Rough idle when warm/hot in gear and ac on

[Pclancy]

I have a pretty good mechanic, and he's rebuilt my original TQ carb on my 77 Newport with a 400 more than once in an effort to address removal of the lean burn system. He knows the car, and my wallet. But he's a serious and honest man.

Here's the issue: My 77 Newport starts in the winter really well--one long pump of the accelerator and she fires right up. And she runs really smooth in all conditions--fast, slow and in between. She likes the colder weather. The choke works just great.

But as the warmer weather approaches, several things seems to happen:

She will idle and run really smooth, except in gear like stopped at red lights;
She idles rougher when the ac is on than when not;
She can be a bear to start when really warm. If I have my technique just right, then no problem. But if not, then I need to crank her.

Literally everything has been replaced--spark plugs, wires, rotor, distributor, resistor, little orange box, etc. The carb has been rebuilt. My mechanic is not a young man, and he has decades of experience. He takes my car as a personal challenge, and he shows care and TLC.

The roughness is not severe, and she does not stall. But among you that know these beasts better than I, and in the hopes that I can say--gently-- "why don't you check X" when I take her in, what would you suggest?

Thanks

 

[Welder guy]

Well might I suggest a vapor lock issue. The hotter it gets the worse the issue . I had a ongoing problem that sounded a lot like this one that took me a long time to figure out. My gas was percolating in the steel gas line. Try to insulate your fuel line from the pump to carb. It’s cheap to do and worth a try.

 

[Davea Lux]

I have a pretty good mechanic, and he's rebuilt my original TQ carb on my 77 Newport with a 400 more than once in an effort to address removal of the lean burn system. He knows the car, and my wallet. But he's a serious and honest man.

Here's the issue: My 77 Newport starts in the winter really well--one long pump of the accelerator and she fires right up. And she runs really smooth in all conditions--fast, slow and in between. She likes the colder weather. The choke works just great.

But as the warmer weather approaches, several things seems to happen:

She will idle and run really smooth, except in gear like stopped at red lights;
She idles rougher when the ac is on than when not;
She can be a bear to start when really warm. If I have my technique just right, then no problem. But if not, then I need to crank her.

Literally everything has been replaced--spark plugs, wires, rotor, distributor, resistor, little orange box, etc. The carb has been rebuilt. My mechanic is not a young man, and he has decades of experience. He takes my car as a personal challenge, and he shows care and TLC.

The roughness is not severe, and she does not stall. But among you that know these beasts better than I, and in the hopes that I can say--gently-- "why don't you check X" when I take her in, what would you suggest?

Thanks

Sounds like you heat riser might be stuck closed, this over heats the carb to where it can "boil". Fuel vapors from boiling will cause a rough idle and fuel can spill out into the intake manifold and flood the engine on turn off. Also check to see if you have a "fast Idle solenoid" On most of the lean burn systems, this would have been incorporated as part of the system to speed up the idle when the A/C was on. If the connections to that item are gone, it no longer functions and you will have a rough idle when the A/C is on due to reduced engine RPM.

If you still have the original '77 Thermoquad on the car, there are are a significant number of modifications that need to be made to get the engine to run without all of the lean burn electronics. You would probably be better served to either replace it with a '75 year Thermoquad or a more modern AVS.

Dave

Dave

 

[CBODY67]

I don't believe it's really hot enough up there for vapor lock (or similar) to be happening, even with ethanol'd fuel blends. There can be a period of each "spring" where the gasoline is still "winter" and the weather is "spring", though.

If the TQuad is orig to the car, THAT could be an issue, but not really at idle. Most of the "lean burn" things happened once the rpm rose enough for the carb to be on the main system metering circuit. Idle was lean as normal, but nothing more.

I'd make sure the egr valve was closing fully at idle. Pull it off, get a new gasket for it, and wire brush the seating surfaces for the pintle in the valve. Don't use a "replacement" egr valve as they are "will fit" and their flow calibrations are less accurate than what the OEM valves were.

If not already done, check for vacuum leaks in the vacuum line connections and on the "vacuum amplifier" (if so equipped). You can use different fluids to check for this, depending on where the connections might be. Some like water, others like spray carb cleaner. If there are some plastic vacuum tees in the vacuum lines, those can fissure at their connections and cause small vacuum leaks. Don't forget about the interior a/c vacuum actuators, but that might be a long shot situation, but they're still in the vacuum use circuit.

