The Solex P40-I Spill Tube Carburetor
The Solex P40-I is the first carburetor on the Porsche 911. The six-cylinder Porsche 911 was first offered with this carburetor in 1964. The Solex is a factory fit for 1964, 1965, and 1966 model 911s. But in 1967 the Weber IDA carburetor is a factory fit.
For many years this carburetor was taken off the early models and replaced with Weber carburetors. At the time the Weber was the top of the line and considered an improvement over the Solex P40-I carburetor. Because of this, a lot of carburetors were shelved. Many were lost, and the remaining sets often have missing parts.
The Revival of Solex P40-I Carburetors
There has been a revival of early Porsche cars that is tapering off a bit now in 2020, but a few years back the car had to be stock to get the big bucks. So sets Solex P40-I suddenly had more value, not just full sets, but every special little part on them became essential. The demand has become so strong that many aftermarket companies started producing some of the parts that are in high demand. If this current atmosphere of restoration continues, you may see more and more Solex carburetor sets.
The Solex P40-I carburetor is getting old. Even if it has sat on a shelf, it is still aging. If it is in use, it has seen a lot of miles by now. Typically the repair time and cost to repair these carburetors is much more than other carburetor systems used on the 911.
What makes the Solex P40-I so different
The Solex P40-I was at the time a completely new way of delivering fuel to the engine. Usually, a carburetor has its immediate fuel supply in the fuel bowl located inside the carburetor itself. The problem with this is fluctuations in the fuel level due to acceleration, deceleration, and cornering can cause less than ideal fuel mixtures, as the height of the fuel level in the bowl will affect CO levels. Solex tried to eliminate these issues with the spill tube carburetor. Unlike the Weber or Zenith carburetors, the intake manifold and linkage system have to be considered part of the carburetor as well as the recirculation pump.
The Spill Tube System
The spill tube system allows for the fuel bowl to act as both the fuel supply as well as an overflow reservoir for the carburetor. The fuel bowl is part of the intake manifold. It is fed fuel from the gas tank via an electrical fuel pump. Fuel is then circulated through the carburetors via a mechanical fuel pump that is mounted on the front of the engine. This pump drives off the 1-3 bank camshaft. The mechanical fuel pump contains two pumps, one for each bank of carburetors.
By circulating the fuel through the carburetors, Solex was able to maintain the desired amount of fuel directly around the jet pack while allowing un-needed fuel to drain back to the fuel bowl. By maintaining a steady supply of fuel around the jet pack and returning un-needed fuel volume away from the jet pack meant that the effects of acceleration, deceleration, cornering, and altitude would be minimized.
Six Single Carburetors
The Solex P40-I carburetor set is like having six individual carburetors. The carburetors are also positioned directly above each respective intake runner on the engine. This provides optimal airflow characteristics. Each carburetor is also individually tune-able. Not only for not only idle mixture control but airflow and accelerator pump discharge volumes. Though many of these significant features should have revolutionized the carburetor industry, I believe they were also the downfall of this short-lived carburetor. The complicity of the fuel supply system can be challenging to diagnose as you effectively have three fuel pumps in the car. The large number of adjustments available on the carburetors made it possible to perfectly tune the engine. But it also means more moving parts to wear and lose their adjustment. The throttle linkage system is also quite complex with many moving parts and adjustments needed to really make the car perform well.
Restoring the Solex P40-I
When restoring a set of Solex P40-I carburetors start by removing the complete assemblies from the vehicle and setting them on the workbench. Make sure to use a bench that has plenty of space as there are a lot of parts in these carburetors. When dis-assembling the carburetors you should be looking for signs of wear and damage as you go. Most of these carburetor sets will need a lot more than just a gasket set and quick clean. As you dis-assemble, each set lay out the parts in the order in which they came off. Laying them out will help you both with re-assembly as well as cleaning and checking all of the parts
Starting with the linkage system look for play in the cross bar and individual linkage rods, these are usually the first things to be removed when dis-assembling the carburetors. Take a good look at the pivot ball areas on the transfer rod. On almost every set I find that the ball wears into the socket of the rod. This allows the rod to move sideways when it should just rotate and begin to open throttles on the carburetors. This will play havoc when tuning as you can usually make the engine idle smoothly yet with a light throttle the engine will be rough due to imbalance in the air flow through all six carburetors. If the shaft and ball do show signs of wear put them off to the side and continue disassembly.
With the linkages removed next remove the top air filter adaptor plate and the fuel inlet line that is connected to each carburetor. Un-bolt each carburetor and lay them out on the bench. Disassembling each individual carburetor is pretty simple, start by removing the four top-cover screws then pulling off the top cover.
