Idle speed issues and where to start

[quinneyz]

Recently picked up a 75 Duster with a good-running slant. PO had installed an Offenhauser intake, Summit Racing knock-off 600 CFM holley, and a DIY kickdown linkage. When I bought it, carb was full of soot and obviously overfueling. Found that all the stud threads had been pulled out of manifold for carb mounting and it was literally just sitting on the instake- massive vacuum leak causing extreme overfueling, but enough adjustments had been made that car idled and ran just extremely richly. Additionally, carb mounting flange on intake was so warped from PO overtorquing studs, I was unable to machine it flat.

So far, I have replaced the manifold with a new Offenhauser unit and confirmed exhaust manifold is good, no cracks. Installed manifolds with new Remflex gasket and proper hardware, torqued accordingly. Installed a new Holley 0-8007 390 CFM 4 barrel. At this time, I can not get idle below 1500. I have soaked the base of carb, all vacuum fittings, and manifold to head mating surfaces with starting fluid with no avail... I have found ZERO vacuum leaks. Even isolating every vacuum circuit with caps, etc has warranted no change. When first starting the car, it idles where it should and over about 5 seconds idle speed slowly picks up. I know the linkage is all the way closed with new Lokar throttle cable, and even with vacuum advance disconnected car slowly idles up to about 1500 where it is very strong and steady. Is there a chance that the previous owner advanced the timing so much to compensate for the extremely vacuum leak and overfueling that the advance is pulling the idle up? Ignition timing has not been touched by me yet, nor have any of the ignition components been replaced yet, as I'm trying to not introduce too many variables at any one time. Yes, fast idle speed screw is free and idle speed screw is backed out with no more adjustment. Where am I forgetting to check for vacuum leaks, or what do I need to do next? I cannot for the life of me find a leak... all suggestions welcome!

 

[RustyRatRod]

Recently picked up a 75 Duster with a good-running slant. PO had installed an Offenhauser intake, Summit Racing knock-off 600 CFM holley, and a DIY kickdown linkage. When I bought it, carb was full of soot and obviously overfueling. Found that all the stud threads had been pulled out of manifold for carb mounting and it was literally just sitting on the instake- massive vacuum leak causing extreme overfueling, but enough adjustments had been made that car idled and ran just extremely richly. Additionally, carb mounting flange on intake was so warped from PO overtorquing studs, I was unable to machine it flat.

So far, I have replaced the manifold with a new Offenhauser unit and confirmed exhaust manifold is good, no cracks. Installed manifolds with new Remflex gasket and proper hardware, torqued accordingly. Installed a new Holley 0-8007 390 CFM 4 barrel. At this time, I can not get idle below 1500. I have soaked the base of carb, all vacuum fittings, and manifold to head mating surfaces with starting fluid with no avail... I have found ZERO vacuum leaks. Even isolating every vacuum circuit with caps, etc has warranted no change. When first starting the car, it idles where it should and over about 5 seconds idle speed slowly picks up. I know the linkage is all the way closed with new Lokar throttle cable, and even with vacuum advance disconnected car slowly idles up to about 1500 where it is very strong and steady. Is there a chance that the previous owner advanced the timing so much to compensate for the extremely vacuum leak and overfueling that the advance is pulling the idle up? Ignition timing has not been touched by me yet, nor have any of the ignition components been replaced yet, as I'm trying to not introduce too many variables at any one time. Yes, fast idle speed screw is free and idle speed screw is backed out with no more adjustment. Where am I forgetting to check for vacuum leaks, or what do I need to do next? I cannot for the life of me find a leak... all suggestions welcome!

Remove the carburetor. Flip it upside down. Look closely at the secondary throttle blades. I "just bet" they are too far open. There's an adjustment screw (either small slot screwdriver or allen head) on the passenger's side of the base plate. Adjust it so the secondary blades actually stick closed and then adjust them so the JUST don't stick anymore. Next, you'll want to adjust the primary throttle blades so the idle transition slots are perfect squares. Then reinstall the carburetor. When you adjust the primaries, you'll want to make SURE the choke linkage is down on the lowest step and the choke idle screw is not still contacting that lowest step. Sometimes they come adjusted way to high. They used to come all adjusted good enough to slap on and run decent. Now, you have to go over all that stuff, because they don't do it anymore.

 

[quinneyz]

Fingers crossed. As soon as I'm home tonight I'll make the adjustments and hopefully have no more issues.. Will report back.

 

[67Dart273]

While it is off I would take a REALLY REALLY close look at the manifold ports, any carb ports AND THE GASKET setup as well as anything projecting below the carb, that can "****" the carb on the mount and prevent sealing. Remove the gasket/ any spacer and place them/ it first on the carb and examine closely for any leak points, then do the same putting it/ them onto the manifold.

Double check the carb mounting studs/ bolts, to make sure you are not bottoming into the intake (bolts) and or either studs or bolts, not running out of threads on a bolt shoulder

Play around squirting around with a squirt bottle and water to see if idle changes. If not, try a propane torch that is unlit, and get yourself a short scrap of hose, 3-4 ft or convenient, and hold to your ear, probe around. You can often hear either a vacuum or exhaust leak with that

 

[quinneyz]

Well, I feel like a fool. Fast idle screw was in fact hitting the cam and openning the throttle. Car idles great now. Thank you for your help

 

[RustyRatRod]

Well, I feel like a fool. Fast idle screw was in fact hitting the cam and openning the throttle. Car idles great now. Thank you for your help

Happens to the best of us. Glad you found it.

 

[67Dart273]

AND NOW, yet another annoying story from the old days.

My first Mopar, around 1971 ish, was a 69 RR 4 speed, 383. At some point I had installed a "pizza pie" air filter. I cannot find a photo. These were sorta like a filter bottom plate, shaped like a very flat velocity stack, and used a flat oiled foam filter. And a VERY SHORT STUD

One day I was somewhere, in traffic, aimlessly blipped the throttle, and it came up like it was on the fast idle cam. I kicked the pedal a few times and it idled back down. What I DID NOT KNOW is, that the very short stud had vibrated out, gotten caught in one throttle butterfly, holding it open. When I kicked at it, the stud fell right through and about 1.385 milliseconds later, "kachunck kachunk kachunk!!! as the valve and piston ate the stud!!!!

 

[RustyRatRod]

AND NOW, yet another annoying story from the old days.

My first Mopar, around 1971 ish, was a 69 RR 4 speed, 383. At some point I had installed a "pizza pie" air filter. I cannot find a photo. These were sorta like a filter bottom plate, shaped like a very flat velocity stack, and used a flat oiled foam filter. And a VERY SHORT STUD

One day I was somewhere, in traffic, aimlessly blipped the throttle, and it came up like it was on the fast idle cam. I kicked the pedal a few times and it idled back down. What I DID NOT KNOW is, that the very short stud had vibrated out, gotten caught in one throttle butterfly, holding it open. When I kicked at it, the stud fell right through and about 1.385 milliseconds later, "kachunck kachunk kachunk!!! as the valve and piston ate the stud!!!!

Whoops.