This page deals with the new type of engine breather unit which is now available from the usual sources. The importance of the engine breather on a 2CV engine is dealt with in the section describing the original type of breather and we suggest you read that page first

This new breather unit has nothing which can go wrong, unlike the original type which used rubber valves. On this unit they are replaced by a metal reed valve but the principle of how it works is the same.

The diagram below shows the complete unit on the right and the filler cap assembly which screws into it on the left. The red lines indicate the flow of gases under pressure from the crankcase through the unit when the pistons are on their downstroke. The areas numbered are as follows:
click here
The gas is forced from the crankcase into the oil filler tube then at (1) passes through holes in the filler tube into an outer chamber which surrounds it.

The gases are in the outer chamber at (2).

At (3) the gases leave the outer chamber through more holes and then into the filler cap assembly via holes in a recessed section of the screw thread and into the hollow body of the filler cap assembly.

The pressure of the gas opens the reed valve on the side of the assembly at (4) and the gas is then forced into the rubber tube to the air cleaner.

When the pistons start their up stroke they try to suck air/gas back into the crankcase but the reed valve then closes to prevent this and the pressure in the crankcase therefore remains below atmospheric pressure.

Any fluid which finds its way into the outer chamber drains down the small tube at the base of the unit and then into the dipstick tube, exactly the same as on the original type of breather.
The photo on the left shows the unit installed in the car with the filler cap assembly removed. The red lines indicate two of the four holes in the thread (which line up with the recess in the thread of the filler cap assembly). The gases pass through these holes and enter the filler cap assembly, (3) in above diagram.
The car in the photo has the single choke carb. When you first start to screw the filler cap in it slightly fouls the screw on the front of the float chamber cover but as you screw it in further it clears it. We understand that these new units will not fit Dyanes.
Technical pages
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