1974 Quadrajet Idle Problem
Thread Starter
Le Mans Master
Joined: Apr 2013
Posts: 5,003
Likes: 1,056
From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
2023 Restomod of the Year Finalist
2023 C1 of the Year Finalist - Unmodified
1974 Quadrajet Idle Problem
I decided to get some opinions on this, not a big deal but it just bugs me. My Q-jet runs okay, finally, but it just doesn't come to a complete idle. Coming to an idle after warmup, it's about 850 rpm. Not a big deal, but if I press the throttle by hand under hood it goes to 650 rpm. It moves very little, but it does move. I can do the same by flicking the accelerator pedal. I don't want to put any heavier spring on it than I have now and I shouldn't have to.
When the engine is not running, it doesn't do that. The throttle shaft & butterflies are not binding and move freely when the engine isn't running. It's as though vacuum is holding it open but I don't see how. Anybody got any ideas?
When the engine is not running, it doesn't do that. The throttle shaft & butterflies are not binding and move freely when the engine isn't running. It's as though vacuum is holding it open but I don't see how. Anybody got any ideas?
Tech Contributor
Joined: Aug 1999
Posts: 14,384
Likes: 6,411
From: At my Bar drinking and wrenching in Lafayette Colorado
Engine vacuum can cause the throttle blades to get pulled into the throttle bores a little "shifted," not allowing them to seat correctly. You can do a couple of things:
- Loosen up the throttle blades and re-align them on the throttle bores. This is best done with the carb removed and the throttle plate removed from the carb. The staked screws can be loosened about 1/4 turn without breaking the screws. This will allow the plates to be re-centered in the throttle bores for more consistent return-to-idle.
- Rather than having the throttle return spring pulling backwards from the bottom of the throttle lever, re-position the spring to a point above the throttle cable attach point and make the spring pull forward. This equalizes the forces on the throttle shaft and will allow the throttle to close consistently every time.
- If there is a lot of slop in the throttle shaft, this will cause the problem you describe. If the slop is excessive, you'll need throttle shaft bushings to repair the issue.
Last edited by lars; Nov 25, 2016 at 02:37 PM.
Thread Starter
Le Mans Master
Joined: Apr 2013
Posts: 5,003
Likes: 1,056
From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
2023 Restomod of the Year Finalist
2023 C1 of the Year Finalist - Unmodified
I admit I have trouble with the choke. I've tried to follow Lars & others recommendations, but sometimes the engine starts & runs at 1200 RPM and sometimes not at all.
Can you be more specific as to what needs to be adjusted? I have adjusted fast idle to 1200 RPM, and adjusted the divorced choke rod to spec. Checked the choke pulloff. I don't know what else to do or check.
Can you be more specific as to what needs to be adjusted? I have adjusted fast idle to 1200 RPM, and adjusted the divorced choke rod to spec. Checked the choke pulloff. I don't know what else to do or check.
Thread Starter
Le Mans Master
Joined: Apr 2013
Posts: 5,003
Likes: 1,056
From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
2023 Restomod of the Year Finalist
2023 C1 of the Year Finalist - Unmodified
Engine vacuum can cause the throttle blades to get pulled into the throttle bores a little "shifted," not allowing them to seat correctly. You can do a couple of things:
- Loosen up the throttle blades and re-align them on the throttle bores. This is best done with the carb removed and the throttle plate removed from the carb. The staked screws can be loosened about 1/4 turn without breaking the screws. This will allow the plates to be re-centered in the throttle bores for more consistent return-to-idle.
- Rather than having the throttle return spring pulling backwards from the bottom of the throttle lever, re-position the spring to a point above the throttle cable attach point and make the spring pull forward. This equalizes the forces on the throttle shaft and will allow the throttle to close consistently every time.
- If there is a lot of slop in the throttle shaft, this will cause the problem you describe. If the slop is excessive, you'll need throttle shaft bushings to repair the issue.
1. With the carb off, I looked at the throttle plates (primary & secondary) and there is no gap and they open and close freely. I don't tighten the carb to manifold excessively so I don't think it binding. I'm going to check the gasket though.
2. The double spring is as you say. I'll reinstall the spring on the forward side and see if that helps.
I don't, however, understand why the throttle shaft slop would cause this condition. The accelerator pedal will simply follow the throttle lever, right? Please explain?
Thanks
Racer
Joined: Apr 2006
Posts: 477
Likes: 43
From: Millbrae California
In addition to the comments from Lars, if someone has rebushed the throttle shaft (with bushings that are longer than the original design) or the throttle shaft contact area in the base plate ("bushing area") has worn to the point that the contact area is has increased, the throttle shaft will often stick during high vacuum conditions such as idle yet move freely when the engine is off. We have seen far too many throttle sticking problems with factory rebuilt carbs that have 1" long bushings on both the throttle and choke side of the base plate
FYI, We use a 1/2" long bushing on the throttle side and a 3/8" long bushing on the choke side with 5/16" diameter throttle shafts and shorter bushings on the 3/8" diameter secondary throttle shafts.
Henry @ oles carb
FYI, We use a 1/2" long bushing on the throttle side and a 3/8" long bushing on the choke side with 5/16" diameter throttle shafts and shorter bushings on the 3/8" diameter secondary throttle shafts.
Henry @ oles carb
Thread Starter
Le Mans Master
Joined: Apr 2013
Posts: 5,003
Likes: 1,056
From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
2023 Restomod of the Year Finalist
2023 C1 of the Year Finalist - Unmodified
In addition to the comments from Lars, if someone has rebushed the throttle shaft (with bushings that are longer than the original design) or the throttle shaft contact area in the base plate ("bushing area") has worn to the point that the contact area is has increased, the throttle shaft will often stick during high vacuum conditions such as idle yet move freely when the engine is off. We have seen far too many throttle sticking problems with factory rebuilt carbs that have 1" long bushings on both the throttle and choke side of the base plate
FYI, We use a 1/2" long bushing on the throttle side and a 3/8" long bushing on the choke side with 5/16" diameter throttle shafts and shorter bushings on the 3/8" diameter secondary throttle shafts.
Henry @ oles carb
FYI, We use a 1/2" long bushing on the throttle side and a 3/8" long bushing on the choke side with 5/16" diameter throttle shafts and shorter bushings on the 3/8" diameter secondary throttle shafts.
Henry @ oles carb
Thread Starter
Le Mans Master
Joined: Apr 2013
Posts: 5,003
Likes: 1,056
From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
2023 Restomod of the Year Finalist
2023 C1 of the Year Finalist - Unmodified
I use a thick Base gasket so I got to wondering if the butterflies are hanging up on it. I removed this thick gasket and installed a thin one. Test run says throttle returned every time. Does it matter which gasket I use? Why the thick gasket? Heat insulator?
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