Quadrajet doesn't idle when warm

[andrew-75c10]

I inherited my grandfather's 1975 C10 last year, and I got it running last spring. It only needed a new fuel tank, new fuel lines, new fuel pump, a carb rebuild, and about a dozen other things.

Anyways, the truck starts cold just fine but after it warms up it has trouble idling. I'm assume the problem is with the choke, but I'm really not sure. I took before any after photos and everything seems to line up. Although near the bottom, just left of the large screw with the spring, there is a metal pieces that looks bent outwards. Unfortunately that piece is hidden from view in the before picture.

What do you guys think. Does it sound like a choke or something else?

Before:

You must be registered for see images attach

After:

You must be registered for see images attach

 

[ali_c20]

We would need more details on what is going on. Best would be a video. Check the choke linkage if its moving free and if the choke plate opens completely when warmed up.
https://www.onallcylinders.com/2015...mance-issues-that-can-be-traced-to-your-carb/

 

[WFO]

Are you tapping on the accelerator petal when it begins to warm up, to open the butterfly and cancel the fast idle?
If not, it starts running rich.

 

[WebMonkey]

the divorced choke element (part mounted to the intake) could be seized.

 

[Dmack]

I would start by starting it up, cold, with the air cleaner off. As it warms up, blip the throttle and make sure the choke plate is opening up. If it is, the choke is working. If not, you know at least part of the problem is the choke.

 

[59840Surfer]

SOUND is important here too.

What do you hear when it's idling? Is there a sizzling noise?

Q-Jets can drop a float and run rich - which is evident by the sizzling noise of fuel peeking out of the venturi fuel ports and dripping onto the almost closed butterflies.

That will give you a noise that's not unlike the faint sound of frying bacon --- kinda --- but you gotta use your imagination here.

A good carb mechanic can use his ears and listen to a carb and pretty well tell what it's doing/not doing just by the way it sounds and responds.

For the life of me, I cannot remember when the floor-jet problem was re-engineered out of the carb - but we had a lot of troubles with them too - running ultra rich and even when the choke and float were set perfectly.

Ya kinda gotta know the animal to make and correct assumptions!

Carb-guys are a dying breed - much like 'distributor ignition points-condenser' guys. We are called "Boomers" for the good or bad of it.

 

[59840Surfer]

I would start by starting it up, cold, with the air cleaner off. As it warms up, blip the throttle and make sure the choke plate is opening up. If it is, the choke is working. If not, you know at least part of the problem is the choke.

Actually - the choke plate should open even if you don't 'blip it' at all.

Providing that the choke-break diaphragm is still good, it will pull the choke plate partially open (it's adjustable) the moment there's any vacuum created under the carb plate and this keeps the engine from going over-rich and totally flooding.

Then once the choke stove heats up, the choke will continue to open, but the fast idle cam will stay on the notch where the bimetallic coil put it when the carb was opened to set the choke in the first place when the engine/carb was cold.

These 'modern' fuel injected engines don't need to have the throttle pushed full OPEN before starting the engine - it's all in the computer now.

But older vehicles with carbs had to be floored before turning the key, just to set the choke and help the engine to start when cold.

From that point - and assuming all is well and correctly adjusted on the carb - the engine could conceivably be driven off right away, the choke making it's adjustments as the choke stove further heated and the throttle action and engine heat called for less and less choke.

The AFR would be close, although not perfect, albeit in the ballpark --- er, somewhat --- until the choke was totally OFF and out of the operational circuit at that point.​

The above (^) is making a lot of assumptions:
1> nobody messed with the adjustments (I hear crickets, right?)
2> the carb is the correct carb for the vehicle
3> parts are not worn out or missing (MMD*)
4> rubber diaphragms (choke pull off, dashpot, etc) are still intact, in place and working​

(* MMD: Modified/Missing/Disconnected)

 

[Snoots]

Hot Idle adjustment is on the drivers side.
Cold Idle adjustment is on the passenger side. If it starts and idles fine when the ambient temp is cold,
DON'T MESS WITH THE COLD IDLE ADJ!

Don't ask me how I know this either

You must be registered for see images attach

 

[andrew-75c10]

Thanks for all the advice! I found the service manual for my carb here:
https://www.carburetor-parts.com/assets/manuals/quadrajet_carburetor_manual.pdf

It was raining here in San Diego this morning, but now that it's let up I'm going to check things out.

 

[69 gto]

if you want to learn about 'em check out... Cliff Ruggle’s book: How To Build And Modify Rochester Quadrajet Carburetors,or Rochester Carburetors by Doug Roe. I've read the first and own the second.

 

[andrew-75c10]

Hey everyone! Just following up on my progress so far.

@59840Surfer:
1! I am guilty of messing with the adjustments.
2) This carb came with the vehicle, as far as I know it the original from the factory.
3) All the carb parts are there. I rebuilt the carb so none of the parts are worn out - however, this is the first carb I've worked on. My autoshop professor knew a lot about carbs but classes have been cancelled with COVID.
4) I tested the choke pull off by pushing it in and putting my finger over the opening. It held in place until I removed my finger, so I'm assuming it's good. I don't have a da

@69gto:
Thanks for the recommendations, I'll check them out.

First of all, I want to thank everyone for all of your input on how the qjet choke operates. I'm reasonable sure my choke is a-okay. Keep reading if your interested in my real problem.

