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Tuning Solex 40PII-4 Carburetors

September 28, 2010 | Troubleshooting & Repair
By Harry Bieker



Introduction & Requirements

Solex's are easy to adjust when compared to other Porsche induction systems. But they are much harder to adjust than American two barrel carburetors. It takes a certain feel.

Before attempting to adjust the carbs, the following must be correct:

  • the engine must have correct cam and ignition timing
  • the valves must be adjusted properly
  • the points, plugs, distributor cap and ignition wires should be in like new condition
  • the distributor must be mechanical advance
  • the compression difference between cylinders should not exceed 20 PSI
  • an engine compartment fuel filter should be in place



Note: Red type denotes components most commonly used in adjusting carbs

1 - Retaining screw
2 - Passage where high speed mixture enters air stream
3 - Needle & seat (after market shown, original has a spring loaded ball end)
4 - Fuel inlet banjo bolt (hollow bolt)
5 - Cover
7 - Injection nozzle (squirter)
8 - Body
9 - Idle jet
10 - Float level adjustment screw & lock nut
11 - Accelerator pump level
12 - Accelerator pump quantity adjustment nut & lock nut
13 - Accelerator actuation lever
14 - Accelerator pump control rod
15 - Main jet carrier (main jet screws into inner end)
16 - Mixture screw adjustment
17 - Idle stop screw & spring
18 - Throttle shaft
19/20 - Throttle lever
21 - Pump jet
22 - Diffuser (aux venturi)
23 - Air correction jet


  • Front, back, left, right
    Defined from your position as you sit in the drivers seat


  • Throttle body
    The main part of the carburetor.


  • #15 - Main jet carrier & main jet
    Where Porsche special tool P78 is screwed in to check level of fuel in the bowl.


  • #10 - Float level adjustment screw
    Use to adjust the level of fuel in the fuel bowls.


  • #16 - Mixture screws
    These thread into the base of each throttle bore. They have springs to maintain a setting and there are two of them in each carburetor. Be careful as you turn these in, as they seat in the throttle body. Turning a mixture screw in leans the mixture and turning it out richens the mixture.


  • #17 - Idle screw
    Sometimes called the idle stop screw. There is one on the lever end of each carburetor and they determine minimum throttle setting through their action on the throttle arms. When these screws are turned all the way out, the throttle plates rest against the throttle bores.


  • #12 - Accelerator Pump Adjustment
    Used to adjust the quantity of fuel squirted on each stroke of the throttle.


  • Side to side balance
    Defined as equal vacuum between the two sides of the engine through adjusting the idle screws and throttle linkage.

Tools Needed to Adjust the Carbs


  • Stubby screwdriver
    To adjust the mixture screws and idle stop screws.


  • Synchronometer (Unisyn or similar)
    For measuring flow rate through the carburetor bore.


  • Two 8mm combination wrenches


  • P78 Tool
    A special Porsche tool that enables you to check the fuel level with the engine running.


  • Small measuring vial
    Useful for measuring the accelerator pump injection quantity


Adustments Prior to Installation of the Carbs

While the carburetors are out of the car, fill them with fuel by pouring it into the "pipe" in the center of the carb. This accomplishes two things: (1) you will be able to start the car more easily after the carbs have installed and (2) you can adjust the injection quantity while the carbs are on your workbench.


Another Cheap Measuring Vial

Buy a plastic nose dropper from a local drug store and discard the rubber bulb. Glue shut the small end with super glue or a hot clothes iron. Pierce the side of the vial near the big end and attach about 6" of small wire as a handle. From a local swimming pool supply company or Home Depot buy a small (2") bottle of red or yellow chlorine or pH testing chemical. When turned completely upside down 1 drop of either chemical equals 0.1 cc. Mark your new vial accordingly.

Submitted by Tony Ryan

The injection quantity is checked with a narrow CC measuring vial. You can purchase one of these from Porsche,, or you can make one yourself using an old glass fuse. Remove the top of the fuse, solder a paper clip or piece of wire to it, and you have a working device.

Place the vial in the throttle bore under the squirter to catch the gas as it comes out of the accelerator pump nozzle. Check manual for correct amount of fuel per stroke -- sometimes it's more accurate to use a number of strokes and divide by the number of strokes. When stroking the throtle, open the throttle smartly and hold open until squirting has stopped before attempting another stroke.

The injection quantity can be varied by adjusting the length of the stroke using the nuts on the accelerator pump adjustment threaded pull rod (#12).

Note: Many believe that the factory settings are too high for todays gas, and can cause a flat spot upon acceleration. You may want to experiment with using a somewhat lower volume.

One more thing to do before you install the carbs. Go ahead and make the initial adjustment of the mixture screws (#16). Turn until fully seated, then turn 1 1/2 turns back out.

Preparing to Install and Adjust the Carbs

These instructions start with the engine thoroughly cleaned and manifolds installed with new gaskets. The linkage should be clean and high temperature grease added to the ball sockets. An 8mm open end wrench can be used as a ball joint separator. New gaskets are used between the manifolds and spacers and between the spacers and the carburetors (use only genuine Porsche or equal gaskets. A new gasket should go between the carburetor and the air cleaner.

Next the linkage should be hooked up, except for the short ball jointed pull rods. Find the idle stop screws and turn them out until the throttle plates rest against the throttle bores. Now turn the screws in until they just touch the throttle arm, and then turn 1/2 turn more. Now adjust the pull rods so that they can be snapped on without disturbing the throttle setting. This should give the carbs side to side balance. Now snap on the pull rods.

