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Some notes about the Solex 4A1 carb (320/6)

I've got a 1982 E21 320 carbed 6, originally delivered to Scotland and imported to Australia in the late 1980's. Its done 177,000 miles and going extremely well for its age. As West Australia has no cold winters, but exceptionally hot dry summers, the stock British - European fuel system settings are not optimal. In the Australian climate, I found the system chronically over rich and inefficient, creating relatively poor gas mileage. The popular choice is to ditch the Solex and fit a 2B Webber downdraft, but there is a way to deal with the problem.

The factory carb is the Solex 4A1, basically 2 mediocre dual barrel carbs combined into one major mediocre 4 barrel. Tuning is difficult and in addition, the body can warp. It has a thermal band activated butterfly choke on the primaries and a thermal piston activated enrichening port underneath the primary side. Bloody awful setup. The Solex does have dual acceleration pumps and these deliver a good jet straight into the venturis. There is one redeeming feature of the Solex on the M20 motor. When the motor is pulling over 4500 rpm, and you stamp on the pedal, the large secondaries flip open and really shove the power on. Its a joy to drive upto 6500 until the blue lights appear in the rear view mirror.

If you don't have really sub zero cold weather to contend with, the easy modification to improve low end power and gas mileage is this -
  1. Remove the choke butterflies including the mounting rod over the primary venturis. Don't drop the screws! Bypass the choke water manifold with a connecting copper tube to join the hot water hoses, or rerun a new hose but see point 2 next.
  2. Remove the hot water hoses from the Y manifold of the thermal piston assembly at the rear end of the carb body. The thermal piston, when cold is held in place by a small strong spring. At this position, it permits more fuel into the venturis. When the body heats up after say 10 to 15 minutes of running, the piston moves against the spring and closes off the rich port. If this bugger sticks in the retracted cold position because of water crud insulating the piston, you end up with chronic over rich running. Remove the assembly from the carb. Remove the screws of the spring body from the water body and take out the spring. Insert a spacer or series of washers between 4 and 5 mm in total, with a good fit into the body behind the piston so that the piston is in the extended position. Carefully re-asemble the piston body to the water body, even if the gasket is good you can use a tiny smear of sealant when you refit the assembly to the carb. The Y branch manifold with the inlet from the thermal choke (see point 1) and outlet to the rear part of the engine block is now redundant. You can now refit a one piece hose straight from the front water housing to the rear of the block bypassing both choke and rich mixture piston. That aids cylinder cooling and you now have a carb working permanently on the operating temperature mixture.
  3. The method of starting the engine from cold is simple. The acceleration pumps are very good. Press the accelerator pedal 3 or 4 times to pool gas in the base of the manifold and turm the key. The engine should start immediately and then I feather the pedal for 10 to 20 seconds whilst getting the seat belt on and then its in gear and away. For the first mile or so, acceleration is a bit ragged, but remember its still a cold motor and should not be revved hard anyway. The fuel economy has improved by 15% and with the choke butteflies removed, there is less induction drag. I also removed the air pollution hoses, thats no secret, just see how old race motors were set up, its all vented to atmosphere. Carburettors are for fuel and air not oil fumes! Block the small bore disconnected vacuum hoses with tight fitting screws. The airfilter is the largest size Lynx Ramflo cage dome foam filter, its designed for the Holley 650 and the mounting base has to be snipped to spread over the immense size of the Solex. V8 drivers are astonished at the physical mass of the Solex on a 2 litre motor.
  4. A useful tip is to periodically clean the interior of the carb with a pressure can solvent, when removing the thermal piston and chokes, spray a bit through the interior of the carb. Note also that the diaphragm throttle retreat damper can also misbehave, make sure this is adjusted well back from the throttle closed position, otherwise it can drastically fluctuate the low end running. The Solex has screws and nuts around the peripehery keeping the top of the carb onto the mid section. When stone cold, retension the screws first starting from the centre working outwards, like torquing up a head, but gently. Note the nuts are 12 and 13 mm socket sizes just for additional owner irritation. Due to body warpage, some carbs have been rebuilt with 2 mid section gaskets, but I have got away with 3 Bond gasket goo very sparingly applied to the mating surfaces.
  5. Overheating problems, - For all BMW's fitted with the remote radiator header tank, many owners and general mechanics are baffled by a persistent overheating symptom. With a fully cleaned cooling system, new thermostat and radiator, proper bleeding of air from the system, the engine gradually builds up heat and will fluctuate, often blowing water back through the remote header tank. A simple remedy is to remove the connecting hoses and blow through as an amazing amount of crud accumalates in these after some years. And then, take a small sharp pick or nail and poke out the connecting boss on the tank and radiator because thats where it seems to be worse. This small bore hose allows pressure equalisation and the problem is often mistaken for a cracked head.
Mike Lindroos (mlindroos at

Text originally found in Roadfly forums

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