'81 Long/Random High Idle(s)

[MP81]

Hello there!

Ever since getting the car ('81 Z28, LG4 305) last year, the carb has always exhibited somewhat odd/random behavior.

When you start the vehicle up, it jumps to high idle (maybe a little higher than I'd expect) until the choke opens and the idle begins to come down. However, at some point, you'll hear it change, and it'll start increasing - all the way to about 2500 RPM, if you let it.

The only real way to get idle RPM to drop back down is the rev the hell out of the thing, and at some point, it'll drop down to 750 RPM, where it idles like a champ, though it likes to creep up to about 1k after that...

Sometimes, when you rev it when it's up high doing it's "thing", they're very slow to drop back down.

I remember earlier this year (prior to putting the car up on stands to install my headers - where it is currently still sitting awaiting my lazy ass to install the new exhaust) I was messing with it, because it had gotten really bad after the winter. I'm no carb expert, so bear with me on the terminology/descriptions, but the "rear linkage" (separate from the main throttle linkage) seemed to exhibit exactly the same behavior if I pushed on it while the car was idling - same sound of suddenly rising RPM, etc. I thought I had fixed it by playing with that a bit, but it seems to have returned.

I need to give the linkages all a good coat of lubricant (since I doubt that's been done in...well, maybe never), but I'm not sure if that'll be the entire solution.

Any idea what might be the culprit?

Thanks,

Kyle

[Frank400]

Looks like and electric choke that is not working, or unplugged, or plugged but with no power (12v with ign on).

[MP81]

The choke does seem to operate right after you start it, but checking for 12V with ignition on would be an easy test.

EDIT: Couldn't get a real good reading on the connection to the choke with the ignition on (but getting a good reading on the battery wasn't easy either, so I'll take another stab at it tomorrow) - regardless of that, I'm glad that I checked it, because it definitely was barely attached (terminal was too "open" and loose, so I closed it up a bit, and now it is tight on the spade).

[Mudsport96]

I'm not as good as many of the guys on here, but there is a possibility that the choke coil is going bad. Just like anything else, it can wear out, so maybe its opening and then closing again instead of staying open. Something to look at.

[Cliff Ruggles]

I see these problems frequently and exactly why we always replace the associated parts on those units.

I would replace the electric choke and the choke pull-off.  Adjust them per the factory service manual.  Also lubricate all parts involved with the choke and fast idle, and the problems will be corrected provided that you have a good voltage supply to the choke coil......Cliff

[MP81]

Bumping this one back up - still having this funky idle. It seems to be less prevalant the more the car runs after storing it for winter, but still is present.

I've noticed now that it tries to die right after it starts. Giving it a good bit of gas will usually solve that, and then it's fine after a few seconds of feathering it. It'll idle fine, then light a switch will randomly go up to 1500 or 2k. Sometimes giving it a healthy rev will bring it back down where it should be, other times not.

Replaced the choke last year, so that is new, and confirmed it is opening. Any thoughts on what I should look at with the engine running to see what might be suddenly forcing a high idle?

[Cliff Ruggles]

Is the fast idle cam falling all the way to the lowest position and staying there?

[MP81]

If I'm remembering correctly, the choke should push that down as it opens, correct?

[77cruiser]

No it doesn't the choke can be open & still on full fast idle. Open the throttle & the cam should fall on it own.

[MP81]

OK. What I was thinking of was the choke lever for the blade.

I'll be out there shortly to start on the car for the day (t-top weatherstripping - lucky me), so I can take a look.

[MP81]

All right, so I had a chance to start it yesterday.

Of course, it didn't do the typical thing. As I let it idle, I noticed it was sitting around 1k-1100 or so, until I pushed the lever (arrow in pic) down, and the idle dropped right down to about 700 or wherever it is supposed to idle (did not take much effort, just a light touch).

It also seemed, at least in this instance, that revs "sticking" or being slow to drop was solved just by making sure the throttle linkage was pushed all the way back to closed, maybe a little sticky (though I thought I did lube that last year).

I'd still like to see it jump up to 2k while I'm underhood though and see what flips like a switch when that does happen. Maybe it is related, though...

[MP81]

Playing around with this yesterday, that same "arm" basically stayed stuck up, until I pushed on it, and it dropped past whatever was holding it up. Was holding idle about 1000-1200 or so. Once it dropped down, she sat an idled at about 750 for the whole rest of the time - aside from when the throttle would stick it up higher, but that was easy enough to unstick (and diagnose).

As far as the super-high 2k rpm idle that came on like a switch...haven't seen it the past two times. Got me thinking - what if the vacuum solenoid that opens the flap in the air cleaner for the heat riser has been the culprit, causing a big vac leak all of a sudden - the past two times I've run the car, I've had the air cleaner off, and that vacuum port plugged. I don't need it, as I have headers, so I think I will maintain that and see.

[Cliff Ruggles]

Once the fast idle cam drops to the lowest position it should stay there until the choke raises it again.  If it's coming back up while driving around with a hot engine the choke is defective......Cliff

[MP81]

I have to confirm, but I don't think it's popping back up - I think it is getting hung up, rather than dropping down. When I pushed on it, you could tell it was under pressure to drop down, but wasn't able to because it was stuck up, though I'm not sure on what.

Choke is brand new (ordered from here), and it did this with the old choke, as well.

[Cliff Ruggles]

If it's never dropping to the lowest position and getting hung up, then remove the housing and clean/polish/lubricate all the associated parts.  Make sure nothing is bent and touching or rubbing on something it's not supposed to.

Put it back together, make sure the pull-off isn't hitting anything it's not supposed to, and if it is bend the linkage or at the bracket to get the clearance you need (common practice here).

I'd remove the lock-out and discard it, just another thing that can hang things up and isn't needed.  Make sure the parts on the primary throttle shaft are installed correctly and intersecting the choke parts correctly when they need to.

Once all that is done it will work flawlessly, or it should assuming all the parts are correct on that side of the carb.....Cliff