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: