Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: Zach on February 13, 2023, 07:55:55 PM
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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.
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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.
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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.
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.
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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.
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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.
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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Is this too much light shining through? Picture makes it look worse than it is, cannot get them to seat any better. The shaft as a very miniscule amount of play, but doesn't change how they seat.
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That's a lot, loosen the screws & readjust.
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That's a lot, loosen the screws & readjust.
Any specifics tips to align them better? I gave it some effort before I took the picture but got similar results, is sanding advised, or bushings, etc. I’ll try again later tonight.
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How much transfer slot is exposed under the throttle blades
when hard shut? Shouldnt be much.
Check the throttle plate gasket especially toward the front to
make sure it fits right.
Hold the throttle plate against the fuel bowl
without a gasket to make sure nothing is interfering with the fit.
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How much transfer slot is exposed under the throttle blades
when hard shut? Shouldnt be much.
Check the throttle plate gasket especially toward the front to
make sure it fits right.
Hold the throttle plate against the fuel bowl
without a gasket to make sure nothing is interfering with the fit.
I’ll check transfer slot later when I take it off the car. I sprayed quite a lot of carb cleaner around everything and got no response. But I can double check fitment too.
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You need to get the throttle plates fully closed, indexed and seated.
The problem is most likely the linkage. Not uncommon to have to do some "tweaking" to get them fully closed and open to 90 degrees at WOT.
I've rebuilt tens of thousands of them and almost all of them require some attention in that area to work like they are supposed to.
I offer a service to rebuild baseplates if you can't figure it out and want bronze bushings installed......
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You need to get the throttle plates fully closed, indexed and seated.
The problem is most likely the linkage. Not uncommon to have to do some "tweaking" to get them fully closed and open to 90 degrees at WOT.
I've rebuilt tens of thousands of them and almost all of them require some attention in that area to work like they are supposed to.
I offer a service to rebuild baseplates if you can't figure it out and want bronze bushings installed......
Thanks for the response. I am pretty limited on tools and knowledge, but I'll take a stab at it. I will most likely need to ship it out to you. We will see.
Thanks again.
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Adjust pri to sec link as follows:
Open pri throttle some and when #1 meets the sec shaft arm (do NOT bend top of arm) #2 should be in center of slot. Bend link on throttle blade side for proper position.
Open throttles to WOT adjust #3 for close to 90° opening of the sec throttle blades.
Fully close throttles adjusting closing tab #4.
Done!
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Update, got the throttle plate back from Cliff and it feels more solid.
Issue still remains, no change in performance from before and after the rebuild. I have a vacuum leak somewhere and I cannot find it. Putting my hand over the primaries and completely suffocating it causes no change in idle speed or anything noticeable. Making around 16 vacuum at 1000 rpm. Curb idle screw completely out, idle mixture screws set to the best I can get at the moment 3ish turns out. Only places I can think of are bad gaskets on the carb, they are brand new. Brake booster, brand new, but I can try disconnecting it from the vacuum and plugging it. Intake manifold gasket has been on there for a while, I can try spraying more carb cleaner later. Spraying carb cleaner anywhere else doesn't reveal anything, I've replace most things on it, vacuum lines, PCV valve, etc. not sure what else to do.
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Anyways I did some more playing around, high idle cam was hitting a small bit but I fixed that, but it didn't solve any issues. I think I'm past vacuum leaks, hand over carb does in fact kill it. Now I am wondering if it is a timing issue, still set to 18 BTDC at 1k rpm using the factory vacuum line routes, manifold vacuum from carb through the emissions check valve. Or something is wrong with my idle circuit and it is dumping too much fuel or air. It was a stock rebuild and clean, I didn't add any hop ups, like jets, or idle bypass, etc. Just to restate it, curb idle screw can be completely unscrewed (transfer slot can only be seen a little bit), and the car will still run at 1k rpm 18 vacuum pretty steady.
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Is the Vacuum Advance adding timing at idle with manifold vacuum?......
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Is the Vacuum Advance adding timing at idle with manifold vacuum?......
I quickly checked this one so I could’ve done it wrong but unhooking and plugging the vacuum lines to the vacuum advance causes no change in idle. I could try a different vacuum source and skip the emissions check valve. Think I read somewhere that the check valve emissions equipment on a 77 trans am is timed and acts as ported.
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Still haven’t found a solution, there is an oddity that it will run at 800 rpm but I can’t reproduce this and it eventually builds rpm to stick at 1k, I’m wondering if it’s a distributer issue or the vacuum advance is sticking.
Currently fighting below freezing temps, might postpone this until summer.
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Check the timing when the problem occurs and make sure it's back to the "base" setting. Sticking distributor weighst or mechanical advance will cause exactly what you are seeing.....
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Check the timing when the problem occurs and make sure it's back to the "base" setting. Sticking distributor weighst or mechanical advance will cause exactly what you are seeing.....
I believe I solved the issue, early on it was vacuum leaks, timing, and improper tuning. I got the recommendation for a second tach, used a fancy timing light. Seems to be running at factory spec, my dash tach seems to be off by about 300. Makes sense why I’d see <800 sometimes. Carb is idling great, never heard the car at low rpm’s or knew what to compare it too, not very experienced. Appreciate the base plate rebuild, your book I bought helped a lot when rebuilding the main body, huge help Cliff.
Appreciate the help, seems to be solved!
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Good news!....