Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
Idling Too High
Zach:
Howdy, recently rebuilt the Quadrajet on top of my 1977 w72 trans am with Cliff's SR kit and I am really happy with the results except for some minor issues. Prior to rebuilding the major issue I had was hesitation off idle and WOT, after lots of tuning and ripping the carb apart I noticed the APT screw was completely bottomed out for whatever reason, which was causing it to run really lean and cause a huge vacuum leak. Now the car is running great has never sounded so good.
The issue I am currently having now is it is idling too high, idle screw completely lose not even in contact with linkage and I am running around 900-1000 and I know I should be able to drop that by quite a bit. Choke and high idle engage properly when engine is cold and will come down with a throttle blip. I replaced most the vacuum tubes, sprayed carb cleaner everywhere, and don't see or hear any vacuum leaks, I ordered a new PCV valve grommet cause mine is in rough shape but I do not believe it is leaking. The mixture screws have some effect on RPMs/Vacuum but not a lot, not sure how noticeable they are supposed to be. Vacuum gauge reads 16ish at 900-1000RPM but it is very volatile, lots of bouncing small bouncing. I believe I am running ported vacuum to the advance if that is required information, someone asked me that before. Vacuum lines are completely stock for a 77 400 manual except for a exhaust valve delete but that is plugged.
Carb # - 17057263
Everything is stock, engine, jets, rods, etc, etc, used Cliffs SR kit and replaced with whatever came in that. I am only 19 with very little experience so I am learning as I go, please ask if I missed anything.
bry593:
Likely vacuum leak. Extra air coming in from somewhere and if you adjust your mix screws to match, high idle.
Check your carb to mani gasket. If you use the wrong one, you can get a leak front and center.
If you can't find a leak, it might be that your Idle Air Bypass holes in the throttle plate are too large. In which case you plug with a brass or aluminum 1/4-20 NC set screw with a hole drilled to the appropriate size. .047" is a common size for a mild engine. A real smog cam might not need any IAB at all.
Zach:
--- Quote from: bry593 on February 14, 2023, 11:01:48 AM ---Likely vacuum leak. Extra air coming in from somewhere and if you adjust your mix screws to match, high idle.
Check your carb to mani gasket. If you use the wrong one, you can get a leak front and center.
If you can't find a leak, it might be that your Idle Air Bypass holes in the throttle plate are too large. In which case you plug with a brass or aluminum 1/4-20 NC set screw with a hole drilled to the appropriate size. .047" is a common size for a mild engine. A real smog cam might not need any IAB at all.
--- End quote ---
It is a complete stock smog era 400 with factory 6x heads and I didn't beef up the carb at all. Complete factory, and you are saying I need to change the Idle Bypass Air, shouldn't it be fine on stock conditions? Also vacuum leak wise couldn't find anything with carb cleaner and putting my hand over the carb doesn't surge rpm. Something I am going to look into tomorrow is my timing, it is most definitely wrong since I used it to band aid the poorly running Quadrajet at the time, as in tuning it by ear until it was running smooth. Didn't think about it much until someone else mentioned to take a peak at it.
Kenth:
Disconnect the throttle wire/linkage and idle stop solenoid and check for 100% closed throttle valves.
If still too high idle, you have air passing the throttle valves from some defect vacuum device/hose, flange gasket or intake gasket.
Zach:
--- Quote from: Kenth on February 15, 2023, 12:36:19 AM ---Disconnect the throttle wire/linkage and idle stop solenoid and check for 100% closed throttle valves.
If still too high idle, you have air passing the throttle valves from some defect vacuum device/hose, flange gasket or intake gasket.
--- End quote ---
Not equipped with an idle stop solenoid, just linkage. When I had the carb off it seemed to me they were closed but I can rip it off and check again. I Set timing to 18 BTDC, which is factory for a 400 manual in 77, just to be sure I'm square there. My mind is back on a vacuum leak somewhere, hand over air horn doesn't surge rpm but it doesn't change idle at all, so air is getting somewhere. Carb cleaner around base of carb did nothing.
Question, how is the carb supposed to be mounted to a 1977 trans am. Right now I have a metal plate on the manifold with the cylinders around the primaries going into the manifold, the thick gasket on top of that and then the carb. This is the setup from when I took the carb off originally but I am now aware someone messed with the carb before us. The thick gasket provided in Cliffs SR kit will not mount on the intake in any other configuration, as in intake, gasket, then plate. Also noticed my manifold vacuum is not steady and the dial twitches a decent amount.
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