Ta-Aikah

Quick refresh: my 79 rx7 has what I believe to be an early rx3 carb on it with a carter fuel pump and Holley regulator running 3psi, 1985 ignition (dizzy, ignitors, coils) and no emissions.

A video of what's going on.


It's not hard to start. I run no choke and it still starts very easily. I've been dealing with a stumble at idle, though. It starts off as very faint bump sounds that can be heard and felt in the seat. If you blip the throttle as it starts to die, it'll stay running indefinitely. A bit after starting it, it'll stumble like in the video and just die. Once it gets some heat into it, it does what's in the video, drops to 500 rpms and sounds like it's only running on trailing spark. Any throttle while at 500 rpms will kill it. I spent a while tuning the idle mixture, from 3 turns out all the way to 1 and a half turns out (Mid 11s to high 13s on my wideband.) The bump seemed to get a little better with the mixture being richer, but it's so slight I could be imagining it.

I triple checked my timing, everything is set to factory standard, brand new spark plugs, ignitors have been bench tested with a battery and swapped around on the distributor. Makes no difference. The only thing I haven't tested is the coils, mostly because I don't understand how to test them.

Throttle response is really crisp, and above idle speeds, it is very smooth. I can't hear any misfires above idle speed. Under load, it pulls pretty well. I'm worried this older-gen carb just isn't going to work properly on this car, and I'll have to buy another carb to get this car on the road finally.

Any ideas are appreciated, thanks!

Benjamin4456

So, hmm.... If your engine starts idling at 500rpm like in the video (which you mention it can do indefinitely), is there any way to get it out of that state without restarting it? If so, what is it? Do you have to do anything special to restart it if it does die?

You mentioned checking timing and all, however did you put the light on each plug wire during this scenario and confirm they are all working properly? It does sound like it's either running only on trailing, or perhaps on only one rotor....

....which brings me to: what is your setup regarding the shutter valve? It appears to have been deleted in some respect, although perhaps it's still present and could be falling closed? This doesn't seem to likely to me, but it's another thing to double check.



Oh wait, here's an idea. Have you checked your float bowls? This situation sounds very similar to when my car had sticky float. It would run fine driving around, but then at idle it would flood and run on one rotor/run badly.

While your car is idling poorly (like in the video) note where the fuel is in each sight glass. It should be vertically centered in the sight glasses on both sides. If you can't see a line of where the fuel is, it's either too high or too low. A flashlight can usually confirm which of the two it is. There should also not be any fuel emitting from the boosters at idle, if there is that also indicates a problem (again, likely a stuck float).

Ta-Aikah

Sorry for not being clear, it'll die if it sits at 500 for too long (25-30 seconds.) If you catch it within of few seconds of it dipping to 500 with gentle throttle, it'll come back up.

I've checked to make sure everything is sparking with a timing light, but I was blipping the throttle to keep it going. There's spark at all 4 while it's blipping, but we knew that. I'll see if I can get it to run at 500 rpm long enough to check it.

There is no shutter valve whatsoever in my intake manifold. I don't believe the '79 manifolds had one at all. I could be mistaken though.

There was a time where I had let the car sit a week or two while I was waiting for parts and one of the floats wouldn't take fuel after I turned the ignition on. Temporarily turned the fuel pressure up to 4ish psi until it popped and started filling again. You could definitely be onto something with the stuck float. I'll give these checks a test when I next have time.

Thanks!

Ta-Aikah

Hooked my timing light up to the L1 plug and immediately could tell the misfires were me losing leading spark. Swapped to rear rotor leading and it was doing the same. I also noticed if I gave it any throttle (from just a little to full throttle) I'd lose leading spark entirely. It wouldn't come back until a second or two of being off the throttle.

Hmm. I swapped my timing light to the trailing plugs and neither of them showed any inconsistencies.

Swapped the wires to the coils around on the distributor cap. Now my trailing plugs were showing lost spark and no spark upon throttle.

Seems like a coil issue, then.