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Solex Split Shaft Carburetor Conversion

September 27, 2010 | Troubleshooting & Repair

By John Jenkins


Click for larger imagesThe single shaft Solex 40 PII were first used by Porsche in the 1955 1500cc Carrera engines installed in the 356. The 40 PII-4, came with the first Super 90 engines in late 1959. It was chosen because of the increased performance parameters of the Super 90. The 28mm venturi size of the Zenith 32NDIX used on the 1600 Super engines was not large enough to support the extra breathing capacity of the Super 90 engine. The following equation from the Solex Selection and Tuning manual explains why.

d - required diameter of the carb in mm Vh - the capacity of each cylinder in cm^3 n - the peak revolutions in 1000 rpms per minute

for 1, 2, 3, or 4 cylinders:

d = 0.82 x square root of (Vh x n) d = 0.82 x sq rt (400 x 5.5) d = 0.82 x 46.9 d = 38.5 (for 1720 cc engine d becomes 39.9 mm)

There is a simple multiplier, 0.8, to obtain the choke or venturi size. The results is the Solex 40-PII4, viz., 32 mm.

Split Shaft and Single Shaft, Compared

Now lets look at the differences between the split and solid shaft versions of the Solex type 40.

As the name implies, the original Solex 40 PII-4 had one single solid shaft, with two butterflies. The split shaft consists of two independent shafts, each controlling a single butterfly, in one carburetor. They also have different idling circuts and jetting.

Solex originally designed the split shafts for Porsche for the late 912 series, to address stricter emission control regulations that came into effect during the late 1960's. The split shaft Solex, compared to the solid shaft version, has a better transition from idle to main jet operation. When one looks at the bottom of the carb and opens the butterflies, you will notice a series of holes called by-pass holes or ports. On split shafts, there are 5 of these vs. the 3 on solid shafts. These additional by-pass ports, along with the different idling circuit, give the split shaft Solex the smoother transition from idle to main jet operation.

Split Shaft Weak Point

So, if the split shaft version was so superior why do drivers want the single shaft version? The split shaft Solex's two independent shafts are controlled by a central mechanical device which synchronized the butterflies so the airflow is equal in each throat. When these carbs were new, this device worked well. Now, being almost 40 years old, these devices are broken, have rusty screws, and the plastic block degredates, so ultimately, there is little butterfly control. This mechanical device also added to the complexity of the carburetors, making them difficult to rebuild and maintain, plus requiring a certain skill level to tune that many owner/operators haven't yet acquired.

The Fix Is In

We found a way to convert the split shaft to a single shaft and eliminate the mechanical device. In our modified version, the single shaft is made from chrome moly steel and supported by four bearing bronze bushings (originals were brass). Now you can have all the advantages of the split shaft Solexes; extra power and a smoother transition, with the relative ease of adjustment that single shaft Solex are known for. In addition to new shafts, bushings and butterflies, these conversion carburetors include a new throttle rod, throttle link, and link spring.

(See photos below) The throttle lever used is the same so the split shaft carb looks like a split shaft but if you look underneath you will see a big difference. See the simple throttle link in lieu of the complex adjustment device on the split shaft. Note also the split shaft style idle volume screws.

Photos by John Jenkins, email John@356carburetorrescue.com.


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