carb choke idle-down

[jiho2]

This concerns a gov. surplus '83 Chevy Malibu sedan, with the famous "VIN A" Buick V6 (NA with a humongous computer controlled DualJet carb).

First there was a dead ISC nose switch. To compensate, someone had adjusted both the idle stop screw and the ISC plunger to "hardwire" curb idle at 650 RPM.

Once the nose switch was fixed, there was a time of good idle-downs. The car went from full-choke fast idle to curb idle in nice, regular steps untouched by human hands (well, OK, so rough the car shook like a cement mixer).

Then I had to replace the intake manifold, as the flange for the Torx bolt hidden under the distributor was discovered to be, uh, not there. (None of the bolts offered much resistance to removal, so I guess the intake must have flexed too much. The crankcase was having carbon pumped into it, but no coolant.)

Since the manifold was replaced, the car has not idled down, period. I have to kick the accelerator to get it to move down each step, and then it's very balky about going to curb idle. It doesn't want to go to curb idle after driving, either, but it will if I kick the accelerator enough.

The choke opens normally, but the throttle doesn't want to close. The ISC is working fine, everything seems to have good freedom of movement, the return springs are both fine, the accelerator and pump both work normally. The choke coil and vacuum breaks all work fine.

Now, I know what you'll say: "Squirt carb cleaner on it and it'll work fine." It's off the car, but I did just spray it and can't tell any difference. And even if somehow now it will work, HOW and WHY will it work?!?

I've NEVER understood how the fast idle cam is supposed to move down. I have a 1980 DualJet manual that says:

"The fast idle cam has graduated steps so that the fast idle speed is lowered gradually during the engine warm-up period. The fast idle cam movement (and step position) is a function of choke valve position."

Unfortunately, it doesn't explain HOW it's a function of choke position. There isn't any detailed mechanical theory of operation for the fast idle cam.

From what I see, the fast idle cam is passive, that is, the intermediate choke shaft forces it up where the cam follower lands on the high step. But from there, nothing forces it to follow the intermediate choke shaft as the choke opens, and the cam follower actually PREVENTS it from moving, far from forcing it to move. I can get it to click nicely down the steps by pushing on it lightly with my finger.

And this is no different than back in the days when it actually worked.

What empowers the voodoo economics of choke idle-down?

-- Jim Howard <jiho@c-zone.net>

 

[jiho2]

Well....I still have to kick it down from fast idle. But when I do, it now goes down to curb idle easily, and I no longer have to kick it down after driving. So, cleanliness does seem to have been an issue, for that part of the trouble anyway.

But if the fast idle cam was a cleanliness issue, I ought to be able to demonstrate that by tugging the throttle closed on the driver side; that should overcome any crud and get the follower bumping down the steps, same as pressing down on the cam with a finger on the passenger side. Instead, it won't budge, demonstrating only that the cam is locked by the follower.

The plot (if not the crud) thickens with a look at the '83 Malibu Owner's Manual. The directions for starting the gasoline engines reads like it was written by a team of lawyers:

"Press the accelerator pedal to the floor and slowly release it. With your foot off the pedal, crank the engine....When the engine is running smoothly (about 30 seconds), you can reduce the engine idle speed by pressing down on the accelerator pedal, then slowly releasing it. NOTICE: If the engine runs for a long time (5 minutes or more) without pressing down the accelerator pedal, overheating could cause damage to the engine and exhaust system."

Note it says "can reduce" and "could cause damage", not "must" and "will". How ambiguous. I infer that Chevrolet had so little faith in Rochester's fast idle mechanism that they wanted no responsibility or liability for its function.

Well, maybe. I'm defending my position that it's supposed to idle down automatically.

But I do swear that once upon a time it did, in fact, idle down automatically. And the Delco DualJet manual SEEMS to say it should; it only mentions using the accelerator to engage the choke and fast idle to start, and then to invoke the unloader tang if it floods, NOT to force the idle speed down.