Oil the throttle shafts and accelerator pump linkage (and redo everyb 3000 miles) and check for smooth operation. Next, have someone get inside the car and floor the gas pedal while you are looking into the carbs. This is to make sure that the throttle plates open all the way to vertical (but be sure they don't go past vertical).

You can now hook up the fuel lines. If the original braided fuel hose in the line to the carbs is old and ragged, and you don't want to buy the complete set-up from Porsche, you can remove the entire assembly from the car, cut about 1/2" from the metal lines, and install regular neoprene fuel hose and "European" type hose clamps. Don't forget the filters. The German VW Type 3 fuel hose is safer than the stock hose. Be sure to check the metal lines for wear if they rub on sheet metal, etc.

Adjusting the Carbs


  1. Check initial setting of idle screws (#17)
    Idle screws should be 1/2 turn out after touching arms.


  2. Check initial setting of mixture screws (#16)
    Mixture screws should be 1 1/2 turns out from seating.


  3. Warm up engine
    Start engine and warm to at least 140 degrees F. If you haven't filled the fuel bowls of the carbs it may take some cranking of the engine to do so.


  4. Set the fuel level
    You first install the P78 gauge (see below for alternative method that does not require the tool). It screws into the float bowl after you remove the main jet carrier and jet (#15) on the outside of the bowl. Be sure to remove the jet from the carrier and install it into the end of the gauge before installing the gauge.

    Adjust the fuel level by means of the float level adjustment screw (#10) immediately above the accelerator pump block. Loosen lock nut and turn in to lower fuel level, turn out to raise the fuel level. You should take your time with this as the bowl is large and it takes a while for it to stabilize. When turning in the screw, do so slowly, as you may bend the elevator if you go too fast. Lock the adjusting screw when finished. After setting the fuel level, don't forget to remove the jet from the tool and install it back into its carrier. Do both carburetors.

    Alternative Method that does not require use of the Porsche special tool.

    There is a small bolt located at the top middle of the fuel bowl, on the side of the carb facing the engine. Carefully remove this bolt (good idea to put a rag underneath as it's easy to drop and easy to lose). If no fuel drips out, then you turn out the float level adjustment screw (#10) slowly to increase the level of fuel in the bowl. At some point a bit of fuel will drip out of the hole. At that point, turn the float level adjustment back in just to the point where fuel no longer drips out.


  5. Set idle at 1000 RPM
    Evenly turn the idle screws (#17), equally on both carbs, until the tach reads 1000 RPM. Check the side to side balance with the Unisyn to make sure both sides pull the same vacuum. Adjust the idle screws if need be.


  6. Adjust mixture
    Pick a barrel and turn the mixture screw (#16) in until the engine speed drops and then slowly out until the engine runs smoothly. Remember to let the engine speed stabilize after each adjustment. If you find that the engine runs smoothly at less than the 1 1/2 turn initial setting, then you can turn all the other mixture screws in a like amount and start over. If you need more than 1 1/2 turns to make it run well, then do it -- but if 3 1/2 turns or more are required it usually means you need bigger idle jets.

    If the engine doesn't respond when you turn one of the mixture screws in and out, it means that the cylinder is not getting ignition, or it is not getting mixture. If the ignition and compression check out, then an idle passage is plugged. Remove the mixture screw and spring and blow comopressed air into the mixture screw hole to clear the obstruction. (Caution: DO NOT blow into the top vent.) If this doesn't work, then the idle passage may need to be "boiled out" or the passage checked with a duct gauge.


  7. Adjust throttle plates
    After you have the idle mixture initially adjusted, you can adjust the throttle plates with your Unisyn. Find the barrel that is pulling the most vacuum (on #1 carb) and adjust the floating ball in the Unisyn to the center level by using the adjustment on the Unisyn. Then:

    Split-Shaft Carbs
    Adjust the other barrel of the carburetor to an equal vacuum using the small screw below the fuel bowl and between the throttle bores.

    Solid Shaft Carbs
    You must hold the throttle lever and by means of a 5MM open end wrench on the forward end of the shaft and twist shaft in appropriate direction to create equal vacuum. Be sure the throttle is open a small amount when twisting shaft so as not to jam the throttle plates in the bores. Do both carbs.


  8. Readjust mixture
    Readjust the mixture screws until you get the correct adjustment. Usually you will turn the mixture screws in until the engine slows down and then out again slowly until it runs smoothly -- and then perhaps another 1/2 turn. You want the maximum speed with the least amount of fuel. It takes some practice.

    Sometimes as you turn the screw out, it speeds up the engine enough to advance the timing, which adds 300-400 RPM. When you turn the idles screws back a hair to correct this, the weights in the distributor flop back, which slows the engine. This may mean that you need a distributor rebuild. It's best to get your initial adjustment around 1000 RPM and hope the distributor doesn't intercede.

    Be sure to check the side to side balance each time you adjust the idle screws. remove the right pull rod.


  9. Set idle at 850-900 RPM
    When you have finished adjusting the mixture screws, back off the idle screws evenly to 850-900 RPM and recheck balance. Adjust the rod that attaches to the right carburetor throttle arm until it will snap on without moving the throttle setting.


  10. Check the balance at 3000 RPM
    Be sure to also check the balance with the engine running at 3000 RPM.
That's it -- your carburetors should be purring like kittens. Take a long test drive!


Editor's Note:

This is part of our module on how to tune up your 356. Components include: