Q jet idle issue
Thread Starter
Instructor
Joined: Aug 2008
Posts: 110
Likes: 0
From: Le Roy NY
Q jet idle issue
I've got the original Q jet (17080204) on my 1980 which now has a GM 290 HP crate engine (1995) coupled to a TKO 500 tranny. An RV cam with true dual exhaust is also on the car. Runs great, good fuel economy. Recently I began having an idle problem after coming to a stop. Car idles fine cold (electronic choke) and warmed-up, but the idle drops low when coming to a stop (nearly stalling). It recovers to normal (700 RPM) after a few seconds. Any thoughts? Also, is this the best model Q jet to use based on the mods that have been made? It seems to work fine, and I use manifold vacuum for the distributor advance. Thanks.
Race Director
Joined: May 2006
Posts: 18,689
Likes: 2,571
what are you using distributor wise? Assuming it's an HEI or other electronic unit, check your timing first to make sure it hasnt changed. Make sure the carb. bolts havent loosened. Then I would just readjust the mixture screws to see if that resolves the issue.
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Keep the shiny side up!
Scott
Drifting
Joined: Feb 2005
Posts: 1,879
Likes: 38
From: Los Gatos CA
Idle was fine; but when braking the idle would sometimes drop below 500 RPM and sometimes die. I was never able to bleed brakes properly until I bought a Motive Bleeder. During 1 test drive after successful bleeding the brake pedal was firm and the idle did not drop during braking. It must have had someting to do with vacuum, at least that's all I can think of. My car's braking/idle drop is exactly what the OP mentioned. I hope this helps...
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Idle was fine; but when braking the idle would sometimes drop below 500 RPM and sometimes die. I was never able to bleed brakes properly until I bought a Motive Bleeder. During 1 test drive after successful bleeding the brake pedal was firm and the idle did not drop during braking. It must have had someting to do with vacuum, at least that's all I can think of. My car's braking/idle drop is exactly what the OP mentioned. I hope this helps...
Keep the shiny side up!
Scott
Thread Starter
Instructor
Joined: Aug 2008
Posts: 110
Likes: 0
From: Le Roy NY
Never would have made that connection, but I did some brake work this winter and bled them. The brakes are fine, though the idle drop does occur during braking. I'll investigate further along with checking the carb bolts.
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
I can't see how air in the brake lines could have any effect on idle quality, directly. However, if there is a little air in the lines, and you are pressing down harder on the brake pedal, it would maybe make a small hole in the brake booster diaphragm open up enough to cause a bigger vacuum leak? I know, I know, it's reaching, but hey, anybody got a better guess?
Keep the shiny side up!
Scott
Keep the shiny side up!
Scott
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
Thread Starter
Instructor
Joined: Aug 2008
Posts: 110
Likes: 0
From: Le Roy NY
Well, I did a little experimenting and found that the idle drop does occur during braking, not just throttle closure. I ran the vehicle up to cruising speed and pushed in the clutch; the idle went to normal (750RPM). When the brakes were applied it dropped to 400 and almost stalled.
I then tightened up the carb mounting bolts and repeated the test. The idle only dropped to 550 during braking. Seems like the vacuum-assist power brakes do affect the idle. I probably have a leak somewhere also. The Q jet is in need of a rebuild, and I've had the parts laying around for over a year. Time to get to work.
I then tightened up the carb mounting bolts and repeated the test. The idle only dropped to 550 during braking. Seems like the vacuum-assist power brakes do affect the idle. I probably have a leak somewhere also. The Q jet is in need of a rebuild, and I've had the parts laying around for over a year. Time to get to work.
Drifting
Joined: Feb 2005
Posts: 1,879
Likes: 38
From: Los Gatos CA
What worked for me:
1 - Motive Bleeder - make sure you have hard pedal!
2 - Adjust idle mixture screws for highest vacuum @ idle
3 - Vacuum advance hooked up to full manifold vacuum
Good Luck!
1 - Motive Bleeder - make sure you have hard pedal!
2 - Adjust idle mixture screws for highest vacuum @ idle
3 - Vacuum advance hooked up to full manifold vacuum
Good Luck!
Melting Slicks
Joined: Jun 2009
Posts: 2,343
Likes: 4
From: Anderson SC
Vacuum assist brakes + long throw = more vacuum stolen.
That said - I'd still expect to find a vacuum leak or issue elsewhere, and this is only making worse.
-W
