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MG MGA - vent pipe

I just put the filters on the carbs. I put the one with the vent connection on the front carb. Is this correct or should it be the rear? The old hose seems to want to go to the rear. I'd rather not have them off again.

Art Pearse

Hope so. That's where mine is.


Dennis Suski

Front carb is the correct. Interestingly, I am running an alloy rocker cover, with a breathable cap. As the cover didn't come with a breather pipe, I added one, and ever since have had oil seeping back through the pipe into the carb filter, and dripping out the bottom. I don't know why this is, as I've temporarily blocked off the filler cap hole and blown air through the hose...air comes out through the engine vent pipe as expected, so don't know why I get this "reverse" flow. I've just blocked off the rocker to carb pipe, as the filler cap should be sufficient I think?
Gary Lock

Gary, all the suction comes from road speed air past the bottom of the draft tube on the tappet cover.
Is the draft tube blocked? Or do you have blow-by from the pistons?
Art Pearse

Gary,

I just looked inside my filter and didn't really see oil residue. I am wondering if a rocker might have been throwing oil splatter into the area where your pipe is located. I know when I run my forever-under-restoration MGA with the rocker cover off, its spatter-central.

Dennis
Dennis Suski

I've always thought you can expect a little oil in the vent pipe and therefore in the front filter from the oil vapour condensing.
J H Cole

JHC, I know that the oil in mine is coming from the vapour condensing, and I have even put a mesh baffle just inside the rocker cover outlet/inlet to try and stop this, but to no avail.

Art, the draft tube is not blocked, as the compressed air I put through from the top exited the tube OK.

The engine has plenty of power, so no obvious problems with blow-by. Now I've blocked the rocker to carb air cleaner, I'll see if any oil vapour comes out the filler cap.

BTW, what was the real function of the rocker to air cleaner pipe anyway, assuming the rocker had a breather cap?
Gary Lock

Lots of good info on the subject at Barney's place, as follows:

http://mgaguru.com/mgtech/accessories/at126.htm

http://mgaguru.com/mgtech/accessories/at126a.htm

http://mgaguru.com/mgtech/engine/cv101.htm

http://mgaguru.com/mgtech/engine/cv102.htm
Dennis Suski

The MGA rocker cover is not supposed to have a breather cap. The oil filler cam should be sealed.

When the car is in forward motion the draft tube extending downward from the front tappet cover serves as a venturi to pull a slight vacuum on the crankcase. Air from the air cleaner goes in through the rubber connector hose to the valve cover, then through the crankcase and out the draft tube. This provides adequate crankcase ventilation to prevent buildup of condensation that causes scum in the valve cover and sludge in the sump.

When the car is standing still with engine running there is no vacuum on the draft tube. In that case the small amount of blow-by from leaky piston rings will exit both the draft tube and the valve cover vent. The blow-by gas contains a bit of oil vapor, some of which may sneak past the wire mesh filter in the valve cover to coalesce in the air cleaner. A few drops of oil in the front air cleaner is normal (or at least common).
Barney Gaylord

This thread was discussed between 26/04/2013 and 27/04/2013

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