912 Carburetor

My car has the original Solex carburetors. The common change is to convert to Webers. These Solexes have been rebuilt in the past few years, so making the conversion is not a pressing requirement.

Gas Leaks

I've had to fix gas leaks around the carburetors three times in the last 4 months. The latest fix was to tighten four different brass plugs on each carburetor. In the diagram below Nos. 15 and 21 on each half on both sides were loose enough to leak. The problem has gotten progressively worse over that last month, finally affecting cold starting. These fittings were loose enough that the gas would leak out of the float bowls overnight which makes starting hard until gas gets fed back into the float bowl. You can touch each of these fittings after driving the car. If your finger picks up some wet fuel, tighten the fitting. After fixing this, I was delighted to have the car start instantly after sitting for 15 hours.

Rough Idle

If the car is idling roughly, there may be a problem on the balance between the front and back of one of the carburetors. Check with the air mass gauge. If they do not balance, the throttle valves need to be adjusted so the both close at the same point. 912 cars before 1968 have a single shaft linking the two throats. Newer cars have a split shaft that is linked together. If a single shaft, the alignment is fixed by twisting the shaft until the valves seat at the same time on both throats. Open the throttle half way before twisting to ensure you do not damage the butterflies or the throat. On later models, there is an adjusting screw that can be seen with a mirror. The engine can be running when the adjustment is made to save time.

- Found in Up-Fixin der Porsche Vol III, p 8.

The Solex 40 P II

There is a description of the Solex 40 P II-4 Carburetor in the factory manual.

Readjusting Idle Speed

The following instructions apply only to a simple readjustment of the carburetors. A thorough carburetor adjustment requires the use of a synchronizer for achieving accurate settings on all cylinders.

  1. Check spark plug gap at 0.6 mm or .024 in.
  2. Make sure that all throttle valves close in unison and that the linkage is not binding.
  3. Run engine until warm.
  4. Increase engine RPM by resetting idle stop screws.
  5. Adjust idle mixture on both carburetors by first fully closing the screws and reopening by 1 1/2 turns, then closing or opening, as may be required, until reaching a point where the engine runs smoothest and fastest. However, the adjustment screws must never be left in fully closed position.
  6. Back off the idle speed stop screw until normal idling speed is reached. The engine should not die when depressing the clutch or quickly shutting the throttles.

Idle Speed

Should be 800 to 900 RPM, but my car idles about 1200.

"All carburetors are set at the factory and are adapted to individual engines using premium grades of fuels."

Adjusting the Accelerator Pump

From the factory manual:

  1. Adjust the idle speed.
  2. Run engine to fill float chamber with fuel.
  3. Stop engine, remove both air cleaners
  4. Work throttle arm until air bubbles cease to show at the pump injection nozzle.
  5. Hold calibrated vial (P 25a) at the tip of the nozzle and quickly move throttle arm to times from stop to stop.
  6. Check injection quantity, empty the calibrated vial, repeat procedure on the second injection nozzle.
  7. Injection quantity from each nozzle on two pump strokes should be 0.45 cc during the warm season and 0.65 cc during the cold season.
  8. Check injection in the second throat.
  9. If required, readjust injection quantity by resetting the adjusting nut on the pump rod. If adjustment should not be possible due to lack of threads, insert a spacer between the pump arm and the nut.


Fuel squirting from the pump nozzle should not strike the pre-atomizer nor the venture and must pass through the slit between the carburetor wall than throttle valve.

Should it become necessary to bend the injection nozzle, ensure that its tip remains at the same height.

Injection quantity as well as the moment of injection must be identical on all carburetor throats.

The Following...

Topics are from past issues of Porsche Panorama

Flat Spot

Q: Most (pre-1969) 912 engines have, to varying degrees, an acceleration flat spot at low rpm that in some cars borders on a dangerous situation. When accelerating from a stop the engine will go well up to about 1500 to 2000 rpm and then lose power and falter badly. Private tests with an exhaust gas analyzer have shown that the accelerator pumps on these problem cars inject the fuel in about half the time that should exist. Recommendations? Does Solex make smaller orifice nozzles to increase the injection time?

A: Usual cause is incorrect aiming of the nozzles down the throat. The '69 912 should not have the problem because the carburetors have been redesigned.
Last modified: Wed, 09 Jan 2002


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