Engine Breathing.

[Willie]

Some pointers to standard Minor breathing arrangements. The earlier cars
had a pipe pointing straight down on the tappet chest cover which was the
INLET for engine breathing. The OUTLET was via the pipe on the rocker cover
to the AIR FILTER. The Oil Filler Cap on these engines should be solid i.e. no
wire filter in it. The 'straight down' pipe leaked oil so was changed to a swans
neck shape which stopped a lot of the leakage. Later engines had a different system where the breather pipe on the tappet chest went straight
either to a valve device on the inlet manifold and then to the CARB or straight to the CARB so this was now the OUTLET and necessitated a change
in the Oil Filler Cap which now had a wire filter in it and had become the
breather INLET. So the rule is: If your engine breathes into the AIR FILTER
it should have a 'solid' oil filler cap. If it breathes into the CARB the filler cap
should have the wire filter in it and the rocker cover should not have a
breather pipe on it. These filter type caps are supposed to be
changed every 12,000 miles anyway but can be thoroughly cleaned with
petrol etc. If the revs rise considerably when the filler cap is removed then the wire filter is clogged up.

[bigginger]

Thank you Invaluable...

[chrisd87]

Hmm... I have a tappet chest breather open to the atmosphere, the plastic type oil filler cap and no rocker vent - is this ok?

[RogerRust]

I have this setup on my '71 1098 and it has the cleanest oil I've ever had no mayonaise at all.



Crankcase breather goes to inlet on carb and a solid filler cap.

If I take the filler cap off at tickover it leans out and stalls.

[Willie]

It stalls because you have the wrong oil filler cap fitted and when you remove it you allow the influx of fresh air which immediately weakens the
mixture. You should have the 'wire filter' type filler cap and then set up the
mixture to suit. I take it that cylindrical oil trap behind the water hose is from
the tappet chest cover in which case it is correct for your late car.

[Willie]

CHRIS....your handle says that your car is a 1970 model which should have
a Carb' with a pipe for connection to the tappet chest outlet,no rocker cover
breather, and a 'wire filter' type oil cap. You say 'plastic filler cap' both the
'solid' and the 'wire filter' types are plastic originally.

[les]

Further to all this, although I have the correct set up for the mesh cap, If I remove either the filler cap or the pipe from the crankcase canister or indeed both, there are definate puffs of air being expelled from both, I don't think this is right, any thoughts?

[Willie]

LES, this suggests that you have a bit of blow by past the pistons, not that
uncommon and as long as you are happy with the engine it will last for ages.
Just depends when your oil consumption(as apart from oil leakage) becomes too heavy.

[Willie]

I can see no reason to change the filter type oil filler cap as a wash in petrol will remove any debris and the cap should be easy to blow through. Remember that the filter in the cap is to stop foreign bodies going IN to the
engine not to prevent emissions. Officially if the revs rise a lot when the cap
is removed then it is clogged up and should be replaced. The official advice
was to change it every 12000 miles anyway. I would expect the tickover to
increase SLIGHTLY when the cap is removed on a well set up engine. Just
remember that you should ONLY have a wire filtered oil filler cap if the engine
breather pipe is NOT connected to the AIR FILTER on your type of engine.