76 Cordoba with 400 and Thermo Quad

goodbread · May 30, 2015

Need some advice. I purchased a really low mileage 76 Cordoba. The car was an early Lean Burn car. The collector I purchased it from removed the lean burn and installed a Chrysler electronic ignition. The car runs great, but at idle with the a/c on, the car really idles poorly. We have replaced a solenoid and adjusted the idle without much luck. What is recommended for a carburetor as my mechanic thinks that may be the problem. A rebuilt original carburetor or are there better after market choices. It's a really nice car that I want to enjoy without the terrible idle. Any ideas? and - should I replace the ignition as well?

Photon440 · May 30, 2015

What do you mean by "idles poorly"? Is the idle speed too high or low, or is it running rough? What's the idle speed with the A/C turned off?

goodbread · May 30, 2015

It is running rough at idle. It runs very well and smooth while driving. We have adjusted the idle speed up, but it still idles roughly. Without the a/c on, it still idles roughly although not quite as bad as when the a/c is on. Thanks

WP29440SE · May 30, 2015

so did your "mechanic" do all the basic tune-up proceedures? like plugs, wires , timing , fuel filter , vacuum leak check ,ETC ETC. all those things need to be addressed before chasing a carb issue. if all roads lead to the carb, buy a kit , tear it apart , clean it , assemble it and run it.

rumblefish360 · May 30, 2015

If all the basic tune up things were done, then the carb may need a cleaning up.

Did the prior owner install a vacuum advance distributor?
Did he tap the TQ for a vacuum source for the distributor?
Is it the OE carb? If you don't know there are 4 numbers on the carb mounting pad on the rear. Find those numbers out and post them.
The ignition box is orange? Leave it.

mopar 3 B · May 30, 2015

rumblefish360 said:
If all the basic tune up things were done, then the carb may need a cleaning up.

Did the prior owner install a vacuum advance distributor?
Did he tap the TQ for a vacuum source for the distributor?
Is it the OE carb? If you don't know there are 4 numbers on the carb mounting pad on the rear. Find those numbers out and post them.
The ignition box is orange? Leave it.

Tap for the vacuum advance will have no effect on idle unless drilled below throttle plate. Also there should be no vacuum to the distributor at idle if carburetor has been taped correctly and setting are correct. Assuming everything was done correctly vacuum advance will also have no affect at idle engine speeds.

goodbread · May 31, 2015

Thanks guys. When I bought the car it had less than 10k miles on it. The garage owner did a full tune up. I bought the car this last winter and did not turn the a/c on for a while. Once I needed it regularly, the rough idle became a major issue. The original compressor was beginning to "grab" so we put in a new compressor and converted to the new Freon. Other than the a/c taking a while to cool down, the rough idle is the only thing I can find wrong. Looks like I will go with a remanufactured carb from Summit. I didn't know if the removal of the lean burn could be part of the problem - We have done everything we can think of other than a rebuilt carb. The garage I use is known locally as experts on the older cars and used by most car club guys in the area.

WileERobby · May 31, 2015

Back then, when we used to do these, we'd swap the carb for an earlier 70's thermo-quad. Tuned it & they ran smooth as silk.

HawkRod · May 31, 2015

You mentioned a solenoid, but not if it is working or what it does. There is supposed to be a solenoid that increases idle speed when the A/C is on. Is that installed and working? I recommend you test it without the A/C running - put 12 volts on it and see if your idle increases. If not, there is likely your problem...

dieseldazzle · May 31, 2015

I'd do a basic compression test. Too many times people overlook the basics. If the compression is great across the board then rule out ignition parts. I have to be honest I have yet to see a carb cause a car to miss at idle or at speed.

goodbread · May 31, 2015

OK - thanks. We put the new solenoid to increase the idle with the a/c on. didn't seem to help.

HawkRod · May 31, 2015

goodbread said:
OK - thanks. We put the new solenoid to increase the idle with the a/c on. didn't seem to help.

"Didn't seem to help". The solenoid should noticeably raise the idle. Did you verify that it is working and truly raising the idle?

