34PDSIT Carburetor Adjustment

by Jim Thompson

Craig Standley wrote:

Ok, the way I understand it, the screws on the stock Solex carbs (1974 Bus, 1.8l type IV) only adjust the mixture at idle. The bus is running a bit rich, so I want to lean it out a bit. How do I do this? It is rich at idle, so I'm going to adjust the idle mixture screws (out is lean?), but it is giving off a bit of smoke when I'm driving (plus it smells rich if you happen to be driving behind me).

Jim Thompson responds:

Be careful when doing this with the 34PDSIT carburetors and here's why - The screws into the Throttle Body (base) are used primarily for fine tuning the "Balancing" of the two carburetors so that they are in "sync" - Generally two to as many as 5 turns out COUNTERCLOCKWISE from bottoming out. A device called a "Unisyn" or similar animal is used for balancing the carbs.

Idle mixture control is handled only on the LEFT Carburetor - there are two screws (with O-Rings) on the top of the carb. The large one adjusts the idle (called an Air Bypass Mixture Screw) and the smaller one (called a Volume Control Screw) is the one used to set the idle mixture. The Volume Control Screw is usually set at about 1 1/2 to 3 turns COUNTERCLOCKWISE from bottoming it. OUT is RICH - IN is LEAN and only during idle. There is nothing on the right side carburetor that will allow for setting the Idle Volume Control like the left side does - it's pre-set on that carburetor.

There is an additional screw on each carburetor attached to the linkage called the Throttle Stop Set Scew. This is to be pre-set by turning the screw CLOCKWISE until it just touches the Throttle Stop on the carburetor, then an additional ONE turn further CLOCKWISE. Do this with the Choke in the OFF (Open) position.

The Bentley Bible is the best source for setting the carburetors step-by-step. See Section 3, pages 9 thru 25 for the basic overview. Most other books I've looked at regarding the set-up of these carburetors is rather poor and a waste of time.

Running rich symptoms usually mean some of the Air Bypass drillings or passages, both in the idle and power circuits within the carburetor, are clogged up with dirt and junk - the carburetors should be cleaned out or better yet rebuilt. Loose Idle Pilot Jets or Idle Cut-Off Valves can also cause these problems as well. Other causes are the Intake Needle/Seat isn't closing properly (dirty), Float is leaking (doesn't happen very often, but sheesh - these are over 20 years old) and too high of fuel pressure from the mechanical pump. If you're running an electric fuel pump, fuel pressure may be too high and require a pressure regulator as well - pressure shouldn't be more than 2 to 3 psi.

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