azscottd

Some on this forum were most helpful when I was struggling last year to get an old 83 305 running again. After a lot of tinkering, I got it pretty much all sorted out. My son's been driving the car ever since.

My only persistent issue has been high idle. He always has to shut it off in gear.

I've been over this thing with a fine-toothed comb countless times and can't find a single vacuum leak. Tried smoke test, ether, propane, water, etc. Nada.

MCS cycles smoothly at 50%. TPS et all are adjusted to spec.

Here's what stumps me: I can back out the curb idle screw all the way. Primary blades are closed (looking down bore with light) but throttle stop doesn't meet the casting (where the screw is). There's a gap. It keeps running at this point and I'd have expected it to die (no air). Check TPS at this point and it's out of range. You can't turn it in far enough to get voltage to drop. Screw has to have at least a few turns exposed before I can adjust TPS.

Bottom line is - no vacuum leak and everything adjusted that I know how... why does it idle too high? And before anyone asks, I've disconnected and capped all vacuum ports (booster included) to no avail. I get a little drop from EGR but I just can't get this thing down to where it needs to be.

Any hints? Anything I might have missed?

Thanks.

Damon

The fast idle cam linkage (which is part of the choke system) is one of the the "usual suspects" in these situations. If it's hanging on the last step of the fast idle cam, it WILL be idling high.

azscottd

Damon,

Choke is wide open and throttle is resting on stop screw which is only a turn or two showing (barely making contact).

Damon

Hmmmm..... hmmmm..... hmmmm......

You get a LITTLE drop from the EGR? You should get NO drop from the EGR by unplugging it. It should be fully closed at idle.

If it's hanging slightly open (from mechanical damage or carbon buildup) it will increase the idle speed, but the idle should also be pretty crappy and uneven, though it won't skew the O2 readings much.

Beyond that, I'm into secondary throttle plates not seating in the bores and such oddball stuff as that, if there are no external vacuum leaks and the fast idle cam isn't hanging up.

azscottd

EGR? That's why one should never text, email, or post on forums in a sleep-deprived state. :-)

EGR should have read what I was thinking... PCV. Which is to say that disconnecting PCV and plugging inlet with thumb was only appreciable drop in rpm. Blocks all other ports with no impact.

Like I said, I've come at this think multiple times using starter fluid, carb cleaner, propane torch, smoke, and a water hose looking for a vacuum leak. Can't find a thing.

Choke the air horn with my hand and it dies instantly.

Damon

Does it exhibit any kind of "hanging" behavior where every now and then it will drop to a normal idle speed? Worn out throttle shaft bushings can do that. It's common on older QJets, especially where both the throttle cable and the return spring pull BACK on the throttle arm. Not only could it cause a slight vacuum leak, but more importantly it can cause the primary throttle plates to wedge in their bores before they are closed enough to get down to normal idle speed.

A dead-give-away is if you shut off the engine while it's idling high and then immediately restart without touching the gas. The lack of vacuum pulling down on the throttle plates will let the shaft return to it's "home" position and it'll idle at normal RPMs until the next time you blip the throttle.

SolarGoldRaptor

I have the same problem on my 84 L69 with an 83 carb (correct model, freshly rebuilt). It runs ok when cold, the idle is a little too low in the beginning and I have to help a bit with the pedal for a minute or so. Then the idle is good and smooth.
Once the motor is in temperature, the idle gradually becomes faster and faster. The tach is not working but by the sound, I'd say about 2000rpm.
Since the problem only begins as the motor warms up, I doubt it's a vacuum leak but it could have something to do with the linkage. The choke flap looks widely open, maybe 40°-50°. Choke pull of is brand new.
I haven't driven her much so far, it ran ok but ate too much gas and once I turn the engine off, it diesels for a few seconds...which I assume is due to the high hot idle.

EDIT : I've noticed that if I cover the carburetor with my hand, the idle increases significantly. If I unplug the choke pull off, the idle slows down and seems smoother. Adjusting the choke pull off has little to no effect

SolarGoldRaptor

I've solved the problem, it was several things :
- the choke linkage was dirty and a bit rusted, it's the only thing I hadn't removed when rebuilding the carb... so I've replaced the whole unit with one in perfect condition from another carb. It was simpler as I had two other ones, but if you have only one, disassemble it completely and clean it.
- the idle mixture screws weren't set properly, so I've adjusted them to factory specs (3.3 turns for an L69)
- the fast idle screw wasn't set properly either


Now the idle is super smooth and at a correct speed, can't say how fast as the tach doesn't work. But I really like how it sounds.

Hope it helps