I guess I'll just have to take the carb off again, and give it a real bath, and dance around and chant. But I'd feel better if I knew how it's supposed to function mechanically.

-- Jim Howard <jiho@c-zone.net>

 

[tony moore]

you have to open the throttle to let the fast idle move it has a steped cam on the opposite side of carb from choke.
you have to open throttle again to let it close , when you see it you will understand how it works.

 

[jiho2]

Originally posted by tony moore
you have to open the throttle to let the fast idle move it has a steped cam on the opposite side of carb from choke.
you have to open throttle again to let it close , when you see it you will understand how it works.

Thanks for the reply (I was starting to think no one knew if their carb idled down automatically or not ).

Actually, the stepped cam is between the carb body and the choke, on the same side.

Anyway, I've seen it -- I've taken it apart and put it back together -- but I only understand part of how it works....

Alright, when you say "open the throttle to let the fast idle move", you mean to close the choke and put the throttle on the high step, right? That's to start warmup.

Then when you say "open throttle again to let it close", you mean to get to curb idle, right? That's before you drive off.

Or do you mean you have to open the throttle to move it down each step of the stepped cam?

That's what I don't see. In between fully closed choke and high fast idle, and fully open choke and curb idle, are you supposed to need to kick the throttle to get it to move down each step as the choke opens, or is that supposed to happen automatically?

Or does nobody know because they've never waited around long enough to find out?

I remember that happening automatically once upon a time. The idle speed seemed to go down fairly quickly in 2 or 3 stages, without me doing anything. I do believe I did have to kick it once, after the choke was fully open, to get all the way down to curb idle.

Otherwise, either that was just a bizarre fluke of adjustment and everything being really clean, or I'm mistaken about what I remember for some reason (like an issue with the fan?!?). You'd think you'd have a way to know, but I can't reconstruct the circumstances....

-- Jim Howard <jiho@c-zone.net>

 

[jiho2]

This is kind of humorous, especially since the other thread I got into involved somebody's catalytic converter....

Originally posted by jiho2
Once the nose switch was fixed, there was a time of good idle-downs.....

Then I had to replace the intake manifold.... (....The crankcase was having carbon pumped into it, but no coolant.)

Since the manifold was replaced, the car has not idled down, period. I have to kick the accelerator to get it to move down each step, and then it's very balky about going to curb idle. It doesn't want to go to curb idle after driving, either, but it will if I kick the accelerator enough.

It turned out my cat was so bad it was stalling the exhaust.

Before I replaced the intake manifold, it was letting the exahust divert into the crankcase and back out through the breather and air cleaner snout. After I replaced the manifold, the exhaust had nowhere to go and was causing the intake mixture to stall, which in turn caused the the throttle to float.

I finally got the cat replaced (due to flunking smog), and since then I've had no problem finding curb idle and staying there.

If I hadn't seen this with my own eyes, I never would have believed it. I mean, it sounds kind of ridiculous. Is this even possible, or is it some kind of weird coincidence?

Of course, I still have to kick it down to get to curb idle, otherwise it would stay on fast idle all day. I've given up on that part, since I can't see any way it could possibly step down by itself....

--Jim Howard <jiho@c-zone.net>

 

[jiho2]

Re: Re: carb choke idle-down

Originally posted by jiho2
I finally got the cat replaced (due to flunking smog), and since then I've had no problem finding curb idle and staying there.

Just for the record, I can see that needs a little explanation. Something got lost in the shuffle between that and my earlier posts of a few months ago.

In my earlier posts, when I said "curb idle" I just meant a low idle speed. I hadn't gotten around to checking with a scan tool to see if the ISC nose switch was actually closed and the computer controlling the idle speed. When I did check some weeks later, turned out it wasn't. That's when I started to suspect the cat.

Now, by "curb idle" I mean when the ISC nose switch is closed and the computer is controlling the idle speed. That's what is happening now and wasn't before I had a new cat installed.

--Jim Howard <jiho@c-zone.net>