As the roughness seems to get worse with the lower rpms, make sure the idle rpm is where it needs to be. In "N", plus under full=load idle in gear. Check the manifold vacuum, too, as if it gets too low under load, it might be causing enrichment as the power piston spring is letting the power piston raise the metering rods, making the mixture too rich, possibly.

As the carb's been rebuilt, I'm suspecting the idle mixture limiter caps are history. Which means the idle mixture can be adjusted for "lean best idle" (possibly a bit richer than the "other" setting might have been).

AND, make sure there's a thick OEM-style carb base gasket under the TQuad. It's a better heat insulator than the thinner cardboard gaskets which come in many carb kits. If NAPA can't supply one, then they used to be in the Holley carb catalog.

I developed an idle speed setting procedure back in the late 1960s, which worked fine with single exhaust vehicles, less so with dual exhaust vehicles. After first getting the engine to stabilized operating temp, I'd adjust the idle speed and mixture, starting with the factory specs. Then tweak the mixture for "best lean idle", looking for a 20rpm drop with each idle mixture screw, then adjusting them to just get that 20rpm/screw back. Ignition timing was done first, then the idle speed and mixture.

After this, I'd nail the parking brake, put the vehicle in gear, slowly release the foot brake to make sure the vehicle didn't move with just the parking brake fully deployed. Then with the settings what they were, I'd quickly go to the exhaust pipe, putting my hand in the exhaust stream to see if it was "individual pulses" or "smooth flow". If the rpm needed to rise slightly to get the flow smooth, I did that. Maybe 30rpm or so, not much, usually. And when the flow became smooth, idle smoothness increased.

One time, I was at my shadetree shop and one of the guys a few doors down drove up in his '70 MonteCarlo. The idle was radical, so I asked what cam was in the car. He replied and I was surprised. Another friend had that cam in his car (same size motor and similar compression ratio) and was much smoother. I could tell it was idling slower than normal, so I asked if he had a dwell/tach. When we checked it, it was more like 450rpm than 600rpm. When the rpm was increased to 600rpm, the idle smoothed out to "better than stock". Took all of his "thunder" out of it.

Let us know how it comes out.
CBODY67

 

[commando1]

One time, I was at my shadetree shop and one of the guys a few doors down drove up in his '70 MonteCarlo. The idle was radical, so I asked what cam was in the car. He replied and I was surprised. Another friend had that cam in his car (same size motor and similar compression ratio) and was much smoother. I could tell it was idling slower than normal, so I asked if he had a dwell/tach. When we checked it, it was more like 450rpm than 600rpm. When the rpm was increased to 600rpm, the idle smoothed out to "better than stock". Took all of his "thunder" out of it.

Let us know how it comes out.
CBODY67

lol. I remember adjusting the idle for best stumble at idle.

 

[Pclancy]

Wow, great stuff guys. She'll be in the shop in a couple of weeks, I will figure a way to share, and I will report back. My mechanic does NOT mind gentle suggestions as long as he gets to call the shots.

 

[CBODY67]

Might be good to have a conversation with the mechanic, asking questions (mentioned here) and seeing what he might say to them. Best to ask "for you" rather than identify their source, typically. Once that conversation has happened, discussing possible issues, then hopefully progress can happen. There might be something he'd not considered or thought about or things he knows (from experience) aren't operative in your case. Make this repair progress a partnership between you and the mechanic for (hopefully) the best ultimate result and the best value.

Enjoy!
CBODY67

 

[73 T&C]

If the choke is pulling off earlier in warm weather, and the choke butterfly is set too far open when fully warmed up, it may be the cause of your problem as well. I’d start by checking that and all the above.

I’m having my own problems myself now.

 

[Pclancy]

Might be good to have a conversation with the mechanic, asking questions (mentioned here) and seeing what he might say to them. Best to ask "for you" rather than identify their source, typically. Once that conversation has happened, discussing possible issues, then hopefully progress can happen. There might be something he'd not considered or thought about or things he knows (from experience) aren't operative in your case. Make this repair progress a partnership between you and the mechanic for (hopefully) the best ultimate result and the best value.

Enjoy!
CBODY67

Indeed that's been my experience. Although he knows by now that when I bring technical carb questions they invariably come from someone else, it never goes down well when I quote some unknown dude on the internet. If I just skip that part, he's more than open to considering what I say.

 

[Polaraco]

Thermoquads are known to crack. You may be experiencing a crack that opens when hot somewhere. TQ's were great carbs. I never bothered with them though. Most I ever got were toast. My suggestion is, and I just went through this, Just put a 550 Eddy on there with the linkage adaptor and be done with it. Sounds like you've poured way too much money into this.

