The idiot tries to tune a carb, PT.1:

[vandelay industries]

What, in your opinion, do you think happened here?

Problem:
The engine all of a sudden, will not run below 1000 rpm unless i keep my foot on the gas, AND, the engine either will not start or will only start after lots of cranking and even then it will putter to a start vs. catching positively. This is engine cold or hot.

--Before this, the engine would start within one crank revolution or less. And the engine would idle at 600 in gear no problem.


Background:
Cheap ass Jeg's reman q-jet #15805
Stock 4.3 v-6 with stock cam---like .184/.195 @ .50
Stock HEI distributor

All i did was to bend the tab that the screwdriver is pointing to, to touch the tab just below it----in order to get the secondary plates to open; The carb shipped with the secondary plates NOT opening, among other issues.....

Then, IMMEDIATELY after this, the engine will not run below 1000rpm unless i keep my foot on the gas and it's hard to start or will not start.

So, what do you think happened?

 

[ktmoutfront]

Was it ever working with this carb before your tab adjustment?

Really bad vacuum leak. As bad as you are describing, it might be under the carb bottom plate. Are all of the vacuum lines attached correctly? No open hole anywhere?

 

[vandelay industries]

Was it ever working with this carb before your tab adjustment?

Really bad vacuum leak. As bad as you are describing, it might be under the carb bottom plate. Are all of the vacuum lines attached correctly? No open hole anywhere?

No, the secondaries were not opening at all when i first got it----because it's a POS. The carb itself ran ok---on the front two barrels and started pretty easy hot or cold.

i will recheck vaccuum, although i already had everything blocked off including PCV and brake booster just to eliminate everything.

 

[ktmoutfront]

So it ran on the primary's just fine. The secondary's are giving you grief when you bent the tab down? I would look to see if the secondary's are closing completely. That is a huge vacuum leak and fuel dump. I bet it was blowing a little black smoke as you started it. The secondary's should not have any daylight showing at the edge of the butterflies.

Bend that tab forward (I think) to where it is not contacting the arm, then try running it. Watch the butterflies, if you can, while to bend it.

 

[Wes Harden]

Q jet has a secondary lock out they are not supposed to open until engine is under load and throttle is is open more then 2/3rds, somewhere around there.
You should hear them open under those conditions.

 

[mrk5]

Like Wade was alluding to, could the secondaries now be hanging open?

Also as odd as it may seem, it's possible you have something else entirely different going on. Fouled plugs or distributor issue. If you try messing with the carb a bit more with no success, try pulling plugs just to be sure.

Another thought would be something going on with float bowl level. Like maybe it's getting too full.

You mentioned not running below 1000 rpm. Do you have to flutter the pedal, or just hold the throttle open?

 

[1-ton]

Does this carb have vacuum secondaries or mechanical?

 

[ktmoutfront]

If it is a Quadrajet, it should be mechanical.

 

[tRustyK5]

all Qjets are vac secondaries

 

[tRustyK5]

Q jet has a secondary lock out they are not supposed to open until engine is under load and throttle is is open more then 2/3rds, somewhere around there.
You should hear them open under those conditions.

This

when you tried to "modify" it because you thought the secondaries weren't working you focked it up. Undo what you did...

there are adjustments for how quickly/easily the secondaries open that don't involve bending tabs.

 

[ZooMad75]

This

when you tried to "modify" it because you thought the secondaries weren't working you focked it up. Undo what you did...

there are adjustments for how quickly/easily the secondaries open that don't involve bending tabs.

I was going to say something along those lines. I've never had to bend anything on a Q-jet for adjustment purposes short of float adjustment or the rod from a choke stove. Certainly not the secondaries.

The secondary linkage is mechanical by the way. The air valve on top of the secondaries opens due to the vacuum below it as they open. That is adjustable by a small set screw on the underside of the shaft for the secondary air valve.

Wade is right if the secondaries are open at idle it's going to run like crap due the vacuum leak. Straighten out what you bent and recheck that they are closed at idle. Recheck that they open with enough throttle input.

