Inconsistent idling problem

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davidcribbs

davidcribbs

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I have read quite a few different threads, but didn't hear the same problem I seem to be facing.

I will give the background that I know it for my vehicle. It is a '67 Dart Convertible GT. Originally came with a 273, but now has a 360 in it. I purchased it in May of this year and have been working through the issues as I find them, and as they rear their ugly heads. I have switched out the points ignition for the Proform Electronic Ignition with the Orange ECU. I have rewired the fusible link so it does not run through the bulkhead connector. I have replaced the 750 Holley Vacuum Secondary (3310) with a Summit Racing 600 Vacuum Secondary. I replaced the Carter Fuel pump with the Edelbrock Fuel pump a couple of months ago. The car has a mild cam (do not know any of the spec's), and a dual plane Weiand manifold on it.

I do not really know if this problem with an inconsistent idle has been with me the whole time I have owned the car, due to the fact that it has so many of the parts changed in the ignition and fuel delivery system over the last 6 months, and has been in and out of my garage and the local garage fixing the inconsistency issues.

The problem I am currently facing is that the car does not idle at a consistent RPM in or out of gear. I have adjusted the fast idle on the choke to run at around 900-1000 rpm on initial start-up, and after it sits over night it will start and run at about 600-700 rpm. I also have adjusted the curb idle to operate around 650-700 rpm, and it will operate at that speed sometimes, and other times it will drop down so low that it stalls (around the 400-450 rpm range). I will adjust the carburetor again, and it will again change. Then I will get to the next stop light, and it will idle around 650-700 rpm, with no rhyme or reason why it wont stay consistent. I am not sure whether this is an ignition problem or a fuel problem, but thinking something in the ignition process is not working right. I also have a strong smell of gasoline when I step on the gas, that will stay around for several minutes after jumping on the throttle. My thinking is my coil may be going out, or my spark plug wires are failing.

So much of the information I have gotten for modifications on the vehicle have come from this web site, and they have been great. Any help would be apprciated.
 
A few things come to mind, vacuum leak at intake , advance weights flopping around, fuel pressure too high, junk in tank. 2 carbs , same issue so thinking its not that, but was timing and idle set while in gear? Converter acting ok?

I would do the tune up first since you question it. And buy quality parts.
 
Car seems to be running excellent other than the idling issue. I do not believe the converter is acting up at all. The transmission was rebuilt just before I purchased the car.

As far as the fuel pressure goes, it seems to be running around 7.5 psi at the inline guage. I have purchased a regulator for it that I will install this weekend. I was going to set it at 6 psi, and see if that helps.
 
Could be as simple as the throttle cable binding in its sheathe. Good hunting
 
Haven't even dug into thinking it was something as simple as that Redfish. I will check that out tonight.
 
Carbs with a worn throttle shaft will do that.
(the shaft that the throttle plates are attached to)

They settle in a different position all the time causing a varied amount of air to come in around them, and that changes the idle.
Some carbs can be "rebushed" to fix that.
 
TrailBeast said:
Carbs with a worn throttle shaft will do that.
(the shaft that the throttle plates are attached to)

They settle in a different position all the time causing a varied amount of air to come in around them, and that changes the idle.
Some carbs can be "rebushed" to fix that.
Click to expand...

I chased this issue for a frustrating amount of time with my motor running with a rebuilt Holley. I was convinced the intake must be leaking. The idle would wander and increase 300-400rpm after driving.Tried spraying choke cleaner and starting fluid around the carb and intake with no results.
TXDart was kind enough to put his smoke machine on the crankcase, and when we covered the bowls smoke started shooting out around the throttle shaft. Swapped on a different carb and now it idles and runs great!
Not sure what the history is on the carbs you have run on it, but either way a smoke machine test might be worth paying for.

[ame]https://youtu.be/E68FD_mJ8Ew[/ame]
 
KnuckleDuster said:
I chased this issue for a frustrating amount of time with my motor running with a rebuilt Holley. I was convinced the intake must be leaking. The idle would wander and increase 300-400rpm after driving.Tried spraying choke cleaner and starting fluid around the carb and intake with no results.
TXDart was kind enough to put his smoke machine on the crankcase, and when we covered the bowls smoke started shooting out around the throttle shaft. Swapped on a different carb and now it idles and runs great!
Not sure what the history is on the carbs you have run on it, but either way a smoke machine test might be worth paying for.

https://youtu.be/E68FD_mJ8Ew
Click to expand...

