Quadrajet issues

General engine tech -- Drag Racing to Circle Track

Moderator: Team

benno318
Pro
Pro
Posts: 243
Joined: Sat Apr 03, 2010 7:55 am
Location: gold coast, AUSTRALIA

Re: Quadrajet issues

Post by benno318 »

we had quadrajets on various gmh products here in australia, im far from an expert but have set up quite a few - many of the later carbs in particular will have trouble getting enough idle fuel (the mixture screws will just about fall out you just cant get them rich enough without idle tube mods)

recently set one up for a small 253ci V8, with a decent size cam it only had 10" idle vacuum, needed to go 44 thou!! idle tube size to get the screws about 2.5 turns from seated. and clip the spring for the primary metering rods.
chevyfreak
Pro
Pro
Posts: 493
Joined: Sat Oct 04, 2014 4:23 am
Location: south africa johannesburg

Re: Quadrajet issues

Post by chevyfreak »

rfoll wrote: Thu Feb 25, 2021 9:16 pm It's still completely insensitive to Idle mixture screws. Closing them off completely has no effect, but the engine idles like it should. I'm starting to wonder about an intake manifold leak, but the exhaust is completely free of smoke.
From that info , has the bottom well plugs been checked if they are leaking?

Chevyfreak.
Bowtie for life
Geoff2
Guru
Guru
Posts: 1980
Joined: Mon Nov 09, 2015 4:36 pm
Location: Australia

Re: Quadrajet issues

Post by Geoff2 »

You may need to enlarge the idle screw discharge ports in the t/bores. They were quite small on later QJs. This could be restricting the idle mixture that the engine is getting. I would enlarge these holes to 0.096".

Nozzle drip. That needs to be fixed or you will never get the idle/off idle running to be ok.

Not many things cause n/drip. F/level too high, blocked idle air bleed & most common: t/blades open too far at idle. Engine may need more bypass air to set the t/blades in the correct idle position: 0.040" or less of t/slot below blades at idle.
Woodruff
New Member
New Member
Posts: 14
Joined: Sat Oct 12, 2019 12:04 am
Location:

Re: Quadrajet issues

Post by Woodruff »

If you have the idle mix screws bottomed and it still runs, then you are most likely idling off the main circuit. (Booster drip) Does the carb have idle bypass holes at this point? And where is your initial timing?
Dave Cunningham/ Woodruff Carburetor Specialties
Carburetorrestorationyoungstownoh.com
rfoll
Guru
Guru
Posts: 3023
Joined: Mon Jan 11, 2010 12:44 pm
Location: St. Helens, OR

Re: Quadrajet issues

Post by rfoll »

I had an occasional drip, maybe one drop every 5 seconds at 1500 rpm, but resetting the float made it go away. The throttle blades are closed all the way, it idles about 900 rpm. Insufficient idle feed could certainly make it too lean with headers and the previous open exhaust. I wonder if the larger primaries on the 800 cfm QJ come into play here. Most slow speed driving uses only the primary circuit, so getting it right will be necessary if I put this engine in my daily driver this summer. I know I can make a Holley or AFB style carb work. I have dozens in ready to run condition, but I would like to have a chance at getting reasonable fuel economy. If I have to go that route, Would a 3310 with down leg boosters produce better economy than the later 3310 carbs, an 1850, or my 750 cfm Street Demon?
So much to do, so little time...
77cruiser
Guru
Guru
Posts: 1484
Joined: Fri Dec 29, 2006 10:32 pm
Location: I Falls MN
Contact:

Re: Quadrajet issues

Post by 77cruiser »

I would think the mains should be feeding @ 1500. That's the advantage with a Qjet mains start right off idle.
Jim
rfoll
Guru
Guru
Posts: 3023
Joined: Mon Jan 11, 2010 12:44 pm
Location: St. Helens, OR

Re: Quadrajet issues

Post by rfoll »

77cruiser wrote: Fri Feb 26, 2021 11:24 am I would think the mains should be feeding @ 1500. That's the advantage with a Qjet mains start right off idle.
I will look again, but I'm sure they don't. Another oddity is with the secondary rods. If I lift them slightly at idle, the engine stumbles and starts to die. I have to wonder if the secondary throttle plates are not seating. Time to run some more experiments.
So much to do, so little time...
77cruiser
Guru
Guru
Posts: 1484
Joined: Fri Dec 29, 2006 10:32 pm
Location: I Falls MN
Contact:

Re: Quadrajet issues

Post by 77cruiser »

Check your secondary plates to make sure they are seated & the sec. air bleeds aren't plugged.
Jim
rfoll
Guru
Guru
Posts: 3023
Joined: Mon Jan 11, 2010 12:44 pm
Location: St. Helens, OR

Re: Quadrajet issues

Post by rfoll »

77cruiser wrote: Fri Feb 26, 2021 11:46 am Check your secondary plates to make sure they are seated & the sec. air bleeds aren't plugged.
I probed all of the holes when I had the carb apart yesterday, but everything is clean. I will be looking things over more carefully today.
So much to do, so little time...
Tuner
Guru
Guru
Posts: 3184
Joined: Sun Apr 01, 2007 5:26 am
Location:

Re: Quadrajet issues

Post by Tuner »

rfoll wrote: Fri Feb 26, 2021 11:32 am
77cruiser wrote: Fri Feb 26, 2021 11:24 am I would think the mains should be feeding @ 1500. That's the advantage with a Qjet mains start right off idle.
I will look again, but I'm sure they don't. Another oddity is with the secondary rods. If I lift them slightly at idle, the engine stumbles and starts to die. I have to wonder if the secondary throttle plates are not seating. Time to run some more experiments.
Are you just lifting the hanger or actually opening the air door slightly?

