Carb and Idle Issues

[JBeau]

Hello,

I have a 1978 124 with a 1.8 liter engine and a Weber 32 ADFA. In the past, I've had some strange stalling and fuel starvation issues that I thought I had resolved, but they came back. In attempting to resolve those issues, I removed the carb and intake (dropped a nut down the plenum, got SUPER lucky that it was in the intake and not the cylinder), and replaced the intake gasket and fuel pump (single plane intake). I made sure to torque the intake bolts and studs to the specified 18 ftlb.

Since I have gotten everything back together, I cannot get the car to idle or run correctly. It idles very rough and low (poorly adjusted tractor is the best metaphor I can come up with) and the throttle response is weak. Mixture screw seems to have no effect, and when I disconnect the idle solenoid (confirmed working), the car continues to run. What I've read is that this behavior indicates it's idling on the main jets, but I am not sure how to correct this. I took the carb apart for cleaning when it was off the car, but I was careful to put everything back properly.

I've also adjusted the ignition timing, and everything looks to be within spec.

After reading as much as I could here, I am at a loss. What is the best way to make sure my carb is idling on the proper jets? Are there any other adjustments I should make?

Thanks in advance!

[18Fiatsandcounting]

My guess is one of two issues (or both): You have an air leak into the intake somewhere, or you are idling off the main circuit as you surmised. The latter could be caused by a blocked idle circuit in the carb.

Here are the steps I would take, in this order:
1. Look around the intake manifold for any hoses that don't lead anywhere or are disconnected, frayed, look like they might be leaking, etc. Fix any issues if found. If the car has been de-smogged, make sure the various hoses/tubes to the intake and the carb are all plugged. The only one you really need is the big hose to the brake booster (and sometimes that hose develops leaks).

2. With the engine off, push on the brake pedal moderately hard. Then start the engine with your foot still on the brake. Your brake pedal should depress another 1/2" to 1" with keeping the same pressure. If it does, your booster is working OK and likely doesn't have a leak.

3. With the engine off and with the choke plates moved open by hand, peer down the carb barrels with a flashlight as you work the accelerator. You should see a stream of gas coming from the accelerator pump jet, but it should stop when you stop working the throttle. Then start the engine but don't work the throttle. This time, there should be no fuel dribbling from the pump jet outlet. If all that appears OK, then your accelerator circuit appears to be working OK.

4. With the engine idling and choke plates held open, peer down the barrels again with a flashlight. You want to look at the circular "strut" that goes across each barrel, two inches or so below the top of the carb. Both barrels. You should not see any fuel dribbling from that center area, or pooling up on the top of the bronze-colored butterfly valve at the bottom. If you see fuel, then yes, you are idling off the main circuit and not the idle circuit.

If all of the above appears OK, then my guess is that the idle circuit is plugged or inoperative for whatever reason. It could simply be that your idle shutoff valve is stuck or bad. Is the idle mixture screw (the one pointing left at the base of the carb, upwards at an angle) screwed all the way in by chance? If all this seems OK, then it might be time for a thorough carb cleaning and rebuild.

-Bryan

[JBeau]

Bryan,

Thanks for the detailed response. Here's what I've found:

1. Hoses are all connected. Everything except the brake hose is relatively new and in good shape. The brake hose is still braided, so it may very well be original. I thought it was OK until performing these checks, but now I'm not so sure. I'll change it as the next step in diagnosing the issue.

2. Brake booster was working as you described until I tested tonight (everything was OK on Saturday, which is why I thought the brake hose was still serviceable). Now, there is no change in pedal position with the engine running. Admittedly, it ran VERY poorly, so it may not have been building enough vacuum to make a difference. I don't know enough about it to know if that's a possibility.

3. Pump jet works properly. Jet of fuel when working the accelerator and the engine off, no dribbles when running.

4. There is fuel dribbling from the center of the strut in the primary barrel (the one with the choke plate). The secondary seems to be dry. I suspect this is the source of some, but not all of my worries.

So, with all this in mind, what is the best way to ensure that the car idles from the idle circuit? My plan is to replace the brake booster hose first, but what (if any) adjustments can I make to set it properly?

Also, the mixture screw is currently set two turns out as a baseline, based on suggestions from all of he guides I've read. I confirmed again that the idle solenoid has power and is working (solid "click" when plugging in the 12V source with the ignition on). Another (newly discovered) wrinkle is that the engine seems to try to smooth out a bit while it's idling. Just intermittently, but it's like the rpms want to be in the correct window. I don't know if that's useful information.

Thanks again!

