bmwe21.net forums

I pulled the air filter housing off to give it a tidy up & found a spring loaded flap which diverts air intake from above the exhaust or behind the o/s headlight, depending on the temperature.

It all looks a bit rusty. The flap can be moved with a screwdriver but I've no idea if it still moves automatically. My car now has a Weber carb with manual choke so I don't know if that affects things. How can you check? Obviously, taking cold air in all the time would be preferable from a power point of view.

You can check if the valve still moves by making a vacuum at the hose which comes from the inletmanifold towards the airfilter. Hopefully the temperatureswitch is still working ofcourse.

The goal is to warm up the engine faster offcourse.

Off the car the spring is holding the flap closed so it would draw air from above the exhaust. There is what looks like a sensor which must determine the temperature of the 'cold' air from the headlight. Question is, how does that sensor open the flap? Or does it somehow prevent the flap from closing?

There does not seem to be any external connection to influence the changeover.

Normaly there goes a tube from the flap towards a unit which is mounted in the circle from the airfilterhousing (at the carb.) From there a tube goes to the inletmanifold. I always had the idea that that unit determined if the flap opens with the vacuum.

http://bmwfans.info/parts-catalog/E21/E ... cartridge/

I don't have that vacuum hose. My car's build date is May 1982 & seems to be the same as a 1981. Could this be because of the Weber conversion?

My 320/6 is also from early 1982 and has it for sure.

I first had the weberconversion also but if you did not made that tubeconnection towards the airfilter it will not work.

the small brass/copper sensor is similar to what is used in a radiator thermostat ....when the brass/copper gets warm the pin gets pushed outward and moves the flap ....try some warm water on it to see if anything moves....edd

I tried a hair dryer on the sensor but the flap didn't move. You can push it up & down by hand so it isn't seized.There is a small port next to the flap but it looks factory sealed (it's flush with the casing). I guess that's where a vacuum pipe would connect.

Is the any way to remove the flap assembly without pulling the casing apart? A cold air intake would be much better. Another good reason to go for a Motronic M20

If the flap is already closed, and you'll apply heat to the ceizing unit (the brass cylindric thing ) with a hairdryer, it wont open but even close more firmly.

The flap has to go open at low temperatures to prevent real cold air going in the carb.

Mine has a adjustable hex bolt and nut accecable at the cold air intake side.

Motronic is superior to the carbs indeed. And a megasquirt conversion gives even more tuning options and better performance.

I thought about removing the whole flap assembly but ot looked like it might get messy so I drilled a small hole in the casing & used a self tapper screw to hold the flap fully open (i.e. closing off the hot air intake). It probably won't make much difference but psychologically it's got to be worth 5-10 bhp !