Low RPM idle when in gear and weak uneven idle

[Evan Frucht]

My idle drops ATLEAST 200 rpms or so when I shift the car from PARK or N, into DRIVE. And starts to become a very sluggish uneven idle. Starts to run crappy basically and often stalls.

It will go from 800 rpm in N, to 500 rpm in Drive

If I adjust idle down it wont stay running and will always die when engaged
If I adjust idle up to let's say 1000, it will still go down to about 700 in drive but still occasionally misses.

Car running in NEUTRAL I cant detect anything wrong and it doesnt seem to miss a beat (even if I turn down the idle speed screw) But when I put into gear something is clearly wrong.

My original service manual also states the curb idle should be at 500 rpm (curb idle means in NUETRAL)... but everyone tells me it should be at about 700? What's up with that? Anyways it would always die at stop if I set it too 500

1964 plymouth fury. All stock basically. The previous owner may have done a refund on the cam. It also has a factory 4 barrel intake that was added with a newer Carter AFB... I think 650.

I've rebuilt the carb when I got the car about a year ago and am confident that is fine but will look into it as last resort.

Could it be a sticky lifter ?

- I've have checked carefully for vaccum leaks and have found none. I checked visually, made sure everything was tight, as well as sprayed carb cleaner all around carburetor base and intake manifold while running.
- Brake booster was rebuilt by Dewey's so that rules a vaccum leak coming from that.
- The symptom still occurs with dist. vac adv. Plugged
- Intake manifold was re-painted and installed recently. It is very well sealed and It made no change from before.


I started another thread that sort of related to this where I have been chasing electrical issues in my car and more specifically, the charging system. Basically I thought this was related to my alternator BUT now I do NOT think so. So I'm trying to isolate and tackle this symptom now that I'm confident in the alternator, regulator and the charging circuit

 

[Lefty71]

If you had an air leak, I doubt it would sound good in neutral as well. It should be hunting all the time if you had an air leak. Are you sure it's not just too lean on the idle circuit, like something got a little dirty?? And maybe the timing is not quite right perhaps??

 

[DLevel]

Hate to be the first to say it, but there are numerous variables that will cause the issue that you're experiencing. You didn't state your base timing for one. Have seen too low of a setting cause many problems if the engine runs vacuum advance. Once the car is put into gear a slight loss of vacuum takes timing away, rpm drops, could have slight surge, and the carburetor is blamed for being too rich.
If the converter is not too tight for the cam then the air, fuel settings on the carb could be a problem also. It sure helps to be able to make idle bleed changes if you find you're at the limit of the carbs adjustment screws. If the converter is just too tight for the cam, you can find yourself chasing that carb and ignition and still not be happy with the transition rpm.
Hope some of this helps, but I'm sure many others have suggestions as well. Good luck.

 

[Evan Frucht]

Hate to be the first to say it, but there are numerous variables that will cause the issue that you're experiencing. You didn't state your base timing for one. Have seen too low of a setting cause many problems if the engine runs vacuum advance. Once the car is put into gear a slight loss of vacuum takes timing away, rpm drops, could have slight surge, and the carburetor is blamed for being too rich.
If the converter is not too tight for the cam then the air, fuel settings on the carb could be a problem also. It sure helps to be able to make idle bleed changes if you find you're at the limit of the carbs adjustment screws. If the converter is just too tight for the cam, you can find yourself chasing that carb and ignition and still not be happy with the transition rpm.
Hope some of this helps, but I'm sure many others have suggestions as well. Good luck.

My base timing is just shy of 15 degrees TDC. When I had the trans rebuilt they rebuilt my original torque converter so it should be the stock one for my car (which originally was 318 but 2 barrel)
But my car has
- 4 barrel now
- dual exhaust
- when I was looking at the car to buy it PO said that the shop who rebuilt the engine for him "did a mild regrind on the cam" so that's all I know about the cam. So I'm pretty sure that's it's been ground down but I have no idea to what extent.

Been playing around with tuning more and it seems that if I bring up my RPMS to 900 in NEUTRAL then it drops to about 675 rpm in DRIVE and seems to sound ok now?

I'm going to keep my eye on it.