That's why I brought it up, was to possibly keep someone else from going through the same thing you did.
( we never used smoke machines, but more a solid set of diagnostics reinforced with experience) :D
 
TrailBeast said:
That's why I brought it up, was to possibly keep someone else from going through the same thing you did.
( we never used smoke machines, but more a solid set of diagnostics reinforced with experience) :D
Click to expand...


Precisely why I posted.
I asked around the site for months and nobody ever offered up the throttle shaft as a possible leak... or really offered any detailed explanation of what would cause the idle to change. I'd say using smoke to pinpoint a leak IS solid diagnostics...and experience comes from trial and a lot of being wrong. ;-)

Hopefully the OP has an easier time finding the problem.
 
This sounds very familiar. If you don't have a good ground from the engine to frame and ECU to the body, your electronic ignition won't work with a crap.

I had this problem, and a 50 cent grounding strap cured it all.

Rod
 
KnuckleDuster said:
Precisely why I posted.
I asked around the site for months and nobody ever offered up the throttle shaft as a possible leak... or really offered any detailed explanation of what would cause the idle to change. I'd say using smoke to pinpoint a leak IS solid diagnostics...and experience comes from trial and a lot of being wrong. ;-)

Hopefully the OP has an easier time finding the problem.
Click to expand...

Well I don't know where I was when you were going through that, but you also know how often a suggestion falls on deaf ears, or gets lost in a sea of suggestions.
I agree on the smoke machine and I am sure it would have been used if we had one, but it just never came up.

And as far as "being wrong" my wife must think I have a ton of experience in that case. :D
I could tell her that the sky is blue and she would go to Google for a second opinion.

That reminds me of this.
 

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TrailBeast said:
Well I don't know where I was when you were going through that, but you also know how often a suggestion falls on deaf ears, or gets lost in a sea of suggestions.
I agree on the smoke machine and I am sure it would have been used if we had one, but it just never came up
up.
Click to expand...

Man , I wish you had thrown that out there !
I figured vacuum leak, and that was mentioned. The last post I ran was titled "Symptoms of an intake leak?" ,and I got no real input. I never thought of the carb leaking other than an unplugged port.
 
In response to the throttle shafts being worn, I failed to mention the 600 CFM Summit carb is only a couple of weeks old. I just recently replaced it, and it seemed substantially better from the start than the 750 Holley due to the fact that the engine is probably around 200 horse, and that was way too much carb for that engine.

The floats on that carb are clear, and it appears as though the fuel level is just below half on the sight plug, which they mention is the correct height.

I have tried figuring out the fuel smell, and last night when I drove it home from work I kept the car running in the garage, and went back and forth between the engine bay and the cab of the car and realized the smell is in the cab, but it is not strong in the engine bay. I traced all the lines from the tank to the fuel pump, and nothing appears to be leaking. I may have to drop the tank to see if I am missing something.

I did check the ground between the ECU and the battery and it seems to be solid. That was one of the areas I thought of also, since eratic spark would be the symptom.

I unfortunately think this may be a case of several things going wrong at the same time. I just had the same situation when my voltage regulator, ballast resistor and ignition switch all went out at the same time.
 
davidcribbs said:
In response to the throttle shafts being worn, I failed to mention the 600 CFM Summit carb is only a couple of weeks old. I just recently replaced it, and it seemed substantially better from the start than the 750 Holley due to the fact that the engine is probably around 200 horse, and that was way too much carb for that engine.

The floats on that carb are clear, and it appears as though the fuel level is just below half on the sight plug, which they mention is the correct height.

I have tried figuring out the fuel smell, and last night when I drove it home from work I kept the car running in the garage, and went back and forth between the engine bay and the cab of the car and realized the smell is in the cab, but it is not strong in the engine bay. I traced all the lines from the tank to the fuel pump, and nothing appears to be leaking. I may have to drop the tank to see if I am missing something.