Because there is a very slight but normal air leakage around the secondary throttle blades which results in a slight depression below the air door the vapor escaping from the secondary main well is drawn into the engine with that small amount of air.

The diameter of the largest step of secondary rod which is in the SMJ when the air door is closed controls how much fuel can enter the secondary main well to replace the fuel escaping in the form of vapor.

Among the nearly 100 secondary metering rods available the diameter of the initial step which rests in the SJM when the air door is closed varies from .135" to .131".

A couple years ago I dallied with a nearly zero miles 100% perfect '72 Corvette with a QJet on the 454. The carb was the nicest unmolested QJet I have seen in many a moon but it had a problem with the E-10 fuel and the Corvette's nature of high underhood temperature.

When the carb temperature reached 170F boiling point of the ethanol the engine got so rich it would struggle to idle or simply stall. Fiddling with float level had no effect and the nozzle wasn't dripping in the first place.

Eventually I noticed that if the secondary air door was opened, even just a crack, even though it lifted the metering rods the engine would not become rich.

Opening the air door let the fuel vapor escape instead of drawn into the engine with the air leaking around the secondary throttle.

The cure was to change the secondary metering rod from the original DA rod which has .1333" diameter 1st. step to a BG rod which has .1352" diameter. This reduced the amount of fuel entering the secondary main well to replace the ethanol which boiled at 170F and the engine no longer suffered the rich problem. The circumstance was an AHH-HAAA moment.
Tuner
Guru
Guru
Posts: 3184
Joined: Sun Apr 01, 2007 5:26 am
Location:

Re: Quadrajet issues

Post by Tuner »

Geoff2 wrote: Fri Feb 26, 2021 3:26 am You may need to enlarge the idle screw discharge ports in the t/bores. They were quite small on later QJs. This could be restricting the idle mixture that the engine is getting. I would enlarge these holes to 0.096".

Nozzle drip. That needs to be fixed or you will never get the idle/off idle running to be ok.

Not many things cause n/drip. F/level too high, blocked idle air bleed & most common: t/blades open too far at idle. Engine may need more bypass air to set the t/blades in the correct idle position: 0.040" or less of t/slot below blades at idle.
With some QJets .096" for curb idle discharge will result in waaaaay too big for the needle tip diameter. You need to compare the adjusting needle to the situation before drilling.

In many (most) cases it is better to reduce the idle air bleed size than enlarge the idle jet.
F-BIRD'88
Guru
Guru
Posts: 9802
Joined: Tue Mar 27, 2007 6:56 pm
Location: Ontario, Canada

Re: Quadrajet issues

Post by F-BIRD'88 »

Hot idle compensator port valve stuck open?
novafornow
Pro
Pro
Posts: 205
Joined: Mon Feb 04, 2019 2:23 pm
Location: california

Re: Quadrajet issues

Post by novafornow »

chevyfreak wrote: Fri Feb 26, 2021 12:21 am
rfoll wrote: Thu Feb 25, 2021 9:16 pm It's still completely insensitive to Idle mixture screws. Closing them off completely has no effect, but the engine idles like it should. I'm starting to wonder about an intake manifold leak, but the exhaust is completely free of smoke.
From that info , has the bottom well plugs been checked if they are leaking?

Chevyfreak.
This is quite often the problem with these carbs.
rfoll
Guru
Guru
Posts: 3023
Joined: Mon Jan 11, 2010 12:44 pm
Location: St. Helens, OR

Re: Quadrajet issues

Post by rfoll »

All of the plugs are leak tight. I measured the idle discharge hole at about .060". It's .020 smaller than the other carbs I compared it to. When I played with the secondary rods, I did not move the air door. The secondary butterflies are sealed tight. I'm going to put a new set of plug wires on to be sure they are not an issue before I proceed further. Thanks for the replies and suggestions so far, Rick..
So much to do, so little time...
rfoll
Guru
Guru
Posts: 3023
Joined: Mon Jan 11, 2010 12:44 pm
Location: St. Helens, OR

Re: Quadrajet issues

Post by rfoll »

Some success at last. While trying 3 different carbs yesterday, I started hearing popping from the exhaust. I started swapping ignition parts and finally found severely fouled spark plugs. They were new when the engine went on the stand, but I have always had fouling problems as a result of the inability to load the engine. I drilled the idle discharge holes on the 800 QJ out an additional .020" and the carb runs much better. Thanks to all, Rick
So much to do, so little time...
Post Reply