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I have a 69 Corvette with a 427 390 hp Holly carburetor. I backed off the idle screw as far as possible and it won’t lower the idle low enough is there something internally in the carb or is it a way I can lower the idle.
Remove air filter. look down the carb. are both throttle plates closing completely? A bent or sticky shaft could cause a throttle plate to hang up. Now if both butterfly's are closing completely. Start looking for the vacuum leak.
While we're talking vacuum leaks and throttle plates.....spray carb cleaner on the butterfly shafts, while running....engine note change??? Worn throttle shafts and vacuum leak THERE.
Check vacuum can for distrubutor...is it coming back to normal? Lube vacuum advance plate.
Not hardly going to believe this,
Back in the day our PCV valves were somewhat calibrated for the engine.
Today, Idle air control make things happen.
Those PVC's we buy are great big vac leaks.
Take a pair of needle nose and pinch off the PVC line and watch all your problems go away.
I use a harbor freight paint gun inline regulator to dial down the vac amount and still pull out
case pressure.
Now anybody have carrier brg CAPS for our 82 dana 44? PM me please.
A picture of the carb from each side, without air cleaner, would help us all speak a common language. Pics are very important because what we think you have and what you really do have could be very different and misleading. I believe the original carbs on non-tripower cars were Rochester Quadrajets, so the Holley must be a replacement. Earlier C2 427/390s did come with Holleys.
What is the idle speed rpm we're talking about? (while in drive if auto)
New problem or old problem? If new, has the carb been out and disassembled recently or any other changes to the carb or engine made?
What carb model is installed? (Numbers are stamped on the airhorn)
Disconnect the vacuum advance line at the distributor and plug the hose. What is the rpm like this?
Are you sure the choke linkage on the side is fully "kicked down" and not affecting anything? (Not just a fully open choke plate, but no linkage touching the stepped cam-thingy on the side) - this is why pics are important because you may not have the choke that I'm picturing.
Answering these questions should help identify a focused direction to head in.
In my L-82, I had the same problem and I noticed I would need to rev the motor to get it to idle down. It was sticky centrifugal advance plates in the distributor. Easy fix.
From: At my Bar drinking and wrenching in Lafayette Colorado
Originally Posted by 67:72
I believe the original carbs on non-tripower cars were Rochester Quadrajets, so the Holley must be a replacement. Earlier C2 427/390s did come with Holleys.
Owen is correct: '68 and '69 427 390-hp cars were originally equipped with Q-Jets, so your Holley carb is not "correct." It is an aftermarket installation, so somebody may have done some "funny-tuning" to it...
That said, there are several things that can cause a high idle. Any carb should have enough idle speed adjustment to be able to kill the engine, regardless of timing adjustment - if you have a timing problem, it is not the cause of your high idle and inability to adjust it. Timing, and sticky timing advance, may cause erratic return-to-idle, but it should not prevent you from being able to kill the engine with the idle speed screw. Here are a few things that can cause high idle and inability of the idle speed screw to lower it:
Vacuum leaks. Very common on these cars with vacuum operated headlights and wiper doors. The power brake system can also cause vacuum leaks with high idle.
Incorrectly adjusted or installed throttle cable. Try disconnecting the throttle cable to see if it is preventing the throttle from fully closing. Since your car originally came with a Q-Jet, the cable and the cable brackets are not intended for use with the Holley carb. They must be correctly modified to be able to use the Holley. If this was done incorrectly, the cable can hold the throttle open.
Choke & fast idle system not operating correctly/misadjusted. If the choke is not fully opening, or if the fast idle cam is not dropping all the way down, the fast idle screw and cam will keep the idle at an elevated rpm. It is common when people put an aftermarket Holley on an engine that they do not get the choke system operating correctly, especially on divorced choke systems.
Uncapped vacuum port/fitting. Although this falls under the category of "vacuum leaks," it is not associated with the vacuum line systems on the car. Aftermarket Holley carbs have a couple of vacuum fittings below the primary float bowl in the throttle plate, and these are difficult to see. It is common for one of the ports to be open and uncapped, causing an elevated idle speed.
Throttle blades not centered in the throttle bores, preventing full closure of either primary or secondary throttles. It's not uncommon for "carb builders" to remove the throttle blades and the throttle shafts for a rebuild (although this is usually entirely unnecessary). If meticulous care is not used in the re-assembly and alignment of the throttle plates, the throttle cannot be fully closed, resulting in high idle. To check this, you need to remove the carb, flip it upside-down, fully back off the idle speed screw, and look up the bottom of the carb (with choke open) at a light source to observe that the throttles are fully closed with no light gap around them.
Incorrectly set secondary idle speed. All Holley carbs have a secondary idle speed adjustment, and this is used to correctly set the transition slot exposure at idle so that excessive t-slot exposure does not cause idle and tip-in issues. When perfectly set up, the primary throttle blades should be set up with an idle speed that exposes less than .020" of the transition slot, and the secondary idle should be set to hold the secondary throttle open the exact same amount as the primary side. This is set and observed using the "light gap method" as outlined in detail in my Holley Carb Setup papers - essential for proper Holley operation and performance. E-mail me for my papers on how to set this up if you don't know how to do it. If this secondary idle speed has been set too high, you will not be able to lower the idle speed using the primary idle speed screw.
Incorrect carb base gasket installed. If the engine is equipped with a factory 427 4-barrel manifold, the manifold will have an exhaust groove crossover (I call it the "smiley-face manifold") under the carb's primary side. Use of gaskets not intended for the "smiley-face" will cause a vacuum leak, and may cause exhaust gas ingestion into the engine.
These are just a few of the items that I commonly see cause a high idle and inability to lower the idle.
Lars
Can’t get idle low enough