[18Fiatsandcounting]

What you describe in #4 above makes me pretty sure that the engine is idling off the main circuit, not the idle circuit. Your idle shutoff valve could be plugged even if it is clicking on and off, so I'd unscrew that valve and carefully inspect the small holes at the end. It doesn't take much to plug those. Use a flashlight and magnifying glass to make sure the jet at the end is truly open. It wouldn't hurt to shoot a squirt of carb cleaner in that hole in the carb to flush out anything that might be at the bottom of that hole.

I would temporarily disconnect the brake booster hose and plug the hole on the intake manifold with something. Like a rubber cap or the like. Just to make sure it's not a leak issue with the hose or the booster.

Your observation about the idle smoothing out occasionally sorta confirms that there is either something plugging the idle circuit or causing an on/off again leak into the intake manifold.

The best course of action would be a carburetor rebuild, but let me know if that's not feasible and I can come up with some other ideas.

-Bryan

[JBeau]

Bryan,

Thanks for your response. I really appreciate your insight.

I'll throughly inspect and clean the solenoid and blow out the port as well. Good idea about removing the brake booster hose to isolate the issue,

Is there any relationship between any of these issues and the fast idle adjustment screw? Are there any recommendations about setting/adjusting that screw that would impact the idle issues I have?

I don't have a rebuild kit (yet), but that might be the next step. I plan to call around to see about the cost of rebuilding from a shop, but one place I called quoted $500, which seems like...a lot. I've never rebuilt a carb (I'm sure that's a surprise, but it's true!), are there any resources to recommend?

Thanks again!

[18Fiatsandcounting]

Your comment about the fast idle switch got me thinking: Is the choke stuck in the on position? Perhaps not the flaps at the top of the carb, but the fast idle screw is hung up on the cam inside the choke mechanism, thus holding the throttle open?

The fast idle screw I'm thinking about is the one that sticks sideways into the choke assembly, at the end of a 3" or so lever.

Check the brake booster hose issue, and inspecting the idle jet and let me know what you find.

I don't know where you're located, but if you are interested, you could ship me the carb and a rebuild kit and I would rebuild it for you. No charge for labor, and I likely have most of the little bits and pieces that often go missing. No charge for those either, and I might even have an ADFA rebuild kit. PM me if you're interested.

-Bryan

[JBeau]

Bryan,

Sorry for the delay in response, I haven't had a chance to spend any time with the car since the middle of the week, plus it's been super hot here, which hasn't helped.

I disconnected and capped the brake booster vacuum line, but all of the conditions persisted. I removed the idle solenoid and blew out the passage with carb cleaner (both directions) and connected it to power and shot cleaner through the holes in it as well (grounded to carb to open them). I am also not getting any increased brake movement with the engine running, but I think that may be due to low vacuum.

I noticed that there was no fuel dripping into the secondary as the car was idling today, but disconnecting the solenoid still had no effect. The mixture screw also seemed to make no difference (except for one small point in the rotation, just for a second, and I couldn't replicate it). It's currently set at the baseline two turns out. The idle speed screw is just kissing the throttle lever.

The fast idle screw seems to be contacting the cam properly, as far as I can tell.

Thanks for the offer of help on the rebuild, I am struck by your generosity. I'm located outside of Philadelphia, so I don't think it's really feasible to ship the carb. I really appreciate the offer, though. I think my next step is a rebuild kit and new solenoid (just in case).

Thanks again for the advice, please let me know if you think of anything else that would be relevant.

[18Fiatsandcounting]

Well, there is one other thing I can think of. The ADFA carb actually has another fast idle device, separate from the one associated with the choke. This other one is an assembly under the float bowl in the front, and the diaphragm inside often gets crusty and sticks in place, which has the effect of holding the throttle open.

This other fast idle device is on the opposite side of the carb from the idle solenoid, and a bit lower. It's held on by two screws, and you might be able to remove the whole thing without removing the carb from the manifold.

Other than that, other possibilities are a bent throttle shaft, a sticking or misadjusted gas pedal linkage that prevents the throttle from closing completely at idle, or another linkage somewhere that's holding the throttle open.

And, one more thought, are you sure your ignition system is all working correctly? Good spark plugs, wires, cap / rotor, all that stuff?

-Bryan

[JBeau]

Just wanted to close this out, so that others in the future can know that the issue was resolved. I did wind up getting a rebuild kit, and taking the carb to a local shop to do the rebuild (I realized quickly that I would have been over my head with the task). Got the carb back on the car and it immediately ran 1000% better! Now I need to sort out the ignition timing, and I think it should be in good shape. I haven't had a chance to do that, since I've been trying to solve a leaking oil issue, but that's the next task.

Thanks again to Bryan for his insights and offer of help.

[18Fiatsandcounting]

Thanks for letting us know. Yes, these spiders are wonderfully engineered, but if one little thing in the carb or ignition gets out of whack, well, not so well engineered. Good luck with your spider!

-Bryan