When I rebuilt my carb, the engine was idling too fast. I know I messed with the cold idle adjustment. I think that was before I installed the cam follower incorrectly. I pulled off the carb and got the cam follower on the correct way, and was glad my engine didn't sound like it was going to blow up.

However, it did have trouble idling. Cold idle wasn't great, but it didn't idle warm at all. After reading some manual and watching a lot of videos on qjet turning, I figured out I had backed out the cold idle screw almost all the way. I adjusted it by turning the cold idle screw in until it contacted the cam followered, then gave it 3 turns in as recommended by one of the manuals I read. This idled too fast so I backed it out a half turn at a time until the idle sounded reasonable - I don't have a tach yet so I wasn't able to set it to a recommended RPM.

I just bought a vacuum gauage, and I'm shopping around for a tach. I'll be happy to get my idle adjustments dialed in.

 

[Ricko1966]

Not trying to derail anything, but take a pause and make sure all the vacuum lines are connected and not only connected, but connected correctly.It may idle cold but not warm due to a thermo vacuum switch opening that say should be opening an air cleaner door but is actually opening an EGR valve or anyone of another combinations.

 

[andrew-75c10]

Thanks Ricko. Most of the lines coming off the carb "look" correct. I labeled each one before and after the carb rebuild, and I also replaced most of them with new hoses. There are a few hoses I haven't got to yet.

The vacuum line that looked most suspicious went goes to the crankcase and EGR. There is a valve and a couple T connectors, and I really don't know what it is doing. I'll take some time this week to take some good photos and provide a diagram of that particular connection.

I'm just happy I was able to drive the truck around the block a few times. Gave me enough confidence to take it to Home Depot and pick up a Christmas tree

 

[Bextreme04]

Hey everyone! Just following up on my progress so far.

@59840Surfer:
1! I am guilty of messing with the adjustments.
2) This carb came with the vehicle, as far as I know it the original from the factory.
3) All the carb parts are there. I rebuilt the carb so none of the parts are worn out - however, this is the first carb I've worked on. My autoshop professor knew a lot about carbs but classes have been cancelled with COVID.
4) I tested the choke pull off by pushing it in and putting my finger over the opening. It held in place until I removed my finger, so I'm assuming it's good. I don't have a da

@69gto:
Thanks for the recommendations, I'll check them out.

First of all, I want to thank everyone for all of your input on how the qjet choke operates. I'm reasonable sure my choke is a-okay. Keep reading if your interested in my real problem.

When I rebuilt my carb, the engine was idling too fast. I know I messed with the cold idle adjustment. I think that was before I installed the cam follower incorrectly. I pulled off the carb and got the cam follower on the correct way, and was glad my engine didn't sound like it was going to blow up.

However, it did have trouble idling. Cold idle wasn't great, but it didn't idle warm at all. After reading some manual and watching a lot of videos on qjet turning, I figured out I had backed out the cold idle screw almost all the way. I adjusted it by turning the cold idle screw in until it contacted the cam followered, then gave it 3 turns in as recommended by one of the manuals I read. This idled too fast so I backed it out a half turn at a time until the idle sounded reasonable - I don't have a tach yet so I wasn't able to set it to a recommended RPM.

I just bought a vacuum gauage, and I'm shopping around for a tach. I'll be happy to get my idle adjustments dialed in.

Get a timing light with a tach readout on it. The timing will also affect the RPM quite a bit. Here's the order I would do things to get it dialed in and driving right:

1)Hook up timing light with tach readout on a cold engine.
2)Screw in the idle mixture screws on the front of the carb base plate(both of them, one on driver side and one on passenger side of PCV inlet) until they LIGHTLY bottom out, and then screw them both back out two turns. This will get you to a start able setting.
3)Pump the gas pedal all the way to the floor and then slowly let up to ensure the choke has set.
4)Start the truck
5)Wait for approximately a minute for the engine to be all the way up on the cold idle cam and at peak RPM
6)Adjust the high idle if it is outside of 1200-1500rpm(this will be the small screw on the passenger side of the carb that can be seen between the carb body and choke pull off diaphragm)
7)Let the engine fully warm
8)Check the current distributor timing
9)Disconnect and plug the vacuum advance hose from the carb to the distributor(plug the carb side)
10)Set the Initial timing somewhere between 4-12° BTDC. I usually find 8° is my sweet spot with 87 octane. Any more and I get detonation under load.
11)Set your hot idle to 750-800rpm(this will be the larger set screw on the drivers side)
12)Set the parking brake and chock the wheels
13)If this is a manual transmission, skip the next step
14)Place the vehicle in drive and ensure it doesn't move with your foot off the brake(even with a little throttle)
15)set hot idle to 750rpm
16)Slowly screw out(counter-clockwise) the drivers side idle mixture screw until the engine RPM maxes out and starts decreasing again. You should adjust the idle set screw to keep the engine RPM between 700-800rpm while doing this.
17)Once you have found the peak RPM, set the idle RPM to 750rpm.
18)Slowly turn in the mixture set screw(clockwise) until the engine RPM is down to 650rpm.
19)Repeat steps 16-18 for the passenger side mixture screw.
20)Put the truck back in Park.
21)Now set the idle set screw to your final rpm. I recommend 750-850rpm out of gear. You should have no less than 650rpm in gear. I would sacrifice a higher out of gear RPM to make sure my In-gear rpm is high enough to run smoothly and charge the batteries with the headlights on.