choke, and it definitely engages when cold, but I have not checked if it slowly opens as the engine warms or not.
I suspected fuel filter as well, I have purchased a replacement but have not yet installed it.
The carb is pretty busy on this motor, since it has all the vacuum lines running all around it.
I havnt changed the idle air mixture, just the speed, but I've kept track of any changes made - dont want to mes one thing up while trying to fix another!
Ill post any updates as I find them

 

I'd say fast idle screw. When you hit the foot feed without it started, does the choke plate, and throttle cam advance to "forward" (cold)? The choke plate should be closed, but the fast idle cam should move a notch. When properly adjusted, 1) set the choke/fast idle by hitting the foot feed before cranking. 2) start engine, it should idle at 1k rpm or so, 3) after warm, hit foot feed once, that releases the fast idle, and if warm (thermal choke control) will open according to how warm the choke manifold spring is.

 

Your choke heater should plug into the rear of the alternator.

Also make sure your choke butterfly can move freely when you touch it. It should be nearly horizontal when the engine is cold, and nearly vertical when the engine is fully warmed up.