Carburettor Breather

[Carl Reading]

I seem to have 2 carburettor vent systems fitted to my car. One is a small bore steel pipe system with rubber elbow and tee fittings very much as shown in the workshop manual. This system connects to a fitting on the cast inlet elbow on each side of the engine and, according to the manual, should connect to the back of the air filter.
However on my car it isn't connected as there is already a hose connected.
The second system consists of 1/2" hose and plastic tee-pieces and is connected direct to the carb body on both sides and then to the air filter. See attached pics. I had assumed that these larger hoses were part of the engine breathing system, but they seem to connected together and then connected to the filter box.

Any advice? DSCN1328.JPGDSCN1329.JPG

[DJT]

The pipes and elbows should be fitted to the air filter. These are vent pipes. The other, larger hoses fitted to the carbs should ‘T’ together to a longer single pipe in front of the carbs, below the air filter, and run down the front of the engine. These are overflow pipes. At present, if the carbs overflow, the fuel will go straight to the air filter.

https://rimmerbros.com/Item--i-GRID005207

[Wisey]

My understanding is both pipes are "breather" pipes for the carbs. At idle, or with engine turned off, the bigger pipes vent the carbs out to the front of the engine. For some folk, they will notice a small puddle of fuel on the floor under this pipe in the summer when fuel in the float chamber vapourises due to local heat and no fuel flow. This vapour will exit along the pipe and condense out at some point along the pipe run, thus giving a small puddle on the floor under the cold end. When you open your throttle (i.e. drive) a shuttle valve moves inside the carb and changes the vent route to the small pipes running to the clean side of the air cleaner. With fresh cool fuel arriving in the float chamber, and also being drawn into the carb, excessive vapour is not so much of an issue so this route then balances the pressure in the top of the float chamber with atmospheric (nearly) pressure in the air cleaner box. If the shuttle valve sticks one way, you have the potential to pump fuel vapour into the air inlet side of the carb system and give hot start issues. If it fails the other way, it will still balance out the float chamber pressure to ambient air pressure.
Hope that makes sense, and I'm sure a grown up will illuminate any missunderstanding in the above...
Steve