Quadra Jet stuck open at idle

[chev-obsession]

Hey guys,

So my truck has sat for a couple of years. I went to start it up and gas started pouring out of the Holley carb. So I put the original Quadra jet back on to see if I could get it to fire and move it around. After putting what I thought was fresh gas into the tank and a new fuel filter, the truck fired up, idled and ran great. Almost to the point to where it was too good to be true! Turned it off and left it for a couple days.

Then I go out this morning, put some good tires on it to get it moveable, the truck started but “idle’d” at 2,500-3,000 rpm’s, almost like something in the carb is stuck open. Noticed the gas had turned green in the clear fuel filter. Would the gas have caused something to get plugged up with in the last 2 days of sitting or is it a simple fix like a lever is stuck open or closed?

I looked around, rev’d the engine up to see if the choke was stuck open and it did not idle down. And the idle screw is backed down quite a ways to where I know that is not the issue.

Any ideas what it could be? I’m at a loss

[68gmsee]

I doubt it was the gas that caused the high idle. On a quadrajet, I'd suspect linkage sticking or choke mechanism not releasing properly. In my opinion the most complicated part of the qjet is the choke mechanism. Look at it carefully to see if it allows the butterflies to close after choke opens all the way.

[truckster]

How long did it idle that high? Did you verify the engine speed with a tachometer? It sounds like it was on the fast idle cam. Once the choke warms up it should drop off the fast idle cam as soon as you touch the throttle.

[chev-obsession]

Originally Posted by 68gmsee

I doubt it was the gas that caused the high idle. On a quadrajet, I'd suspect linkage sticking or choke mechanism not releasing properly. In my opinion the most complicated part of the qjet is the choke mechanism. Look at it carefully to see if it allows the butterflies to close after choke opens all the way.

I checked the choke it was wide open

[chev-obsession]

Originally Posted by truckster

How long did it idle that high? Did you verify the engine speed with a tachometer? It sounds like it was on the fast idle cam. Once the choke warms up it should drop off the fast idle cam as soon as you touch the throttle.

Idled like that for 5-10 minutes, long enough for the t-stat to open and get hot. I didn’t check it with a tach but I know it was close to that rpm range. I’ll go back and check the high idle cam but i hit the throttle a couple times and nothing changed

[61K10]

What intake manifold do you have?

[68gmsee]

As high as it's idling there's definitely something sticking or maladjusted. Be sure and check the fasty idle cam and the adjusting screw. It's hard to see behind the choke mechanism.

The video below shows you both the main idle adjusting screw and the fast idle adjusting screw around 3 minutes into the video.

https://www.youtube.com/watch?v=yX_hCujFQ_I

[chev-obsession]

Originally Posted by 61K10

What intake manifold do you have?

Edelbrock performer... the one for the Rochester

[chev-obsession]

Originally Posted by 68gmsee

As high as it's idling there's definitely something sticking or maladjusted. Be sure and check the fasty idle cam and the adjusting screw. It's hard to see behind the choke mechanism.

The video below shows you both the main idle adjusting screw and the fast idle adjusting screw around 3 minutes into the video.

https://www.youtube.com/watch?v=yX_hCujFQ_I

Ok I watched the video, backed the screw out all the way, didn’t change anything

[68gmsee]

Maybe someone else has some ideas for you.

The only thing I know that causes a high idle aside from what was mentioned above:

Internal carb problems, butterflies or secondaries not closing, vacuum leak and timing. Barring that, I'm lost on the cause on your high idle -especially since you mentioned it ran good at first.

As you said in the first post, maybe the gas caused some internal stuff to get gummed up. A cleaning and fresh gas may clear that up.

[chev-obsession]

Originally Posted by 68gmsee

Maybe someone else has some ideas for you.

The only thing I know that causes a high idle aside from what was mentioned above:

Internal carb problems, butterflies or secondaries not closing, vacuum leak and timing. Barring that, I'm lost on the cause on your high idle -especially since you mentioned it ran good at first.

As you said in the first post, maybe the gas caused some internal stuff to get gummed up. A cleaning and fresh gas may clear that up.

That’s along the lines of what my next step would be. Take the carb off, put a new gasket on the intake to carb mating surface, spray some carb cleaner up in it and throw some fresh gas in the tank.

[Coley]

Originally Posted by chev-obsession

Ok I watched the video, backed the screw out all the way, didn’t change anything

" backed the screw"....do you mean you backed both screws out?

Noting that there is one on both sides...one for the high idle (driver's side) and one for the (choke) idle on the passenger side.
Either of those screws can push the idle speed up so they both have to be checked.

Next up is you have to check the linkage to make sure nothing is binding on a hose or bracket or anything....when you look down into the carb (when running) with a flashlight (not comfortable or easy to do) you should see the bottom/lower throttle plates closed at idle (while the upper choke plate is open).
As you 'flick' the throttle those plates will open accordingly thus revving the engine as more fuel/air are put through.

All good
Coley

[chev-obsession]

Originally Posted by Coley

" backed the screw"....do you mean you backed both screws out?