Someone told me on here that idle should only drop about 50 rpm in gear, is that true in all cases? Mine drops about 200

 

[66 Sat]

Mine drops more than 50 rpms in Drive I'm sure. It's hard to tell as the tachometer is all over the place at these low revs as I have a fairly hot cam in it and the idle is a bit lumpy. I'd guess at 100-200 rev drop. If you're at a happy place now just leave it. The factory manual specs for idle revs would be for standard cars, not modified.

 

[Evan Frucht]

Ya maybe it's just a bumpy idle that I'm feeling from the cam regrind. Good to hear. It runs well on the street with good power. Turning the idle up seemed to make it smoother and it hasnt stalled yet. The idle is just a bit more uneven in gear especially anywhere under 700 rpms or so. Normal I guess?

 

[miller]

I don't recall if anything has been said about the distributor.
So, what distributor, and is the mechanical advance working right?

 

[68Moparmaniac]

Deviating from the stock cam profile the factory base settings become more of a suggestion. You'll need to play around with timing, carb settings, advancement curves and the such. Most of the time if you haven't deviated too far from factory specs the changes in adjustments are minimal from base settings and of course the further away you go the more these settings will deviate from factory settings. This is the part of the hobby that takes time and patience to tune your set up... good luck

 

[Kowal]

Deviating from the stock cam profile the factory base settings become more of a suggestion. You'll need to play around with timing, carb settings, advancement curves and the such. Most of the time if you haven't deviated too far from factory specs the changes in adjustments are minimal from base settings and of course the further away you go the more these settings will deviate from factory settings. This is the part of the hobby that takes time and patience to tune your set up... good luck

I agree with the above. That big of a drop, based on what you are saying on timing, is probably carb settings (too lean). Though I would bump the idle timing a bit as well to 16-18. This all assumes you still don’t have a hidden vacuum leak somewhere. But each car is different. All can do is play around. Just be disciplined, don’t do timing and carb at the same time, do one and drive, then try the next thing. Keep a log of your changes.

 

[Mick56]

Vacuum leaks can be hard to find, sometimes spraying ether around doesn't seem to work. The best way is with a smoke machine.

 

[AR67GTX]

I fought this awhile on my 67 GTX until I pulled the transmission a few years ago and rebuilt it and put in a new, 11", slightly higher than stock stall torque converter.
Turned out it had an original 12" torque converter out of some other model that apparently was real tight. Now the rpms probably drop about 100 rpm at most going into gear. Before it pulled my idle down so low that at night my lights would dim and even my FM converter would drop out due to low alternator output.

 

[Rusty 72]

Not sure if you’re running points but I had that same issue when my points went bad. It would slowly die unless I put it in neutral at a stoplight. Drove me nuts, I kept thinking it was a idle circuit problem. Good luck.

 

[66 Sat]

Mine drops more than 50 rpms in Drive I'm sure. It's hard to tell as the tachometer is all over the place at these low revs as I have a fairly hot cam in it and the idle is a bit lumpy. I'd guess at 100-200 rev drop. If you're at a happy place now just leave it. The factory manual specs for idle revs would be for standard cars, not modified.

I've had another look and I was talking crap before - the revs don't drop at all. Once the car is warm the tachometer settles down and it idles about 700 rpm whether in drive or neutral. There's a bit more rock and roll when in drive, but no change in the revs. Sorry for the wrong info last time.

 

[Evan Frucht]

Not sure if you’re running points but I had that same issue when my points went bad. It would slowly die unless I put it in neutral at a stoplight. Drove me nuts, I kept thinking it was a idle circuit problem. Good luck.

I am running points but they are less than a year old with maybe 1000 miles on them. I've re checked gap, cleaned the whole distributor assembly with contact cleaner, re set and checked timing to about 16 degrees initial. I've turned the idle up to about 900 and it drops to about 700 in gear, that seems to stop it from dying out. Maybe it's my cam ? Maybe it's the whole cam, carb, TC combo. They may have done a pretty significant regrind on the cam... I honestly have know way of knowing. The car runs fine and has good power while driving it's just at low rpms when in gear... then idle gets a little choppy, rough, bumpy.

The lights still slightly dimming at idle but better than before and ammeter bounces just slightly at idle but once I step on gas the ammeter steadies out completely and works perfectly. Its spikes toward C rights after starting the car then the settles down to the middle of the gauge after a minute.