The other thing that just occurred to me is the electric throttle advance. They had a solenoid on the throttle to step up the idle when the AC was engaged. Edelbrock has one for their carbs. You may be able to retro that on there. However, I would test the theory first

 

[CBODY67]

The "throttle kickers" were a good idea, to keep the idle speed the same when the a/c compressor was working as when it was not. BUT they don't always work, by observation. Just add about a 1/4 turn to the idle speed screw to compensate, see if it will shut down cleanly when you turn off the ignition switch (tweak as needed), and let that be "it".

Plus, they usually don't always have enough power to raise the rpm by themselves (extend their plunger), when activated, and might need a manual increase in throttle to let them work.

 

[Polaraco]

True. I don't think I ever saw one working! Aspen had them on the 79 and 80 6 Cyl. Never saw them work there either. But I have seen the edelbrock. It's all leverage.

I didn't tell him to raise his idle because I don't know his skills. But that is the best way. Keep it under 850, no load, and you shouldn't do any damage to anything

 

[CBODY67]

850rpm in "N" hot, no load???!!!! Hot base idle should be more like 700rpm or less, if I remember correctly. There were some engines, like 3x2bbl motors, where the idle speed was over 800rpm to meet emissions, but not a '77 Newport 400 TQ.

What does the rpm drop to in "D"? With the a/c on?

CBODY67

 

[Polaraco]

I know that. I'm talking temp with the AC on. Easy there. I've been doing this for 55 years. CBarge and a several others can tell you.

 

[commando1]

Even the factory has two idle rpms spec'd. One with, and one without, the AC running under load.

 

[Pclancy]

Thermoquads are known to crack. You may be experiencing a crack that opens when hot somewhere. TQ's were great carbs. I never bothered with them though. Most I ever got were toast. My suggestion is, and I just went through this, Just put a 550 Eddy on there with the linkage adaptor and be done with it. Sounds like you've poured way too much money into this.

The other thing that just occurred to me is the electric throttle advance. They had a solenoid on the throttle to step up the idle when the AC was engaged. Edelbrock has one for their carbs. You may be able to retro that on there. However, I would test the theory first

You are correct, I've been (via my mechanic) messing with this carb for over a year now. I HAD suggested to him that we simply put on another carb, but he's a cranky old guy and said that even a new or reman other carb would be just substituting one set of problems for another. And then, when it kept on having problems, he started not charging me to adjust it. Real dedication; like I said he sees it as a personal challenge. But suppose I can convince him that it's time to take another approach (or that he agrees the TQ is toast): What is the consensus choice for a replacement carb--the simplest, easiest, least likely to have problems, easiest to install carb. New or reman? Where to buy? What exact model number. The engine is a 400 with the lean burn removed and literally every ignition and spark component replaced. Orange ICN. Thanks guys.

 

[Pclancy]

Sorry, meant to say orange ignition control module--ICM. Think it is a MOPAR unit.

 

[kmccabe56]

I have a pretty good mechanic, and he's rebuilt my original TQ carb on my 77 Newport with a 400 more than once in an effort to address removal of the lean burn system. He knows the car, and my wallet. But he's a serious and honest man.

Here's the issue: My 77 Newport starts in the winter really well--one long pump of the accelerator and she fires right up. And she runs really smooth in all conditions--fast, slow and in between. She likes the colder weather. The choke works just great.

But as the warmer weather approaches, several things seems to happen:

She will idle and run really smooth, except in gear like stopped at red lights;
She idles rougher when the ac is on than when not;
She can be a bear to start when really warm. If I have my technique just right, then no problem. But if not, then I need to crank her.

Literally everything has been replaced--spark plugs, wires, rotor, distributor, resistor, little orange box, etc. The carb has been rebuilt. My mechanic is not a young man, and he has decades of experience. He takes my car as a personal challenge, and he shows care and TLC.

The roughness is not severe, and she does not stall. But among you that know these beasts better than I, and in the hopes that I can say--gently-- "why don't you check X" when I take her in, what would you suggest?

Thanks

I think you have one or more vacuum leaks. Start by plugging hoses. The supply to the HVAC and if you've got a quiet street or deserted shopping mall parking lot, plug the vacuum tree nipple that supplies the power brake booster. Just remember that you're going to have to push like hell to get it to stop. So just try holding it in drive at idle when it's hot to see if the idle improves. Don't pull ALL the hoses off at once. In addition to the HVAC and the brake booster, there's also the cruise control (if you have it) and the auto level control (if you have that too). This should be a simple process of elimination. Have you also replaced your PCV valve lately?