Issue with choke sticking on 12a
 

After searching threads, I have an idea of what my problem may be but am looking for some clarification.

The choke knob on my 12a (stock Niki carb) moves freely and seems to open the choke, but does not pop back into place once the engine warms. When I manually push it back in, it doesn't seem to be closing the choke as the idle remains high at roughly 1200-1500 RPM. The choke lever on the carb seems to move freely and doesn't seem to be bound or sticking.

It's my understanding that a magnetic switch operates the choke and closes it when the coolant temp sensor reads the proper temp. I'm not sure if the issue is with the magnetic switch, the temp sensor, or the choke cable itself. I don't think the temp sensor would have any control over manually closing the choke, so I'm guessing it's the switch or cable. Anyone have suggestions on how to fix this or where to start?

If all parts of the carb are confirmed to be moving freely... and it's not the cable... Could it be the sensor on the back of the water pump? This controls whether the magnetic choke knob holds or not, based on coollant temp.

When you pull the choke out (on cold starts), the magnetic -switch part holds it in place (open) based on the temp at the sensor that Jeff20B mentioned. Once the coolant temp gets warm enough, the temp sensor lets that electro-magnet release and the choke opens (knob pops back in). I don't have a nikki anymore but I think there is a spring of some kind on the choke lever/mechanism that pulls it back to open. Just like your throttle return spring closes the throttle when you let off the gas pedal.

I think that spring on the linkage is at fault. If your temp sensor was bad it wouldn't hold the choke closed. You seem to have the opposite problem... your choke won't open properly once the engine warms up. I think your problem is somewhere in the linkage on the carb.

Looks like I am referencing open vs.closed in the wrong order in my original post, so I'm glad you understood what I meant!

I'll take your suggestion and start at the carb linkage and work my way back to the knob and try to find what's causing the obstruction. Hopefully it is something as simple as a spring. My fear is chasing electrical gremlins because I'm not sure how complicated the magnetic switch is to diagnose.

Okay here's the choke cable where it meets the carb (dead center of this pic). Back at the knob end there is just a flat box with some wires to the harness. When the sensor on the back of the thermostat housing is cold, it allows current through which charges the magnet in the knob box thingy. That holds it in position, ie pulled out / choke closed. When the temp gets to a certain point the voltage / current (?) drops to "half choke" and the knob goes half way in. When the temp is all warmed up the sensor/switch stops letting juice flow, and the magnet is dead, ie it has no hold on the choke knob any more.

All the while there is a spring on the choke butterfly shaft, fighting to open the choke butterfly (second pic). If your choke isn't closing when the engine is warmed up there is some binding or disconnect in the spring/linkage/cable. When the temp sensor or magnet go bad the default is to fail open, as in choke open. Yours is failing closed, so I believe it is a purely mechanical failure and not electrical.

GSLSEforme is our resident Mazda mechanic,... He should be here any time now.

https://cimg2.ibsrv.net/gimg/www.rx7...5f91d4e0f3.jpg
https://cimg4.ibsrv.net/gimg/www.rx7...2a4b8473f3.jpg

This makes total sense. Thank you! Not long ago I had the intake off to install freeze plugs and seal up a coolant leak. When I pulled the intake, I left the carb attached and just kind of moved it all out of the way. I'm willing to bet I messed something up with the linkage during the time I had everything apart.

Just wanted to clarify that only some models have the "half choke" function so if you get it unstuck and your's doesn't do it, there is no need for immediate concern. At least for the 80 model year (probably others too), the two stage choke only came on the Cali spec cars. If you're curious about it once ya fix the problem, a simple check to find out is to look at how many wires are running to the knob assembly (which isn't the easiest thing to see, but if the curiosity bug gets ya, there is it). If there's two it's only a one stage choke, four wires is a two stage.

There, now you've learned one more thing today :)

I finally had a chance to get under the hood and figure out my choke problem.

I should preface that since I bought the car last year, I've had issues with high idle. The fix ended up being pretty simple. The previous owner somehow had the choke cable routed around the front of the oil filler neck. I never noticed it until I started tracing the cable from the carb back to the knob. The cable had been rubbing against the filler cap enough to bind the steel cable.

I removed the filler cap and freed the cable, this was enough to allow the choke to operate as it should. Engine idle has dropped to around 850 rpms. I gave the linkages a spray of WD40 and wrapped the cable with some tape. Now it's good as new. Not sure how I didn't notice that when I changed the oil, but can't say I would have recognized it as a problem either.

Pic of the damaged cable before I wrapped it:

https://cimg4.ibsrv.net/gimg/www.rx7...e254af8087.jpg

Thanks Benjamin, I didnt know that. I've had two '85's which bot had the two-position choke knob. Here's a pic of the knob assembly with 4 hot wires (5 counting the black).
https://cimg3.ibsrv.net/gimg/www.rx7...6f3a554b90.jpg

and the temp sensor in case somebody didnt know what I/we were talking about..
https://cimg3.ibsrv.net/gimg/www.rx7...a185c75cc9.jpg
https://cimg4.ibsrv.net/gimg/www.rx7...4ad95fdf11.jpg

Dirtfool that's good news! It's nice when it turns out to be something simple.