WP29440SE · May 31, 2015

dieseldazzle said:
I'd do a basic compression test. Too many times people overlook the basics. If the compression is great across the board then rule out ignition parts. I have to be honest I have yet to see a carb cause a car to miss at idle or at speed.

ive had it happen more times than I can count. if one of the floats/needle and seat are stuck or dirty , it will cause a rich condition that the plugs cannot burn every compression stroke at idle , but will fire as RPMs increase. a severe rich condition will cause a miss at speed , and a rough idle and hard starting. if a carb is too lean , it can do the same at idle and speed. in fact , a bad or dirty / flooding / ill-adjusted carb is usually the suspect . hell , bad gas or rust in the float bowls can do the same. carbon built up in the metering screw jets can cause major problems. I would ditch that thermobog and slap a 1406 performer on there and I bet all those problems go away. problem is , you will need a different intake or have to use an adapter.

dieseldazzle · May 31, 2015

WP29440SE said:
ive had it happen more times than I can count. if one of the floats/needle and seat are stuck or dirty , it will cause a rich condition that the plugs cannot burn every compression stroke at idle , but will fire as RPMs increase. a severe rich condition will cause a miss at speed , and a rough idle and hard starting. if a carb is too lean , it can do the same at idle and speed. in fact , a bad or dirty / flooding / ill-adjusted carb is usually the suspect . hell , bad gas or rust in the float bowls can do the same. carbon built up in the metering screw jets can cause major problems. I would ditch that thermobog and slap a 1406 performer on there and I bet all those problems go away. problem is , you will need a different intake or have to use an adapter.

He needs to do a compression test. Carbs dont cause misfire. Sure they can end up fouling your plugs which will lead to misfire but that is from the plugs, not the carbs.

.........he could easily look at his plugs as he pulls them to do a basic compression test......

Im sure the thermoquad is donkeyed up, most are but try not to discourge him from doing a cheap and usually free basic check.

Billschroeder5842 · Jun 6, 2015

I scrapped my Thermoquad as I could never dial it in. I bought a demon and have had great experience. Big improvement.

goodbread · Jun 29, 2015

Still in a quandry. Have put two different rebuilt carbs on that were supplied by a company in Atlanta. The rough idle continues - much worse when the a/c is on. With both rebuilt carbs, we added hesitation to the mix. When you accelerate, it hesitates and will some time cut off. Mechanic thinks we should go with an original oem carb. is there a good aftermarket carb that I can buy that doesn't require manifold changes? These thermoquads seem tempermental. It's amazing, the car ran so well before the a/c was needed. Once I began turning it on, the rough idle began and now we have the hesitation added to the mix.

rumblefish360 · Jun 30, 2015

There a PITA to set up correctly and get them dialed in well. Once there set up, there excellent carbs to drive on. Another round of Q's;

By chance, are the correct gaskets used to put the carb together?
Did you disassemble and blow it out with compressed air?
Are the float drops set?
Both posts wonder. Just wondering on how the carbs state of health is and how it was rebuilt. I've seen funny mixing of parts work but more often not work.

Considering what you wrote, it just seems like there is dirt somewhere or the throttle shaft was bent during A/C operation causing a air leak.

chykov's76doba · Jul 15, 2015

Have you checked your timing chain? A loose chain can cause rough idle. I believe those vintage chain sprockets were coated with vinyl and overtime, they can break down and cause slippage throwing your timing out of wack. Just a thought. BTW, I have a 76 Cordoba with a 400 also.

coloradodave · Jul 15, 2015

IMO, if you spend more money to try another carburetor (that would be #4?) you're going to be disappointed in the results. The fact that someone did a tune-up recently doesn't mean that everything is alright. To track down a problem you start with the basics. The first thing is a compression check. The 2nd thing is a cylinder balance test. Every plug wire should be checked for the correct resistance, even new ones. The plug insulators should be closely inspected. The distributor cap should be checked for carbon tracking. If the car sat over the winter, was fuel stabilizer used?
The point I'm trying to make is that you can spend a lot of time and money throwing parts at a problem and never find it. Good diagnosis will narrow it down and get to the root of the problem.

rumblefish360 · Jul 16, 2015

chykov's76doba said:
Have you checked your timing chain? A loose chain can cause rough idle. I believe those vintage chain sprockets were coated with vinyl.

Coated with vinyl?!?!?!!?!!

The idea of a worn out chain is plausible
Vinyl is not.

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