Before the 1980's almost every engine used a carburetor to supply the required mixture of gasoline and air. Over the years carburetors evolved to become complex assemblies with hidden internal passages designed to keep the engine running smoothly across it's rev range. As a result they can seem a mystery to those unfamiliar with their workings. This guide specifically addresses the Solex 34 PICT carburetor used on the 1971 – 74 VW. However, the carburetors on earlier models are similar so the general advice provided here should prove useful to owners of vintage VW's..
Before you start
A word of caution: most suspected carburetor problems usually turn out to be ignition problems. In other words, if the engine in your VW is not running well, the carburetor is the last component you should work on. Instead, start by following these steps:
Set valve clearances, with the engine cold
Set the points gap (replacing points if necessary)
Set the ignition timing
Only when you these are correct should you set about modifying carburetor settings.
Application and history
The 34 PICT-3 was used on the Volkswagen “Beetle” from 1971 until 1974. It evolved from the 28 PICT which became the 30 PICT in 1968. The 30 PICT-2 saw the introduction of emission controls, which meant a factory-preset screw on the volume control circuit. The subsequent 34 PICT-3 then gained a fuel cutoff solenoid, which necessitated relocating the air bypass screw.
Carburetor model numbers can usually be seen stamped on the side of fuel bowl.
In the 34 PICT-3, in common with all carburetors, air and gasoline are drawn in by vacuum created by the pistons descending in the cylinders. Two jets, termed the pilot jet and the main jet, allow gasoline to flow from the fuel bowl. A throttle butterfly valve in the carburetor controls engine speed by restricting airflow. Upstream from this sits another butterfly valve, the choke. With the engine cold the choke is partially closed off, making the fuel-air mixture richer so the engine can run.
The choke is also connected to the throttle butterfly to provide a “fast idle” function, (meaning it holds the throttle open,) via a stepped cam. This allows idle speed to be progressively reduced as the engine warms up.
At idle the throttle valve is almost completely closed, shutting off airflow. So that the engine can idle a bypass circuit allows the fuel-air mixture into the inlet tract just below the throttle butterfly. This circuit consists of two drilled passages, each of which has a screw governing the volume of fuel flowing. These two screws can be seen just above the fuel cutoff solenoid. The lower, smaller diameter screw was originally preset at the factory and capped off. This is the volume control screw and consists of a long, thin needle. The larger screw governs the air bypass and was always intended to be technician-adjustable.
The only tools needed to set up the carburetor are a flat-blade screw driver and a tachometer.
Carburetor adjustment procedure
Here are the steps involved in setting up the carb. (Notice the iterative nature of finding the optimal screw positions):
Fast idle adjustment
Volume control screw initial adjustment
Idle speed initial adjustment
Volume control screw final adjustment
Idle speed final adjustment
1. Initial checks
With the engine thoroughly warmed-up, remove the air filter and check the choke is fully open. If not, the choke mechanism should be adjusted before making any other changes.
Verify that all vacuum hoses are connected.
With the engine running, look for leaks in the gasket at the base of the carb. One way to find these is by spraying a small quantity of carb cleaner around the gasket. A leak will draw this in, momentarily increasing engine speed. Any leak must be dealt with before proceeding.
2. Jet cleaning
Unless the carb has recently been rebuilt the two jets should be taken out and cleaned to ensure there are no blockages. (Even just a partial blockage will prevent the carb and engine from responding to the changes you are going to make.) A blast from an air line is usually sufficient although stubborn deposits may need an application of carb cleaner. Replace the jets when they are dry.
3. Fast idle adjustment
At idle with the engine warm the throttle butterfly should be almost completely closed. This is set by the fast idle screw which acts on the stepped cam.
Adjust the screw so there's clearance of 0.003”, then turn it inwards a half turn. This sets the throttle opening.
4. Volume control screw initial adjustment
Carefully screw this in until you can feel it seating. Do not over tighten as that will damage the carb body. Then back it out 2 ½ turns. This is the initial setting.
Hook up the tachometer, replace the air filter and start the engine.
5. Idle speed initial adjustment
Idle speed is set by adjusting the larger screw. Turning it in reduces the volume of fuel-air mixture flowing, turning it out increases the volume. Turn the screw slowly while monitoring engine speed. The target is around 850 rpm.
6. Volume control screw final adjustment
Return to the small screw and turn this slowly to increase engine speed. (Usually outwards.) It should only need less than a full turn.
Find the highest speed then turn the screw inwards so that the revs drop slightly.
7. Idle speed final adjustment
Go back to the bypass screw and turn this to set the idle speed back to 850 rpm.
If your engine does not now idle smoothly there may be a deeper underlying problem. If all has gone well though, it will be running like a sewing machine.