Noting that there is one on both sides...one for the high idle (driver's side) and one for the (choke) idle on the passenger side.
Either of those screws can push the idle speed up so they both have to be checked.

Next up is you have to check the linkage to make sure nothing is binding on a hose or bracket or anything....when you look down into the carb (when running) with a flashlight (not comfortable or easy to do) you should see the bottom/lower throttle plates closed at idle (while the upper choke plate is open).
As you 'flick' the throttle those plates will open accordingly thus revving the engine as more fuel/air are put through.

All good
Coley

Yep, backed both screws out to the point to where they were almost out of the carb body, the idle speed went down a little bit not enough to deem that the “root cause”.

Like I said before, when I fired it up the first it ran/idled almost flawlessly after warming up. Then 2 days later it was as if someone was standing on the throttle and yes I checked to make sure the throttle linkage isn’t bound up. The throttle lever on the carb body rests on the idle screw like it should.


I have not looked down the carb with the engine running. I suppose that’s not a bad idea, and will try that before taking the carb off.

I have never had a vacuum leak due to a bad gasket on an intake. Would that cause a high idle? Or possibly something in the brake booster?

[chev-obsession]

Originally Posted by Coley

" backed the screw"....do you mean you backed both screws out?

Noting that there is one on both sides...one for the high idle (driver's side) and one for the (choke) idle on the passenger side.
Either of those screws can push the idle speed up so they both have to be checked.

Next up is you have to check the linkage to make sure nothing is binding on a hose or bracket or anything....when you look down into the carb (when running) with a flashlight (not comfortable or easy to do) you should see the bottom/lower throttle plates closed at idle (while the upper choke plate is open).
As you 'flick' the throttle those plates will open accordingly thus revving the engine as more fuel/air are put through.

All good
Coley

Yep, backed both screws out to the point to where they were almost out of the carb body, the idle speed went down a little bit not enough to deem that the “root cause”.

Like I said before, when I fired it up the first it ran/idled almost flawlessly after warming up. Then 2 days later it was as if someone was standing on the throttle and yes I checked to make sure the throttle linkage isn’t bound up. The throttle lever on the carb body rests on the idle screw like it should.


I have not looked down the carb with the engine running. I suppose that’s not a bad idea, and will try that before taking the carb off.

I have never had a vacuum leak due to a bad gasket on an intake. Would that cause a high idle? Or possibly something in the brake booster?

[Coley]

To look down the carb you can always use a mirror as well, but you might need someone to give you a hand...and you'll probably need a flashlight to help illuminate the throttle plates.

As for vacuum leaks they can cause a variety of things.
Sometimes they will increase the idle speed IF the mixture in the carb was/is too rich....prior to the leak.
Then as more air is 'leaked' (sucked) into the carb it evens the running mixture out, improving the idle which usually results in increased rpm/idle speed.
If the mixture was running lean (not enough fuel) then a vacuum leak will worsen the idle causing to be very rough and stumble.
In other words, prior to the leak or under normal running conditions...the carb wanted more fuel to idle better....or less air (one of the other). However, the vacuum leak was inducing (sucking) MORE air into the mixture....so given that the mixture was not great to begin with....it just got worse in terms of a nice air/fuel ratio by adding even more air.

All of this is based on a vacuum leak 'messing' with the air/fuel ratio of course.
....so the running result will be based on how well the carb was set before the vacuum leak....ie: 'Rich' or 'Lean'.

Regardless, check that out and post up the results. After that, we might have to take the (top) air horn off the carb to see what is going on in the float/bowl area.
Note: this isn't too difficult but is best accomplished by removing the carb and putting it on the bench to review...which is a task that doesn't take more than 15-20 minutes under most conditions.

All good
Coley

[cadillac_al]

I hope you verified the secondaries are closing all the way. I just mentioned it in a thread a couple days ago. I have had secondary linkage stick and the secondary throttle plates hang up on the intake manifold or the gasket.

[burnin oil]

I have a holley that did the same basic thing. It is a blast when you give it throttle to climb a hill then the throttle sticks right where it was and you already lifted your foot. Grab the keys and cut the ignition! The throttle shafts themselves were sticking from sitting. A liberal spray down with PB Blaster fixed it right up. Flipped the carb and sprayed it. Let it soak then worked the throttle. Repeated once more for good measure.

[Dead Parrot]

Since it ran properly the first time with the Q-jet, suspect a loose or rotted vacuum hose gave up on the second start. This can include the brake booster as well. Don't forget any transmission modulators that use engine vacuum. Easy to test by simply removing the hoses and plugging the ports.

With the motor off, look down the carb and verify the throttle plates are closed all the way. With both idle screws backed out, the plates should be fully closed. Possible a small critter or dried bits of one wedged in place and is holding the plates open.

[tdangle]

I suspect the float either soaked up fuel and is heavier than it should be (if it is not a brass float, if so small leak so the float filled with fuel) . Or the float needle seat is full of crud. With it idling look into the throat of the primaries. If you see a lot of fuel coming out of the boosters this is the problem. Fuel level is way too high causing it to pull fuel out of the bowl.