My Quadrajet backfired when working on ignition and I got problem with very high idle rpm later on. It's almost normal now after pushing the fast idle cam down. But I'm missing Idle vacuum on lower port used for distributor. Removed, cleaned, new rubber port plugs, installed with new base gasket. 10* initial + idle screw fully out = 900-1000rpm as lowest = 0 idle vacuum. Vacuum gauge reacts on heavy accelerations. Had to use RPM gauge to try set the air idle screws. New vacuum plugs on all ports except connected power brakes and pcv valve. Removed power brakes valve and tested it, seems to be working. Haven’t checked the PCV valve on intake manifold. Guess I should plug pcv and brakes also just to see but I’m curious if there is something more that could be wrong?
If you have zero vacuum at the port and gauge reacts on acceleration, that is ported vacuum. You should have full time vacuum on the upper port on the front passenger side of your Q-Jet
What is your Quadrajet code? Like Jason said, the port you are checking must be ported vacuum. Ports on the base plate are manifold, check them for your readings.
If your idle speed screw is fully out and your at 900 rpm the Carb is going to respond with a bad stumble when you go to take off from a stop! It sounds to me that yes you are trying to find Vacuum at a ported vacuum take off port, but I also think that the back fire you had blew out the Carb or a Intake Gasket making for a vacuum leak and the need to have the idle speed screw backed out all the way.
Looks great, but how are we going to help you from these photo's? Does opening up the throttle make the vacuum reading go up?
Maybe you see something I’m missing. This is how I connected the vacuum gauge and not getting any reading until I do some drastic accelerations. Just watched youtube and that pipe could be ported or manifold. But even if I find better place for the vacuum gauge, I still have problem with “high” idle. When you “blow” a carburetor, are we talking just external things like vacuum caps or something inside?
Your vacuum port there in the base of the Carb is working as intended with no vacuum reading until the throttle is opened. If your throttle blades are fully closed due to your idle speed screw being fully backed which also means you can not get the motor to stall out as it should with a setting like that then you have a vacuum leak . Get some spray Carb cleaner and with the air cleaner on the motor spray around the Carb base , the Intake runners and vacuum hoses, and increasein rpm means that spot is sucking in air. Also unhook the pvc valve and block that hose and see if the changes the rpm.
Or slowly cover the air horn with your hand and see if the idle goes up. In other words, slowly choke off the air supply to the carb. If theres a vacuum leak as you suspect, the rpm will go up. So will the vacuum reading Whats the vacuum reading at isle?
As others have said, you need a manifold vacuum port. IT is so EASY to tell the difference between that and ported vacuum. You don't even need a vacuum gauge. With engine running, uncovering a manifold vacuum source will produce an unmistakable HISS because you just created a big vacuum leak. Ported vacuum is non existent at closed throttle and increases rapidly as you open the throttle. Ported vacuum is sourced from ABOVE the throttle blades. Manifold vacuum is sourced from below the throttle blades, so it is always there as long as the engine is running (Except at WOT). As Mark said, the back port is usually manifold as it supplies the climate control vacuum storage container.
Not trying to hijack your thread, but I'm kind of having the same issue as the OP. Mark, is the carb(17058241) you built for me the same configuration? I plugged my vacuum gauge in the same port as OP, and am getting same symptoms. Low idle vacuum, but goes up with light throttle. Also, mixture screws don't seem to change idle vacuum. I'm assuming it's because gauge is plugged into wrong port?
Jim, Read my post right before yours. You need a manifold vacuum port. With the engine running at idle, just pull off the vacuum caps one at a time until you hear a hiss. When you do, that's the port you need to plug the vacuum gauge into.
Switched port to the one on right side of fuelline, all fine. Will try the rear port. (also spray tested, no obvious leaks maybe really small on power brake valve, can't tell) Hot engine. Neutral. ~900RPM. 14* initial. Disconnected vacuum advance. Verified zero/true TDC on new damper. Started with one side and turn in the air idle screw to it stumbled, and out to highest vacuum. Same on other side. And kept trying fine adjust once more. Question 1: Am I supposed to stop when it stops rising on the gauge, or stop when I have turned the screw out so far the vacuum drops? Vacuum shows; ~16.83 – 17.42 inHg. Question 2: When you have some small vacuum movements like this (between ~16.83 17.42), and more on RPM, is this all mechanical (example camshaft) or a sign that I should try fine adjust the idle screws even more? Still can’t lower the Idle RPM as much as the book say: 500rpm in neutral. With idle screw all out I get 800RPM or maybe slightly less. Question 3: Could it be something with the choke? Fully open. fast idle cam down.
On question 1. You want to stop at highest reading point. Unsteady vacuum readings indicate an issue somewhere 3. Choke needs to be completely disengaged. And fast idle cam all the way down. Verify that throttle is closing all the way. Sometimes the throttle cable can hold open a little. When idle speed won't go lower, you are getting air from somewhere. Vacuum leak, throttle blades are common. Check all linkages to make sure they allow full closure of primary and secondary. Many times the linkages prevent full closure and need to be bent to adjust.
Small variations in vacuum/RPM are often caused by the ignition timing wandering around. RPMs affect ignition timing, and ignition timing affects RPMs... so you can get into unstable circular feedback loops where the engine can't quite settle on a rock solid idle speed. What book are you referencing? 500 RPM is quite slow for any condition. You generally get a 150-200 RPM drop when shifting from park/neutral into drive... if you're at 500 in neutral and drop to 300 in drive... the engine is just going to stall. The spec is about 850 in park, 650 in drive with your foot on the brake. Or are we talking about a manual transmission here?
I started to lower the fast idle to something much nicer. With cold engine and choke activated, I could make the rpm really low. But I set it to 1000. To be honest, I have to finger down the last step on the fast idle cam. But it could be that I’m in a hurry, that 10.000rpm fast idle stress me. Will test to wait longer time. I plugged the power brake on intake manifold. And PCV on carburetor. Sprayed, thought it reacted on backside of carb. Checked the nuts holding down the carburetor, lightly tighten them with two fingers on my ¼” ratchet. I think this lowered my idle. Tested to reconnect PCV, and disconnect. I think it runs slightly better with pcv disconnected. Not sure. I have 1968 430 with 850cfm quadrajet, new MSD distributor and TH400. My 1967 workshop manual and buick.net tune up page say 500rpm idle on 1968 430. However, the carburetor comes from 75-76 when they used 600rpm. Both in neutral. Perhaps not possible to go lower with newer larger carb on older engine?
Stock idle specs for a 68 430 are irrelevant. You have a different carburetor and distributor. 650 in gear is more like it IF the cam is stock. A 500 RPM in gear idle is completely unrealistic IMO.
I just went through this with another board member from Chicago's '70 GSX ported cast iron intake and he ended up finding a vacuum leak caused by a missing bolt/plug on the top-front-right side of the 2-4 intake runners .... closer to the outside where the wiring harness runs. I know this should be plugged when the runner there is opened up enough to break into the threaded portion there, but I missed checking for it. The car came to me with open headers so not much to hear except the thunder in my shop. The heater hose was laying directly on top of the open hole, .....not an excuse, I know better and should have caught it. I'm not positive but I've seen that location used for a heater hose bracket. I know the owner had the plenum divider removed and port matched. I just checked another customers ported TA intake and sure enough it's also broken through (pictures). Just something worth everyone checking, especially those using tall ports. Larry