It's possible that your original perceived problem of the secondaries not opening could have been the secondary air valve was stuck or adjusted too tight and not opening at all. It might be easier to confirm this with the carb off and make sure all the linkage for the throttle blades are not tweaked/binding due to the adjustment that was made.

One of the things that trips folks up on q-jets is expecting the secondary air valve to open if they rotate the throttle linkage to wfo with the engine not running. I guarantee all 4 barrels will open in that scenario, but with the engine not running the upper air valve stays closed, which causes one to assume the secondaries aren't opening.

 

[ktmoutfront]

When we modded the q-jets back in the day, the vacuum butterfly's were removed. So I forgot about those. He had a mechanical linkage he installed to open the secondary needles. I had to go into my wayback machine to remember that.

 

[Blue85]

In Q-Jet world, "Secondaries open" means the second throttle shaft at the base of the carb is opening (you can see the end of the linkage, but you can't see the throttle blades!). For the TOP flaps to open requires having the car driving or on a dyno. It's not something that will/should happen in the driveway.

The secondary throttle blades are to supposed to stay 100% shut until the choke is completely open, including the vacuum choke pull-off, so engine at full temp, choke coil and linkage properly adjusted. No point working on secondaries until those things are right or (temporarily) bypassed.

The main reason people bash Q-jets is that they don't understand them, not that there is something wrong with it.

 

[vandelay industries]

So it ran on the primary's just fine. The secondary's are giving you grief when you bent the tab down? I would look to see if the secondary's are closing completely. That is a huge vacuum leak and fuel dump. I bet it was blowing a little black smoke as you started it. The secondary's should not have any daylight showing at the edge of the butterflies.

Bend that tab forward (I think) to where it is not contacting the arm, then try running it. Watch the butterflies, if you can, while to bend it.

The secondary plates are closed fully. No light.

Like Wade was alluding to, could the secondaries now be hanging open?

Also as odd as it may seem, it's possible you have something else entirely different going on. Fouled plugs or distributor issue. If you try messing with the carb a bit more with no success, try pulling plugs just to be sure.

Another thought would be something going on with float bowl level. Like maybe it's getting too full.

You mentioned not running below 1000 rpm. Do you have to flutter the pedal, or just hold the throttle open?

Plugs are new AC R43T. i think the float, needle+seat are ok.

i have to keep my foot on the pedal to keep it above 1000rpm. Although Friday i was able to get it 800ish for a few seconds with no pedal intervention, the it sputtered out. The only change i made was to increase curb idle screw. MAYBE what happened was:

In order to adjust the secondary blades (not air flaps), you have to bend tab "A" rearwards, towards the back of the carb/car so that it contacts tab "B" Tab "B" is connected to the rod that connects to the secondary throttle shaft etc. If you want more opening bend it further, less opening bend it less.

However, you'll soon notice that when you try to bend "A" rearwards, the throttle arm "D", ALSO moves rewards /clockwise because they are connected to each other. So, you have to find a way to stop the throttle arm from moving to have any chance to be able to bend tab "A."

So what i did was grasp the throttle arm and hold it still as i attempted to bend tab "A" with needlenose pliers towards the rear of the carb.

In doing this, i MAY have judiciously turned the throttle arm counter-clockwise---in order to counteract the forces of trying to bend tab "A" rearwards. In doing THAT, tab "C", the throttle arm stop that contacts the curb idle screw MAY have bent rearward, toward the rear of the carb-----thus allowing the primary throttle plates to close more than they usually do? ( Because by bending tab "C", this allowed the throttle to turn more counter-clockwise and thus closing the throttle plates more) i didn't think that tab "C" could bend by hand, but i just tried it on this above carburetor and yes, they can bend without a tremendous amount of force.

NOTES: 1) The carb in the picture is not the actual carb, but it's exactly like the one on the engine.
2) i think my above theory now may be bunk.

Irregardless, i feel we should suspend this discussion for now as i now have a more urgent problem----i can't get it started at all.