I have a 1984 RX-7 GS with the 12A motor. For some reason, when the car is started, the engine will immediately rev itself to redline.

Background:
I purchased the car several months ago. It is my first Mazda/RX-7/rotary powered vehicle, and while I have owned a carb'd vehicle in the past, I never did dabble in it, so I am essentially new to carburetors as well.

The car's body is in fantastic shape which is why I got it. The engine and various internals are in less than fantastic shape, however. With other projects in the way, I knew I wouldn't have time to start on it right away, so I figured the least I could do was start the car up at least once a week and take it for a drive. To start the car I had to pour a little gasoline into the carb, otherwise it would just crank and never start (I did change the fuel filter btw).

The last time I took it for a drive, after taking a corner and accelerating a bit, I noticed the car was accelerating on its own. Pressed the clutch in and the engine immediately shot up to redline and triggered the overrev buzzer. This scared the **** out of me so I killed it and pulled over.

I thought it was a stuck throttle cable, but on inspection, the throttle cable was moving normally. Oddly enough, I was able to start the car without having to pour fuel into it, however, it immediately revved up to redline. After pushing it home, I decided to try it once more, and again it revved to redline.

It has been at least 4 months since I have tried to start it, but now that my other big project is done, I have the time and space to work on it. I really don't want to try starting it again until I have resolved whatever is causing this.

Any ideas?

 

The throttle on the carburetor is stuck open. It's the only explanation I can think of. The throttle cable might not be sticking, but the linkage on the carburetor itself very well may be. If you inspect it and play with it as you make it open and close by hand, you will figure out pretty easily how it should work and what is wrong. At idle the carb plates are barely open. You can hardly tell they let air through.

There is a little screw that, when you adjust it, opens the throttle plates a bit to adjust the idle speed. The rotating part of the linkage should be resting against it. yours probably isn't go to sterlingmetalworks.com There are tons of close up pics of how the carb linkage should look.

 

If the throttle plates are normally closed and move correctly when you push the accelerator, then the problem is likely with the hot start assist. It is located on the driver side strut tower and has what looks like a throttle cable attached to it. If it is working normally, it pulls the throttle open during hot starts to make the hot starts easier. It sounds like it is acting up and pulling it open (and holding) when it is not supposed to. Disconnect the cable and/or the wiring to the hot start assist motor and then see what happens.

 

A buddy of mine had an issue with the secondaires getting stuck open which would bury the tach as well. The primaries worked as they should but the secondaries were stuck half way open. It was a linkage problem like suggested above.

 

I apologize for taking so long to follow up with this thread. It actually slipped my mind

The throttle plates were closed and moved normally. Just to confirm that the problem still existed, I attempted to start the car. It started, and revved up right away.

Next I unplugged the hot start assist like gsl-se addict suggested, and attempted another startup. This time the car started, the tach climbed to 1000rpm for a second, and then the car died. For now it looks like the hot start assist was the cause of the runaway revving.

The car refused to start after that, and the starter cranked more slowly with each attempt. Battery voltage read 11.5v, so I thought I had a problem with the starter. I consulted with Sgt Fox who suggested that my engine had probably flooded, which I am pretty sure is correct because the starter cranks normally again. Somehow while testing the battery I had completely overlooked some damage on the trailing ignition coil. I now think the T2 spark plug wasn't firing due to the damaged coil, and caused the cylinder to flood.

Anyway, I've ordered a new coil and plug wire set. When I resolve my no-start I'll try and confirm if the hot start assist was the culprit and will update this thread with the results.

Thanks for your help guys.

 

Well I'm certainly embarrassed:
After a bit of poking around with a very bright light and a long pair of tweezers, I found a very small rock wedged between one of the secondary butterfly valves and the venturi(?) wall. After removing it, the revving issue is gone.

I can't believe I didn't notice it earlier Now I am just wondering how the rock got in there...