Quadrajet help

[Toro]

My 72, 350 w/Quadrajet won’t idle without feathering the gas, however if the secondary air doors are held open it idles just fine. It needs to be gone through, but wanted to mention this here and see if it sounded like anything in particular. Thanks

[Steeveedee]

Probably a vacuum leak. When you open the secondary air valve this also lifts the rear metering rods, which richens the mixture. Spray around the primary throttle shaft (both sides) and see if the idle speed changes. If that doesn't help, try capping off vacuum lines until you find the leak.

[HO455]

Is this a new issue on a daily driver or on a truck thats being being resurrected?

[Toro]

Originally Posted by HO455

Is this a new issue on a daily driver or on a truck thats being being resurrected?

It’s a resurrection.

[Grumpy old man]

This may help .

https://quadrajetparts.com/

[Wrenchbender Ret]

Sounds like the secondary throttle plates are not closing tight. Try spaying carb cleaner around the linkage & down the throat of the secondary.
George

[rpmerf]

Brainstorming...

Opening the rear doors pulls up the secondary rods richening the mixture.

There are 3 places I would start with. Ignition, vacuum leak, and choke.

Here is what I would do. I've given short descriptions on some things, might want to look into them deeper. This assumes some unknowns, some things I say to set may be more of a check just to be sure it's right. Since you say this is a resurrection, I figure you have either recently checked these things, or they are things you will want to check.

Pull out the plugs, make sure they are clean enough and gapped right. Check the wires for issues where they can short.

Remove any non essential vacuum lines and cap them at the manifold. You will not be driving it for now, so you can remove the trans and brake booster lines too.

Take off the air cleaner. Floor the gas once to set the choke. Verify it is set. Might even do this from under the hood to watch it set. Might not hurt to familiarize yourself with how the choke works. I assume this is similar to the choke on my 70 with a 73 carb. Divorced choke. There is a bimetallic spring on the intake. It will be shorter when cold, and longer when hot.

Start it up. Note the idle RPM, should be around 1200-1600 until the engine is warmed up. If not, you can adjust the fast idle screw on the pass side of the carb. Might just set it to where it will run without dying.

Let it warm up. While it is warming up, look around at spark plug wires for spark to the engine block.

Look for vacuum leaks. The engine needs to be idling somewhat consistently, so you can hear changes. Get some carb cleaner. Spray it around the intake to head gaskets, carb to manifold gasket, sides of the carb, and secondaries while listening for a change in RPM. If you hear a change in RPM, you have found a leak.

As the choke comes off, it will transfer to the base idle. If this is not enough, bump it up enough to keep it running.

Check the dwell, set the timing. Dwell should be 28*-32*. The engine might be higher than normal idle RPM, so the mechanical advance might be kicking in. Doesn't matter too much at this point, anywhere between about 5 and 18* should be fine for now. I would leave the vacuum advance off for now.

Set the idle fuel screws. Shut the engine down. turn the screws in until they just barely bottom. Don't make them tight, just barely touching the bottom. Count the number of turns as you turn them in so you know where they were. Standard is to turn the out 3 turns from bottom. Start the engine, turn them in until you get the highest idle or vacuum. Keep them even. I usually do half turn each side until I get it where I want it.

The timing and fuel should be ballpark, so see if you can get it down to a normal idle RPM. Should be somewhere around 650 for a manual. For an automatic, you want 650 in drive, might be 750-900 in park.

Note the idle vacuum. One at a time, connect the vacuum devices and watch for a drop in engine vacuum. They may take a second to equalize, but once they are, the vacuum should be about the same as before it was hooked up. If there is a drop, that device is not holding vacuum, causing a vacuum leak.

If at this point it can idle at a normal RPM without feathering the gas, I would set the timing and idle fuel again. First time was more of a ball park, set them right where you want them now.

Lars Q-Jet Choke Setup:
http://www.vetteclub.org/warehouse/t...ke%20Setup.doc

[External]

Since yall got quiet.
I have a 72. Original 350, freshly rebuilt same model number Qjet carb, hei distributor, everything else stock. Truck runs fine. I do however think it should have a little more power.
The other day I go pick up a tandem axle trailer. Drives fine through town. I get on the highway and I notice the truck shifting sooner than without the load. 200R4 trans. I hit 3rd gear at 35mph, usually 50mph. At 40 45 it starts nose diving. Losing power. Stalling out and even dies. Thinking it is the fuel delivery system I get a new fuel filter.
Now without the trailer I hit high gear and truck still cuts out at high rpms. Thinking maybe it's the cheaper mr gasket electric fuel pump. *forgot to mention earlier. I go to the parts store to get a better one that has dedicated 4 to 6 pounds of pressure which this qjet requires. Then the guy behind the counter tells me that he thinks it's my carb not being set up right. Like the jets are too small.
Now I am not opening it up. I hear the way it goes is timing first, carb next.
I dont know how to report my timing to you guys. I can tell you that I adjusted it once I received my rebuilt carb by ear. Disconnected the vacuum advance, rotated the distributor until smoothest and highest rpms. #1 wire on dizzy points mostly toward #1 plug.
The truck idles smoothly. Runs smooth down the road.
I find it odd that timing or carb issues are the cause NOW after having it all hammered out prior the trailer incident. What things should I look for?
I'd like to learn and not take it somewhere.

[rpmerf]

Might be good to start your own thread as to not get your issues mixed up with the issues at the top of this thread.

I'd be concerned about the trans shifting earlier. Those trans are really sensitive to the throttle valve adjustment. Might you had just been lighter throttle than normal? I might had just left it in 3 to keep it from shifting up into OD. Do you have lockup on the trans? Might it had shifted it into OD, and dropped the RPM super low.

Hit the basics - plugs, wires, cap, rotor, coil, fuel filter, fuel sock in the tank. Don't necessarily need to replace them, at least give them a look to see if they need to be replaced. Look around at vacuum ports, make sure everything is hook up or covered. Hook up the vacuum gauge and make sure everything looks good.

Test the fuel pump. Pressure is a part of the requirement. Flow is more important. It should pump a pint in under 30 seconds. I prefer to have an old school mechanical fuel pump unless you have a newer engine without the hole for it.

If it's the factory carb (with the original jets, rods, spring, float level), on a factory engine, it should be fine.

How high is high RPM? Is this issue only in top gear, or are you getting stumble at high RPM in any gear?

[External]

Thank you for your reply.
I will go over the list of things you suggested, follow your advice.
Then start another thread.
I really appreciate the help.