Engine won't idle - What else to try?

[bobcity]

I'm really thrashing it out with my 79 ADHA carbed 2000. Idle problems were annoying. I've got some ideas here and tried them, but I'm really stumped now.

First I tried cleaning carb with carb cleaner where ever I could. That seem to make matters worse.

Then after trying to adjust I thought maybe it was the cutoff solenoid. So I replaced with a new one.

Engine would not idle on idle jet when cold at all and would stay way up on the fast idle cam 2500, I'd kick it down and it would just go back up.

I took that choke/fast idle apart and cleaned etc. No joy.

Finally, I found a nicely rebuilt carb here on the board and thought, well this ought to just about do it. No matter what.

Put on new carb today. Would not idle cold. just popped and died under 1200. When it reaches 190 it idles between 1700 and 900, it kind of surges between them, mixture screw most of the way out. This is exactly what the old carb did too. Tolerable to drive, but not happy.

I've checked the vacuum hoses many times for disconnects. The only thing wrong is the dizzy capsule went out and I disconnected and plugged that line.

Now I'm wondering, Is this something more serious? I tried a cylinder balance test and all were the same.


Anybody have any suggestions?

[So Cal Mark]

first, assuming someone has rebuilt a carburetor properly is a HUGE assumption. Most people assume spraying a carb and replacing a couple of gaskets constitutes a rebuild. My bet is most carbs are listed for sale because the owner couldn't make the car run properly with that carb, especially if it's advertised as recently rebuilt. The problem with trying to diagnose the problem without understanding how each circuit in the carb works makes it very difficult.

Now, since your symptoms are similar between both carbs, you MAY assume there is another issue. Since you have a 79, what have you done with the egr valve? If it's still on the manifold, check to see if it's leaking or stuck open. That would create a large intake leak. You say it idles at 1700; are both the fast idle screw and idle stop screw backed out so they aren't touching anything? If so, it sounds like you have a large air leak somewhere. Since your cyl balance seems equal, it has to be at a point that affects all cyls equally. Carb base, egr valve, brake booster, etc. To check for a vacuum leak, you can spray carb cleaner with the engine running. When the carb spray hits the source of the vacuum leak the engine rpm will change

[bobcity]

Ah...thinking... does the EGR valve vacuum source come from the vacuum manifold bank thing like I have it at present or from the carb please?

[So Cal Mark]

on a stock 79 manifold the egr valve sits right on the manifold. One port is drilled directly into the intake manifold, the other port is an exhaust port that crosses over in the cyl head and connects to the exhaust manifold. The small vacuum hoses don't create a large enough leak to create the symptoms your car has. If I were you, I'd remove the egr valve and plug the two holes and see if it changes your situation

[bobcity]

That sounds like a good idea. What would you use to plug the holes with.
Also I sent you a PM about the carb.

[bobcity]

So I made a fake gasket out of brass sheet and bolted the EGR and gasket back in place on top of it.
Still idles way to high 1600 to 1000. surges up and down.

New problem, can't drive car due to hesitation when starting out. Now remember the vacuum advance capsule is broken, but I gather from all I've read here that that wouldn't be the cause. Or could it.?

Carbs are a little different although both are ADHA. New one is missing something called "fast idle diaphragm" which manual says is not used on ADHA .

Decided to put back old carb and see how it runs with EGR plugged.
When cold. Races or dies below 2000 with idle screw all the way in.
When hot can be adjusted to Idle between 1500 and 1000.
mixture screw seems to make little difference past 3 turns out.

Also, has same hesitation as other carb only less.

Also, spraying carb cleaner around the manifold does seem to bump the idle a little.

I've never timed engine yet.

[Blast]

Just shoot me if this is a stupid post, but in my 78 theres a throttle knob under the steering wheel on the left side. If this was pulled out some, could this have anything to do with your problems?

[bobcity]

Just shoot me if this is a stupid post, but in my 78 theres a throttle knob under the steering wheel on the left side. If this was pulled out some, could this have anything to do with your problems?

No shoot me for not having a throttle knob. I've always thought they were a great addition to older cars.
Thanks for the try.

[Jim DeShon]

I agree with what has already been written--vacuum leak ! I would say that it would have to be at or near the base of the carburettor. The reason I say this is that if it were down one of the runners on the manifold [such as a leak between the manifold and the head] one cylinder would be affected more than the others. I would look at the vacuum booster connections carefully. You have already eliminated the EGR as the problem. I also agree that you need to make sure that the throttle linkage is not hanging open. Remove the linkage at the carb. and see if it helps. I would also replace that defective vacuum can on the dist. Although it is not the source of your problem, it will improve throttle response off idle a bit. ...J.D.

[bobcity]

Disconnected throttle linkage, another great idea I should have tried, but now can't keep it running with idle screw all the way in. Wants to backfire up through the carb as is starves out.

Looked a booster line into manifold, no hose clamp on braided line. Hum, maybe I need a clamp there.

It does seem like the leak could be around the base of carb, but how or where? Nuts are pretty tight.

Ordered vacuum capsule today.

[Jim DeShon]

I would pull of the carb. and inspect the gasket. The bolts may be tight but, if the gasket is cracked or worse, it will leak. We are also assuming that the ignition timing is correct. Have you made sure everything is OK there ? If it is too far advanced, it will not idle. Are the centrifugal springs intact ? Does the system advance the timing when the throttle is opened ? Go back to basics. You would be suprized how many problems turn out to be some stupid little thing. ...J.D.

[manoa matt]

If you turn the carbs upside down and look near the throttle lever on the automatic choke housing there is a small "tab" of white/cream colord plastic. If it appears the tab is broke off or non existant on both carbs then they won't idle properly. That part is NLA.

[bobcity]

I would pull of the carb. and inspect the gasket. The bolts may be tight but, if the gasket is cracked or worse, it will leak.

The gasket was very good. I also had a rebuild kit already so I tried the new gasket. But, I went back to old one because of slight difference in shape of holes.

We are also assuming that the ignition timing is correct. Have you made sure everything is OK there ? If it is too far advanced, it will not idle. Are the centrifugal springs intact ? Does the system advance the timing when the throttle is opened ?

Got to go get a light and check it. Your right.

You would be suprized how many problems turn out to be some stupid little thing.

...J.D.
I've got that feeling all the time that I'm missing something very obvious. In case I didn't mention it before here I'm new to the 124 and haven't wrenched in a long time. Some how I'm afraid I fell into the deep end of the pool.

[bobcity]

If you turn the carbs upside down and look near the throttle lever on the automatic choke housing there is a small "tab" of white/cream colord plastic. If it appears the tab is broke off or non existant on both carbs then they won't idle properly. That part is NLA.

I'm not sure what you're saying, what part; what does it do?

Anyway...new carb
or
or
Old carb
or

Plate on top of carb should not be in picture. Just got put there.

[danaspider]

2 carbs same problem, hummm. Have you looked at all of the other vacume lines and the little vacume manifold at the back of the engine, where all of the lines go through. You can even try pulling lines when ideling and see the affect, even putting finger over line to see what changes. Seems to me it is in the mass of detox lines that are on the engine.

just a thought.