The 34 PICT is a good carb but if not set up properly will not tick over. Forget about volume and mixture screw you have to set the throttle arm first or you will always have problems with tickover. Here's an article about setting up your carb.
Before we begin, a short note about the "Haynes VW Beetle & Karmann Ghia Repair Manual": For those using the earlier edition of the manual, there is an error on the exploded views of the 34PICT/3 carburetor. The designations on the adjustment screws on the left side of the carburetor are reversed. The larger screw on the top is the Bypass Screw, and the lower (smaller) one is the Volume Control Screw - not the other way around as the older Haynes Manual indicates. Other than that, we have found the Haynes Manual to be an excellent resource.
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), located on the left side of the carburetor. 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.
Make sure that all the hoses are in place and the gasket at the base of the carburetor is sealing properly (no vacuum leak).
Install the air cleaner (the carburetor expects it to be there).
Turn on the engine and run it until it is warm, then switch it off.
The first step is to set the throttle plate. Back off the Fast Idle Adjuster (also called the throttle screw) located at the top of the throttle arm. You will find the throttle arm on the left side of the carburetor, connected at the bottom to the accelerator cable, which runs forward to the accelerator pedal.
Note: Fast Idle Adjuster is a misnomer - this screw it is NOT used to adjust the idle speed. The Fast Idle Adjuster 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 screw moves down the steps of the choke fast idle cam, reducing the engine idle speed. 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.
With the choke held in the full open position (stepped cam at its lowest point), place a piece of notebook paper or a 0.003" feeler gauge between the lowest step of the choke fast idle cam the the Fast Idle Adjuster screw.
Slowly turn the screw in until you feel drag on the paper or the feeler gauge. Then remove the paper or feeler gauge and turn the screw in preciselly 1/2 turn - no more! This sets the throttle butterfly open the required 0.004", 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.
Next set the volume of gas available at idle speeds. This is done using the Volume Control Screw. Please note that the Volume Control Screw controls the AIR volume, not the fuel volume. Screwing it in reduces the air and makes the fuel/air mixture richer. And of course turning the Volume Control Screw out increases the concentration of air and makes the mixture leaner.
Note: The Volume Control Screw is the smaller of the two adjusting screws, located on the left side of the carburetor just above the Idle Cutoff solenoid (which has a black wire from the positive side of the coil attached to it). The Volume Control Screw is NOT used to set the idle speed - that's the job of the Bypass Screw.
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.
Screw the Volume Control Screw in GENTLY until it bottoms out - you don't want to open up the hole. Now unscrew it exactly 2-1/2 turns. This is the initial 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.
With the Volume Control Screw out 2-1/2 turns, start the engine and let it warm up. (Make sure the automatic choke is fully open.)
Now to set the idle. This is done by controlling the volume of air going by the Bypass Screw. The Bypass Screw is larger than the Volume Control Screw and is located a little above and to the left of the Volume Screw.
Note: Again, the idle speed is NOT set with the Fast Idle Adjuster on the top of the throttle arm as it is on the older 28 and 30 series carburetors - though its name (Fast Idle Adjuster) would lead you to think that its used to set the idle.
As a starting point, turn the idle Bypass Screw whichever way (most likely out) to set the idle at 850 rpm (fast idle if you don't have a tachometer). For a semi-automatic car, use 900 rpm. (It's far better to have the idle speed too fast than too slow.)
Note: See our Tune-Up Procedure for instructions on how to attach and use a dwell-tachometer.
With the engine warmed up and the choke fully open, go back to the Volume Screw and adjust it slowly to obtain the fastest (smoothest running) idle speed (this is usually out - counter-clockwise). You should not turn the screw out 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 Volume Control screw back IN (clockwise) very slowly until the engine speed drops by about 20-30 rpm (slightly leaner). If you don't have a tachometer, listen until you can just hear the engine speed start to drop, maybe as little as 1/8th turn on the Volume Screw.
Go back to the larger Bypass screw again to reset the idle speed to 850 - 900rpm. (Again, the fast idle is better than too slow. You want it just a little on the rich side. Too slow of an idle speed can gause the engine to overheat.)
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. Your engine should be purring like a kitten!
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Hope this helps Robert