On the Solex P40-I, the jet carrier is held in with two captive screws which once loosened will give you something to hang on to remove the jet carrier. With the jet carrier out you want to go ahead and remove the jets so you can pin fit to verify size. With the carrier out, you can clean the jets and carrier. Pin fitting jets is a must when rebuilding any carburetor; about 50% of the jets we see have been drilled and are not the size they are stamped with. I have also seen jets installed in carburetor sets that are all stamped the same size but when measured are all different sizes, which really makes it hard to pinpoint weird drivability problems later.
Accelerator Pump System
Next, disassemble the accelerator pump system by driving out the pivot pin on the pump cover housing. It is easier to do this while it is still attached to the carburetor body. You want to take a good look at the pin as it is another area that usually wears out.
When the pump arm wears a groove in the pivot pin it delays the action of the fuel pump. Which in turn can result in a stumble off idle due to the delay of fuel being injected. You should also have a good look at the linkage rod that comes off the throttle lever. Look for wear where the rod goes thru the pump arm. Often the rod can be half its original diameter due to wear and tear. Next, remove the pump cover and diaphragm and spring and set off the side; you will clean and flatten the pump cover later. To remove the fuel pump rod from the throttle lever you will need to remove the throttle lever from the throttle shaft as the pump rod is screwed into the lever.
Throttle Plates and Shafts
The only part left in the carburetor body now is the throttle plate and shaft. Before pulling the throttle plate out you can check the throttle plate and housing wear. By lightly closing the throttle plate against the bore and then using a flashlight under the throttle plate you will be able to see any worn areas easily. The areas that wear the most are where the throttle plate and shaft meet the carburetor body. The wear here happens as the brass throttle plate rubs against the soft zinc alloy body. Though the throttle plate will never be a completely light free seal, excessive wear must be corrected.
To correct excessive wear overbore the throttle bore and install a new oversize throttle plate. Unmetered air that is allowed to bypass the throttle will cause the engine to idle too high. With unmetered air, you will not be able to adjust the airflow correctly. In the worst case, we have seen the throttle plate bent, in an attempt to reduce the airflow. Also, we have seen the throttle adjusted closed to the point where it impacts the throttle bore opposite the throttle shaft. The effect of this is the plate may bend making the problem worse. Or if it does not bend, it then wears into the progression port area.
Lastly, we want to look at the throttle shafts. Before removing the shaft gently rock the shaft up and down and feel for any excessive play in the shaft. Solex used a combination of bronze and Teflon bushings. Generally, the effect I have seen is that the Teflon will wear out and the bronze bushing will then tend to wear into the throttle shaft. Depending on how many miles the carburetor set has seen, the throttle shafts may need to be replaced.
You may need to replace the throttle bushings as well. Normally I will remove the throttle bushings and install a full-length bronze bushing as it seems to last longer and supports the throttle better. Check the throttle shafts for excessive wear. If the shaft has a lot of heavy grooving I would replace it. If the wear is light and it can be polished up then go ahead and re-use it. As a general rule, I will go ahead and replace the shaft when it measures 7.94m/m or less from its original size of 8m/m.
Cleaning all the parts
With all the individual carburetors dis-assembled everything needs to be cleaned and prepped for a final inspection. Worn throttle bushings need to be replaced and sized to the throttle shafts. Worn throttles need to be over-bored and new throttle plates made. All the gasket surfaces need to be checked for surface distortion and then corrected if needed.
Some carburetors will require more machining than others. The amount of wear you find determines the amount of work involved. This is another reason that the Solex P40-I carburetors are expensive to build. They are also difficult to quote as you will not know the extent of the work until you have taken the sets all apart.
Reassembly of the Solex P40-I
Re-assembly of the individual carburetors is fairly simple; basically it is the reverse of the disassembly process. Starting with the throttle shaft lightly lube it and install it into the carburetor body. Simultaneously installing the throttle lever and pump shaft assembly. Check the throttle shaft end float and shim it to remove excessive travel. Be careful not the shim it too tightly. The shaft may bind in the body when it gets hot. The zinc alloy carburetor has a different expansion rate than the steel throttle shaft.
Install the throttle plate check that it does not bind when it closes and that it has a nice even seal around the throttle bore. Assemble the accelerator pump diaphragm, spring and cover back onto the carburetor body and then install the pump arm and pivot. Start the adjusting nut on to the pump rod but leave the lock nut loose as you will need to set the delivery volumes later.