I did check the ground between the ECU and the battery and it seems to be solid. That was one of the areas I thought of also, since eratic spark would be the symptom.

I unfortunately think this may be a case of several things going wrong at the same time. I just had the same situation when my voltage regulator, ballast resistor and ignition switch all went out at the same time.
Click to expand...


Have you had a look at the filler tube grommet in the trunk area?
If that is rotten or leaking it could fill the cabin with fumes.
 
Trailbeast; I will look at that grommet again. It appeared to at least be present, but I didn't look at it real close to see what shape it was in. You are meaning the one on the floor of the trunk that the filler tube passes through to the tank, correct?
 
davidcribbs said:
Trailbeast; I will look at that grommet again. It appeared to at least be present, but I didn't look at it real close to see what shape it was in. You are meaning the one on the floor of the trunk that the filler tube passes through to the tank, correct?
Click to expand...

That's the one.
 
Lots of possibilities but over the years my experience with wildly changing idle has been traced to erratic vacuum leaks like a bad PVC valve, rotten/split vacuum hose or maybe a leaking power brake booster diaphragm.

Just my 2c
 
As for the fast idle change, do you push the pedal to the floor once before starting cold? You have to do this to kick on the choke and fast idle when starting. (Some guys who grew up on fuel injected cars don't know to do this.)

Also, does this have an electric choke? You may need to adjust the choke setting to get the fast idle to consistently come on. And you should be able to manually manipulate the choke linkage to the stepped idle cam to get the idle adjustment up on the fast idle cam step(s) at any time, to check the fast idle RPM setting.

As for the variations in idle speed, take the carb off and check to see where the primary throttle plates are setting at normal idle. They should be open a tiny bit and the edges of the plates should not resting against the throats. If not, then then secondary plates may not be open the right amount, and the primaries are being closed to rest against the throat, which is wrong. This will cause inconsistent idling.
 
The update on the carb was that it seems to be running more consistent in regards to the idiling after some changes to the idle mixture, and some more time spent working with the fast idle cam adjustment. For right now it seems like the temperature of the engine is making the idle speeds fluctuate 100 or so rpms during curb idle operation. I have not experienced that with my previous carberated cars, but it sounds like it is pretty normal.

Trailbeast; you were right on the money with the area to look for the gas leak. The grommet was fine, but the vent tube had a small hole in it. I replaced that, and the smell is completely gone. Thanks for the heads-up on that.
 
You keep mentioning that engine temp is part of the issue... At a glance I didn't see if anyone suggested or asked if you had the thick insulator base gasket. If your heat crossover in the intake manifold is functional it can get hot very quickly. Also if you have stock exhaust manifolds the heat riser plate in the passenger exhaust manifold may be sticking or partially blocked causing extreme intake manifold temps that will boil the fuel very quickly.
 
and this from a post on warm idle issues...



Originally Posted by nm9stheham View Post

3 ohms is too much to run with a ballast, and with the wrong (higher resistance) ballast, that combo is really lowering spark energy. The coil that works with the Mopar ballast should be around 1.5 ohms. You can run the 3 ohm coil with the ballast bypassed. It would be worth a try to see if that helps.

WOW! What a difference that made...totally fixed the spark issue! Thank you. Cold, the coil is testing at 9.6v...a lot different than the 6.2 it had when the ballast was in place. Cold startup is much easier now, too. Once warm, the spoil is staying at the same voltage, so there's no more miss. Win!
 
mguner; I do have an insulator gasket that I run between the carb and manifold. It is the small (maybe 3/8") one that was supplied with the carb. I do have the stock manifolds, but the intake manifold does not seem overly hot. Would I also suffer from vapor lock on a situation like that?
 
davidcribbs said:
mguner; I do have an insulator gasket that I run between the carb and manifold. It is the small (maybe 3/8") one that was supplied with the carb. I do have the stock manifolds, but the intake manifold does not seem overly hot. Would I also suffer from vapor lock on a situation like that?
Click to expand...

That usually occurs from fuel expansion in the lines and carbs vent easily. A hot carb could transfer heat to the line and cause the fuel between the pump and carb to present a vapor lock type situation but that usually comes from a line routing and or under hood heat issue.
 
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