Here is one procedure for stock Solex's that I snagged from somewhere sometime. I also have a PDF from VW in their Look Listen Do It Better slide series. How can I get it posted to this forum? I use a print I made of the PDF myself.
Adjustment of "Two-Screw" Carburetors
For carburetor description and general information read this post first:
Carburetor Adjustment -- General Notes
Before we begin to adjust the carburetor, the valves, points, and timing should be set. This is important, and they should be done in the right order, as you start with a cold engine, and end with it warm.
Note: The correct idle speed is important with the 34PICT/3 carburetor, which is more complicated (and more sensitive) than the earlier types. It has three separate fuel circuits in it (only two in older carburetors), and the 850-900 rpm idle is designed so the airflow through the carburetor is balanced for the idle circuit fuel flow. That's why it has both Volume and Bypass screws in the side (the earlier ones had only Volume screws). This way the idle speed can be set correctly using the Bypass screw without touching the screw on the throttle arm, which has to be set exactly right.
1. The engine should be warmed up, but switched off. On the left side of the carburetor you will find the throttle arm, which is controlled by the accelerator cable that runs to the accelerator pedal in the cabin. On the top of the throttle arm, facing to the rear of the car, is a screw called the Fast Idle Adjuster. This works with the choke to give a smooth idle on a cold engine. As the choke warms (in concert with the warming engine, hopefully) the butterfly valve in the throat of the carburetor opens and the Fast Idle Adjuster moves down the stepped cam, reducing the engine idle speed.
Note: Screwing the Fast Idle Adjuster screw in more will increase the idle speed, but doing so messes up the Volume Control and Bypass Screw adjustments. This destroys the idle geometry, and the car won't run right.
2. Again, make sure that the choke is fully open and the Fast Idle Adjuster screw is resting on the very bottom of the stepped cam.
3. Unscrew the Fast Idle Adjuster screw until it is clear of the stepped cam. Screw it in until it JUST touches the very bottom of the stepped cam -- NOT on any of the steps themselves. Now screw it in another 1/4 turn -- no more! This sets the throttle butterfly open the required 0.004 inch, so you can use the Bypass Screw (read on) to set the idle speed correctly. From this point on, leave the Fast Idle Adjuster screw alone.
4. The carburetor is adjusted with the Volume Control Screw and the Bypass Screw on the left side of the carburetor.
Note: Before setting the Volume Control Screw per the step below, turn the Bypass Screw (the larger one) out a couple of turns, just to get things started.
5. The Volume Control Screw is the smaller of the two adjusting screws. Screw it in GENTLY until it bottoms -- you don't want to open up the hole. Now unscrew it exactly 2-1/2 turns. This is the starting setting.
Note: Though you want to be careful to not screw the Volume Control Screw in too far, you also want to make sure that it is initially firmly seated before unscrewing it as specified. If you don't start with the Volume Control Screw firmly seated, you may have trouble adjusting the idle with the Bypass Screw, to the point where you may have it turned all the way in and still have the idle too high. This condition will cause stumbling on acceleration if not corrected.
Also Note: The Volume Control Screw is NOT used to set the idle -- that's the job of the Bypass Screw. I would set the Volume Control Screw at 2.5 turns, then go to the Bypass Screw and turn it whichever way (most likely out) that will give you approximately the idle you want -- this is a starting point. Then return to the Volume Control Screw and set it according to the procedure. Then back to the Bypass Screw to set the idle at exactly 850-900 rpm.
6. With the Volume Control Screw out 2.5 turns, start the engine and use the Bypass Screw to set the idle at 850 rpm (fast idle if you don't have a tachometer). For a semi-automatic car, use 900 rpm.
Note: See our Tune-Up Procedure for instructions on how to attach and use a dwell-tachometer.
7. Go back to the Volume Screw and adjust it slowly to obtain the fastest idle (usually out -- counter-clockwise). It should not be much outside the range of 2-3 turns (1/2 turn in/out from the basic 2-1/2 turn out setting). Then turn the screw back IN very slowly until the revs drop by about 25-30 rpm (slightly leaner). If you don't have a tachometer, listen until you can just hear the revs start to drop, maybe as little as 1/8th turn on the Volume Screw.
8. Now use the Bypass screw again to reset the idle speed to 850 - 900rpm.
Note: If you find it difficult or impossible to make these settings, it is possible that you could have stripped threads on any of these adjusters, a damaged hole for the tapered screw, or a damaged needle valve or O-ring.
It is also possible that you have a vacuum leak (i.e., leaking of air into the intake manifold). If there are any holes in the manifold or at any of the connection points, then air can be sucked into the manifold, causing the fuel-to-air mixture to become too lean. This can cause (among other things) adjustment of the carburetor impossible. See our Discussion of Air Inleakage, which includes diagnosis and repair.
That's it - you're done.
You should now have an engine which purrs like a kitten.
This article was submitted by Big Dirty Dog
