Quadrajet issues

[rfoll]

I have a nice 350 on the test stand. Flat top pistons with Vortec heads. The cam is a Bullet 268. 218/268 intake, 222/272 exhaust, 110 lsa. Dual plane intake with headers and no mufflers. The original carb is of unknown origin, idled ok but a little fast. It sounded a little ragged off idle and smoothed out when revved up. It seemed to run fine when on a different engine, but was set aside for a better QJ currently in the car. I really had no intention of using that carb in the car. I have an 850 DP I just assembled and wanted to make sure it was going to work ok before I put it on the 415 in the race car. It started instantly and ran butter smooth throughout the range I ran the motor up to. Idles beautifully. Wanting a better QJ for when the test stand motor goes in a different car, I installed a carb I had rebuilt previously. The original application was 1988 motor home 454. It idles ok, but a little fast. When revved up it runs ragged all the way up until the main circuit comes into play. I'm starting to wonder if I am expecting too much from a stock QJ on an engine with a somewhat healthy cam. I like to run the QJ on my street vehicles for the good fuel economy they produce, but these to carbs aren't going to work in the condition they are now.

[rfoll]

I have

[1980RS]

I have not used a Q-jet in a while but I know that the later ones are harder to tune than the early ones. The last one I messed with had 36B rods and 71 jet in the front and DR rods in the secondary. You may have to clip the spring under the pri rods to make sure they stay down at idle otherwise you will have some low speed troubles inc. idle. Once you get a Q-jet right you will never go back to the dark side. LOL.

[rfoll]

I will have to watch the primary needles when I start it next time.

[F-BIRD'88]

I much prefer working with the older pre1975 4MV style QJets... I had a oem 1968 396-325Hp QJet that ran very well on lots of engines.. Played with the secondary rods and hanger height +/- to good effect. Idled and drove fine stock primary calibration. Should have never parted with it.
Not crazy about any of the emission era QJets.

You can get Edelbrock QJet tuning parts eg: primary power piston springs. .135" high flow needle seat if needed etc.
I made up a guage fixture to tweek my own custom hanger heights.

[F-BIRD'88]

If on a test stand with open headers Add 24"++" of exhaust pipe extension.. EG 2.50" or 3" is great. 2-¼ is fine in a pinch for this. The thing will overscavenge and run lean with open headers.. No use playing with the QJet calibration until you get some exhaust pipe on it..
Longer is better.. Then you can also use a O2 sensor/AFR guage to dial in the idle and critucal off idle transition using the afr guage and vacuum guage as you open the throttle.
Ideally it would be nice to be able to put a load on the motor.. All exhaust joints must seal for the AFR guage to work right.

You are wasting your time trying to dial in a QJet with open headers..

[chevyfreak]

I only use qjets, even altered one from a 327 to work on my 250 i6.
Sounds like the spring under the piston is stiffer than needed and needles not seated at idle.

Best info i can give you is go to cliff's quadrajet performance. Plenty of help on his forum. Cliff will help you to sort the carb out and he does qjet parts as well.

Chevyfreak.

[Geoff2]

If the transfer slot/throttle blade relationship is correct, met rod piston position will not affect idle.

The idle cct maybe too lean. A quick test: if the carb has a choke, close it until the engine nearly stalls; then, rev the engine & see if it runs better; if it does, idle cct is lean & IFRs need enlarging. If no choke, use your hand over the carb. Cliff Ruggles recommends a 1/4" float level, so try that.

[mag2555]

Saying that it ran fine on a different engine without giving the details of that motor is no help in resolving the issue you have now with it !

[novadude]

I'm starting to wonder if I am expecting too much from a stock QJ on an engine with a somewhat healthy cam. I like to run the QJ on my street vehicles for the good fuel economy they produce, but these to carbs aren't going to work in the condition they are now.

I would say "no" - Q-jet should be just fine. I have a vortec 355 with a 270/278, 217/225 @ 0.050", 108 LSA cam. I am using an '82 truck Q-jet with only minor tweaks. It works perfectly, and delivers good MPG.

Here are the carb basics (it's a single main air bleed carb):

MAB: 0.046"
Lower IAB: 0.059"
Upper IAB: 0.067"
Idle DCR: 0.052"
IFR (tube): 0.035"
Idle bypass air: 0.072"
Primary jet: 73
Primary rod: 50M

Try something like that - it should get you close.

[rfoll]

Saying that it ran fine on a different engine without giving the details of that motor is no help in resolving the issue you have now with it !

Mild 350 with the common rv cam, 9.3 CR, Iron manifolds with a nicely done single exhaust, SP2P intake manifold on 58 cc swirl port heads. It produced 19 mpg in my 79 El Camino before it got totaled. It did occur to me the 1 1/2" primary tube headers with really short collectors might be been causing issues. Truthfully, I have never run a QJ with open headers before. I will try the mufflers I have for the test stand. They are huge Magnaflow mufflers that require jack stands to support them. This whole thing got started as a means of testing the 850. Throwing the QJ carbs on was and afterthought since I planned to use one when the engine goes in the car this summer. I have a large collection of other good carbs to use, but the Holley and AFB based stuff will require re-configuring fuel and linkage. The unfortunate part of a home built test stand is the lack of means of loading the engine.

[77cruiser]

Possibly a wrong gasket main body to base.

[rfoll]

Saying that it ran fine on a different engine without giving the details of that motor is no help in resolving the issue you have now with it !

Mild 350 with the common rv cam, 9.3 CR, Iron manifolds with a nicely done single exhaust, SP2P intake manifold on 58 cc swirl port heads. It produced 19 mpg in my 79 El Camino before it got totaled. It did occur to me the 1 1/2" primary tube headers with really short collectors might be been causing issues. Truthfully, I have never run a QJ with open headers before. I will try the mufflers I have for the test stand. They are huge Magnaflow mufflers that require jack stands to support them. This whole thing got started as a means of testing the 850. Throwing the QJ carbs on was and afterthought since I planned to use one when the engine goes in the car this summer. I have a large collection of other good carbs to use, but the Holley and AFB based stuff will require re-configuring fuel and linkage. The unfortunate part of a home built test stand is the lack of means of loading the engine.

Not here. I don't mix and match QJ carb parts other than maybe a rod or jet change. It's way beyond me to second guess the GM carb engineers.I do wish I had the means or information about power valve springs, but i have never seen info on spring ID.

[tenxal]

You are wasting your time trying to dial in a QJet with open headers..

Never knew how dumb all us Stock and Super Stock racers that have to run QJets were until now.......

[rfoll]

Update...I put the mufflers on and the 800 cfm 454 carb improved, but still not quite right. I looked down at the boosters at 1500 rpm and saw an occasional drip from the main circuit. I pulled the carb apart and set the float level lower, It was about 1/16" too high. This carb has a threaded bushing in hole in place of the plug at the front of the choke tower. It appears to be an upper limit for the primary rods. I plugged it thinking it might be an air leak. At 2000 rpm the engine is still a little rough. I'm having difficulty believing the engine wouldn't run smooth at that speed. The cam isn't that big. 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.