1980 Fluctuating Idle
 

Working on a customer's 80 LS and this is the issue I'm having. When the engine is cold and first started the engine idle fluctuates rapidly at 300 rpm intervals. As the engine warms up the idle levels out and the engine idles perfectly. I've played with the carburetor adjustment and nothing seems to help much. The car is all stock other than a full RB exhaust. Not sure where to look on this one. I'm kinda stumped. Any advice or leads?

It does this while using the manual choke? I usually just pull the choke enough to run it btw 1500-2000 rpm. It always keeps a steady pace just like if your using the throttle to evelate the rpm.

Using the choke will cause it to fluctuate instantly. Releasing the choke then makes no difference. It will not stop fluctuating until the engine is warm. Its the weirdest thing I've seen in some time.

So engaging the choke causes it, but releasing it makes no difference. I suppose I'd try to get it started cold w/just the throttle to see if it acts the same. See if you can isolate it btw the carb and choke. I bet someone on here has run into it previously.

Sounds like maybe something related to the fast-idle cam step or the choke thermostat. Time to dig into the carb manual and look at the sequence of events that start when the choke is pulled out, and end when the car reaches operating temperature.

Check each of the vacuum operated parts having to do with the choke. I think you'll find a leak.

Watching with interest here - mine does the same thing….

Stu Aull
80GS
Alaska

I'll take a video tomorrow afternoon and post it up. I'm thinking that it has to be in the carburetor. More often than not its the choke that causes the erratic idle. Once warm there are no issues. There are no vacuum leaks. Oddly enough this issue did not present itself until some time after I installed the RB exhaust. Not sure if its coincidental or not.

Stu, is you SA all original?

Got a video of the issue uploading now.

Noticed that when I manually open the secondaries when this issue is present the engine stumbles a little and then revs much better. The engine was still at normal operating temperature after sitting for about 30 minutes and the issue came back. Once it goes away the engine revs like normal and the car drives excellent.

Here's a direct link to the video of the car running with the issue:

http://vid246.photobucket.com/albums...007_134055.mp4

Got the carb off. I found that only one of the primary squirters was dripping fuel at idle and nothing when the throttle was depressed. I also think there are issues with the air bleeds being clogged. Gonna pull the carb apart and see what's going on with it.

Rebuilt the carb and the engine runs great. Better than before actually.. However, when the engine is first started and either the choke is pulled or light throttle is applied, the engine will rapidly fluctuate. Almost sounds like its ported. Let off the gas or return the choke to the open position and the engine idles fine. Once the engine has warmed up a little there are no issues with the engine fluctuating. I'm at a loss here..

I rechecked and now it fluctuates only when the choke is pulled out. Push the choke in and it will rev nicely when cold. Odd that the choke will cause this issue, especially with a fresh carb.

Could the choke unit itself be bad? Doesn't it have a magnet or something in the plastic case right behind the pull knob.

Yessir it is. Mine runs (on cold start) like its on one rotor, rough, bucking and kicking for the first 30-60 sec, then suddenly the "2nd rotor" kicks in and it smoothes out nicely if I hold the rpm up over 2500. With or without choke. I really need to mind the revs with gas pedal or it stalls out. Once "2nd rotor" kicks in, it idles like its on the showroom floor. I am thinking ignition (not all plugs firing correctly)?

Anyway - don't want to go off topic here - Dave's got the ball...

Stu Aull
80GS
Alaska

The choke does a lot on these cars. When the choke is pulled it sends a signal to the ecu which in turn sends a signal to the vacuum solenoid to open and advance the timing. I thought something maybe up with that. After checking everything, it seems as though the timing is being advanced properly. I'll hook up a timing light to confirm tomorrow when I'm off work. The way the tach jumps leads me to believe its an ignition issue. I honestly am stupmed.

Its age, mine does this if it sits for an extended time with no starts. The second
rotor is not sealing well until enough gas and oil get in there and then it pops and
runs fine. Its is slightly related to ignition as well, since I redid my ignition to use
HEI ignitors and TFI coils, it only does this if it sits a while (2 weeks or longer).
Otherwise now I just pull the choke (which just turns on high idle on my carb),
crank it, and it starts right up at around 1500 to 2000 rpm and I drive away.

I also sometimes have hot start issues, again age. If I shut it down hot and try to
restart very soon after, it will not want to start. Let it sit a bit, and it cranks up.
Age sucks as we all know. :lol:

Made some progress but did not solve the issue. After unplugging the electronic portion of the choke, the issue ceased. I plugged in another choke unit but did not install the choke cable on the carburetor and the issue came right back. Seems as though the choke unit itself is working ok.

Next, I did the same with the emissions control unit. Unplugged it and the issue went away. Swapped in another known good unit and the issue came right back.

The next thing in line was the choke and check relay. When I unplugged the four pin connector from the relay, everything was just fine. Choked worked great, however the warning lights on the cluster were not functioning nor was the tachometer reading voltage with the key in the on position. Swapping in a known good unit did not change a thing.

Then came the throttle switch on the carburetor. Unplugged it and the choke worked great with no issues. Swapped in another good unit and no change. I then went back and adjusted the original unit to factory specs. No change.

I'm pretty lost here. What else is there in the hoke circuit that would malfunction and mess with the idle? again, the idle went erratic even when another choke unit was installed but not hooked up to the carb. Its got me stumped.

Describes my starts to a "T" -except "extended time with no starts" is now just 'overnight'
:(
Rotor motor might just be showing its age (95K). damn.

Stu Aull
80GS
Alaska

Opening circuits in the ECU/warm-up control system is making your problem go away.

Seems like the problem is related to the way your ECU reacts to the car being in a "warm-up" state. Doesn't mean the problem is in the ECU, just that it's in something that is being controlled by the ECU.

Do you have a spare ignitor control box? Only used on the 80, and it has influence on ignition timing during warmup, which is controlled by the ECU.

I'm not sure if it could cause your oscillating idle speed. I did a serious reverse-engineering effort on it a couple years back, but don't have my notes handy, I will see if I can find them.

On the non-cali unit, the BR wire connects the Ignitor box and the Vac Control Valve to the ECU by way of the #2 water temp switch, which is the one on the rad if I recall correctly (CA doesn't have that one). I'd be tempted to see if unplugging #2 water temp switch influences your problem, too.

The way the problem goes away when you disconnect various items that all connect to the ECU makes me wonder if your ECU's ground is maybe bad, too.

https://www.rx7club.com/1st-generati...uzzle-1057351/

My research thread, year and a half ago. This was for an 80 Cali-Spec, but the differences between Cali and Fed seem to all be external to the Igniter box, and are shown in the 80 wiring diagram.

Thank you for the link Glen! I'll investigate the issue more today. I finally have a day off. Going to go through the wiring diagram from the FSM and see what I'm missing.

I recall finding and correcting an error on the 80 Cali emissions wiring diagram. IIRC the federal diagram was OK.