[ROVERNET - UK] Crankcase venting

Fletcher gofanu at usachoice.net
Tue Feb 13 20:25:58 GMT 2007

The purpose of the venting system is to draw air through the engine to 
clear water vapor and other contaminants produced by combustion and 

As such, there must be an air intake, and an outlet. Early systems used 
a vent somewhere near the top of the engine for the intake, and a road 
draft tube exiting near the ground to produce a slight vacuum by venturi 
effect as exit.

Systems with the vent line to the SU between the vacuum chamber and 
throttle plate are called "constant depression" systems, since they 
always have a slight but constant vacuum to draw vapors out of the 
engine. (This is the same "constant depression" that is the basis of 
operation of carbs so designated - SU, Zenith-Stromberg, Bing, and 
others.) This is the exit for the vapors. The system causes a slight 
leaning of mixture, especially at idle, which is compensated in the 
mixture needle selection.

Engines with "PCV" systems plumb the exit into the inlet manifold, where 
the vacuum varies excessively, and it upsets the mixture, so the PCV 
valve limits flow and/or max vacuum. This is usually a pretty crude 
arrangement, though the Smith's diaphragm type PCV valve is quite 
elegant and works very well if in good condition.

Since it is not good to suck dirt into the engine, all vent systems use 
a filter on the air intake line. This was formerly usually in the oil 
fill cap, or was arranged to pull the air from inside the clean air 
stream after the main air filtration. The same holds today; the air is 
drawn from the oil fill vent/filter, the main air filter, or the 
charcoal cannister filter on cars with Evaporative control; modern 
systems are generally more restricted in airflow, so that fuel/air 
mixtures can be controlled within close limits
If you have connected all vents to the carb bodies, then you have only 
exits and no entrances. That gives no ventilation, and slightly high 
vacuum in the engine interior- good for stopping leaks, bad for oil 
consumption. and a slightly rich mixture with the OE needles. Reconnect 
the line that goes to the highest vent on the engine to the clean side 
of the air filter, or fit a vented oil filler cap, with a small 
restrictor hole, about 1/8" (this is a calibrated port, so it interferes 
with fuel/air ratios). The other vent line(s) from the engine should go 
to the carb constant depression ports. You feed the air into the engine 
in the cam cover because that is the coolest place, and thus is where 
water vapor condensation is the biggest problem.


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