Uneven idle
Thread Starter
Advanced
Joined: Sep 2025
Posts: 70
Likes: 7
Uneven idle
1969 C3 427/390 Manual Transmission Quadrajet #7029207
I have an uneven idle as soon as I turn on the engine, similar to what is shown in this video:
https://www.reddit.com/r/C3Corvette/...ow_idle_issue/
I am pretty sure I have a vacuum leak somewhere in the wiper/headlamp system because when more often than not, only one headlamp closes properly. The other stays open. Even wipers are slow to start working when I turn them on.
In order to check whether this alleged vacuum leak in the wiper/headlamp system is impacting my idle, can I I just cap (temporarily) the intake manifold vacuum hose that feeds the wiper/headlamp system?
I have an uneven idle as soon as I turn on the engine, similar to what is shown in this video:
https://www.reddit.com/r/C3Corvette/...ow_idle_issue/
I am pretty sure I have a vacuum leak somewhere in the wiper/headlamp system because when more often than not, only one headlamp closes properly. The other stays open. Even wipers are slow to start working when I turn them on.
In order to check whether this alleged vacuum leak in the wiper/headlamp system is impacting my idle, can I I just cap (temporarily) the intake manifold vacuum hose that feeds the wiper/headlamp system?
Tech Contributor
Joined: Aug 1999
Posts: 14,336
Likes: 6,282
From: At my Bar drinking and wrenching in Lafayette Colorado
Yes. Cap off all vacuum connections on the manifold and on the carb except the PCV and the distributor vacuum advance, including disconnecting and capping the vacuum line to the power brakes. If that corrects the idle problem, start hooking up one vacuum connection at a time until you find the one that causes the problem.
Thread Starter
Advanced
Joined: Sep 2025
Posts: 70
Likes: 7
Thanks! But these relate to the wiper door and headlamp vacuum hose diagram, which, in my photos, all stem from the yellow line. All the other colours (green/red/light blue/pink/white) are not linked with the wiper/headlamp, hence my question what they are related to?
Tech Contributor
Joined: Aug 1999
Posts: 14,336
Likes: 6,282
From: At my Bar drinking and wrenching in Lafayette Colorado
Be aware that you have a hacked, commercially rebuilt carb that likely has a host of problems that can cause a bad idle... e-mail me for a paper outlining just a few dozen of the problems that your carb will have. I can see that your carb has been pieced together from several different year carbs - only your float bowl is a '69 part. It's what's known as a "Frankencarb," and the Frankencarbs can seldom be set up or repaired to run right.
Lars
Lars
Thread Starter
Advanced
Joined: Sep 2025
Posts: 70
Likes: 7
Be aware that you have a hacked, commercially rebuilt carb that likely has a host of problems that can cause a bad idle... e-mail me for a paper outlining just a few dozen of the problems that your carb will have. I can see that your carb has been pieced together from several different year carbs - only your float bowl is a '69 part. It's what's known as a "Frankencarb," and the Frankencarbs can seldom be set up or repaired to run right.
Lars
Lars
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Grand Sport & Grand Sport X Launch Alongside All-New 535hp LS6 V8!
Michael S. Palmer
5 Reasons Bad Drivers Crash & 5 Ways to Avoid a Costly Mistake!
Joe Kucinski
7 Bolt-On Upgrades From Extreme Online Store to Level Up Your C6 Corvette
Pouria Savadkouei
How Likely Are These Five 2027 Corvette Rumors to Be True?
Brett Foote
9 Best Corvettes You Can Buy for Half Price (& 1 You Should NEVER Buy!)
Joe Kucinski
8 Very Best Corvettes of Amelia Island 2026
Joe Kucinski
Top 10 WORST Corvette Engineering Failures of All Time!
Joe Kucinski
10 Records the C8 Corvette Generation Has SMASHED (& 1 Glaring Failure)
Joe Kucinski
7 Wildest Corvette Concepts Ever Made
Brett Foote
Pro
Joined: Jan 2004
Posts: 635
Likes: 55
From: TX
One trick I've used occasionally for finding vacuum leaks which may or may not work for you is this: Take a length of vacuum hose held to one ear. The other end I hover around potential leaking hoses and I've been able to pick up a distinctive 'hiss' - there's the leak.
Tech Contributor
Joined: Aug 1999
Posts: 14,336
Likes: 6,282
From: At my Bar drinking and wrenching in Lafayette Colorado