Assemble the sized main and idle jets into the jet carrier, install the new gasket and O-ring and install the completed assembly back into the carburetor body. Be careful not to damage the gasket or the O-ring. Assemble the top cover and the auxiliary booster into the carburetor body and then set it off to the side until the rest of the carburetors are assembled.
Float Bowl Manifolds
Before installing the carburetors on to the intake manifold the fuel bowl needs to be removed. Check the float as well as the passages. The gasket surfaces for both the carburetor sealing flanges and intake to cylinder head surfaces need also to checked for any surface distortion and corrected if needed. The drain-back bores should also be checked and blown thru to ensure that there are no obstructions that could stop fuel returning from the carburetor to the fuel bowl.
For our builds, we go the extra step on the manifold. After all the machine work is complete, we send the manifold to get a chemical treatment. The treatment re-silvers the manifold. This is expensive and time-consuming but worth it for the longevity and looks. We have seen the Solex P40-I manifold finished in many different ways. The main point is that the manifold is clean, and the sealing surfaces are flat.
With a clean intake manifold and the float bowl installed, now the individual carburetors can be reinstalled. Assemble the linkages first and install the crossbar and adjust the preload on the ball pivots so that there is no end float. Do not make it so tight that the throttle will feel stiff or bind.
Cross Bar Set-up
Previously, I spoke about the wear that occurs in the crossbar. Replace the crossbar if it is worn. You need to do this before trying to install and set up the throttles.
With the crossbar installed and the return spring hooked up (so that it holds the idle stop screw against its stop) install the first connecting rod onto the crossbar. And then carefully snap the other end onto the throttle lever of the first carburetor. Using a flashlight to look down the carburetor. Screw the connecting rod in or out until the throttle is in the closed position. Install the next rod onto the next carburetor and repeat. Don’t worry if it is not perfect at this point. Once the carburetors are back onto the engine the fine balance will be set then.
Bench setting the Solex P40-I
Once all the throttles have been adjusted and the fuel lines have been installed, the next item that should be set up on the bench is a check of the float level. Also on the bench, you can adjust the accelerator pump delivery volumes for each carburetor. To do this operation on the bench is a little tricky. You will need two fuel pumps, one to supply fuel to the fuel bowl, and then one to circulate the fuel through the carburetors.
To check the fuel bowl setting, connect a piece of clear plastic hose to the bottom outlet fitting on the fuel bowl. Bend the hose up and hold it so that the fuel won’t run out and then turn on your supply pump. Make sure that the supply pump cannot exceed 2.5PSI otherwise the float height will be incorrect. Looking at the clear pipe the fuel level should be between 15m/m and 20m/m down from the top of the fuel bowl. To adjust the float level up or down change the sealing washer thickness under the needle valve.
Accelerator Pump Discharge Volumes on the Solex P40-I
To set the accelerator pump discharge volumes you will also need two fuel pumps. The first pump is to supply fuel to the fuel bowl and a second pump to circulate fuel through the carburetors. Once again make sure the pumps don’t exceed 2.5PSI. With both pumps running activate the throttle arms and measure the discharge volume from each of the accelerator pump nozzles.
The discharge should be between .40cc to .65cc depending on the season and your specific area. To adjust the discharge volumes turn the adjusting nut on the accelerator pump rod and then tighten the lock nut down to retain the adjustment.
Installing the Solex P40-I set on an engine
After bench setting everything, the carburetor assemblies are ready to install onto the engine. Make sure that the cylinder head gasket surfaces are clean and use a new base gasket. Make sure that the ignition system is in perfect operating condition. And the fuel in the gas tank and lines is fresh and clean.
Start the engine and warm the vehicle up, it may run a little rough during this stage. Once the engine is up to temperature raise the engine speed to 1200-1400RPM. Check the balance on each individual carburetor body. Using a synchrometer, adjust the throttle position to match the airflows perfectly. After synchronizing each bank let the engine come back to idle and balance side to side using the idle stop screws on the cross-linkage.
Running the Solex P40-I on the motor
With the throttles balanced now check and adjust the mixture screw. Adjust on each carburetor until the engine is its smoothest. You may have to lower the idle speed. Just make sure to adjust both sides evenly to maintain the same airflows through each bank. Finally, switch off the engine and install the remaining linkages to complete the installation. Check that each ball socket is lubed and has no play in the joint. Also, make sure to check and adjust the full-throttle stops. Making sure that the throttle plates are opening completely and not going over-center in the throttle bore.
Install the air filter assembly and time to roadtest ……..
Written By Kurt Donohoe