Rebuild a Q-jet (How to)

[oldgold70c10]

I hadn't seen anything on the board on this, so I thought I'd give it a shot.

This covers a 4M carb, the early style of Q-jet that started in 1965, and was used on cars to 1974, and trucks to 1978. If there is interest, I'll do one on the M4M, which came out in 1975. Let's start by identifying the 4M type over the later M4M Q-jet, which appeared in 1975.

The carb on the left is a 1980 Chevrolet M4M Q-jet. It is typical of the later non-electronic Q-jets. It is rated 850cfm. The one on the right is a 1974 Chevrolet 4M Q-jet. It is typical of the earlier Q-jets and is rated 750cfm, though some BOP 4M's were 850.


So, let's start.
Disassembling
Start by removing the secondary metering rods and hanger. A single screw holds the hanger in place.

Secondary rods &hanger:

Next remove the clip on the choke rod and pull it out of the hole on the choke lever.


The choke linkages on the side of the carb are held on by a single screw:

Remove it and disconnect the link to the secondary air flap:


You can also remove the secondary lock-out lever.
If you've got an older Q-jet the linkages on the side might look like this:

But removal is basically the same.
With that removed, you can take out the inner choke link:

Next, the accelerator pump linkage needs to be removed. The best way to do this is to push the roll pin back just far enough that the lever can be removed. Don't push it all the way back, you'll want to be able to pry it back with a screwdriver later:


Then, you can remove the link where it attached to the throttle lever. It has a little tab that must pass through a keyhole. Rotating the throttle slightly helps get it out:


Now you can remove the 9 air horn screws. Be sure not to forget the two inside the choke are (they are very shiny in this picture):

When you remove the airhorn, pull it straight up and set it aside upside down so you don't damage the tubes that stick down below it.
Next remove the gasket. You'll want to pull the front end up so it goes around the primary metering rod hanger. The gasket should come off in one piece. Save all old gaskets so you can match them up with the new ones in the rebuild kit.

The metering rods and piston/hanger assembly are easy to remove, if it didn't already come out partially with the gasket. Just push it down and let the spring pop it up a few times. A little plastic that collar holds it in place will come out and you will be able to remove it and the spring under it.


Next remove the accelerator pump and the spring below it:

And then the baffle:

Then the float assembly:

[oldgold70c10]

Continued
Now remove the accelerator discharge check ball by removing this screw:

And then turn the carb upside down and catch the ball:

Remove both main jets:

And, using a wider screwdriver than I did here (this is a parts carb, so I didn't worry too much), the needle valve seat and gasket:

That's everything inside the float bowl.
So, turn the carb over and remove the idle mixture screws:

And then the 2, sometimes 3 screws that holds the base to the bowl:

Then remove the gasket:


Your Q-jet is now apart! You can soak all the metal parts in carb cleaner, except the choke pull-off. Next, let's look at a few things to check.

[oldgold70c10]

Things to Check
After you've got everything clean, you can check for problems.
First, check the base. Most important is the primary throttle shafts. Move the throttle shaft up and down and front and back. You shouldn't feel any play. If you do, now is the time to have bushings installed. Also check that the secondary butterflies snap shut when you open them and let go. You do not need to check for play in the secondary shaft, just make sure they snap shut when released, and close tightly. Also use a straight edge and make sure the base is flat, or very close. The thick gaskets used will compensate for some warpage, but not very much.

Next, look at the bowl. If you've got bowl plugs like these:

chances are good you do not have a leaky carb. Until about 1969, the plugs were basically mini-freeze plugs and they leaked very badly. After about 1969, plugs like what is shown above were used. You can check these for leaks by using an air gun from the other side and soap and water on the plugs. A leak will show up as a steady stream of bubbles. You can use epoxy to repair the plugs if they leak. I have never had a Q-jet with this style bottom plug leak.
Check and make sure the dowel pins aren't broken off:

There are 2 on the bottom and 2 on the top. If one is missing, it is no big deal, but use extra care when assembling the carb that everything is lined up.
Check the bowl for flatness just like the base.

Last, look at the airhorn. All the brass tubes pressed in should be tight. If one is loose, use the butt end of a scewdriver and tap it back in.
Look at this airhorn:

One of the secondary discharge tubes is missing!
The secondary air flap should be free and not bind, and snap shut from any angle. The choke flap should not stick or bind.
And of course, check the airhorn for flatness.

Other tips.
Use a brake cylinder hone on the accelerator pump bore, it will increase its performance & life.

Save the gaskets and match the new ones carefully, since several will often come in a kit:

[oldgold70c10]

Putting it back together
Don't you just hate those instructions that say putting it back together is the reverse of taking it apart? Well, putting the Q-jet back together is basically a reversal of taking it apart, however, here are a few hints to make it go more smoothly.

First is the float assembly. This is how the needle should clip to the float arm:


You will also have to adjust the float height during reassembly. To do this, use one finger and press down on the float pivot:

Using another finger, gently but firmly push the needle end of the float down:

While doing this, measure at the point specified in the specs that came with the kit. The brass and plastic floats both have different points to measure from, but the measurement you are looking for will be the same. Use the measurement spec for your carb number exactly.

The next problem point can be the primary rods on the hanger. Sometimes there is a little spring on the hanger to help hold the rods. The spring sould be installed like this:


The little plastic collar on the metering rod hanger should be a snug fit in the hole in the bowl. If it is not, you can lightly peen the metal so it stays in place.

After that, the next frustrating point for some can be the top gasket, and getting it over the metering rod hanger. An easy way to do it is like this:
Bend the little flap down:

Then get it started over the hanger:

And then slide the gasket to the front:

And that's it:


After putting the airhorn back on, the hardest part left to do is re-installing the inner & outer choke linkages.
Get the outer stuff ready first.
The next two images show how the fast idle cam should go on the side choke linkages:


You'll also need to re-install the lock-out lever:


Now lower the link and lever into the slot:

Until it is visible in the hole on the side:

And hold the side assembly up and put the shaft into the hole:

And "pinch" the lever with the slightly pointed end of the shaft:

With the lever pinched, you'll be able to rotate it by pushing the rod down (shown here with the lever & rod outside the carb) so that the flats will line up and the two pieces will go together.


Re-assembled linkage:


After the carb is assembled, you'll still have to do a few choke adjustments.