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MG MGB GT V8 Factory Originals Technical - Engine breathing?

I don't get how the breathing is supposed to work on a V8. I just sourced a Rover 4.2 motor complete, including carb, air cleaner and all that crap, from a guy who had bagged a conversion (no time).

How does (/should) the thing breathe? The passenger side (if LHD) valve cover has a hose running through a PCV valve on the way to the bottom of the air cleaner. Fine. But where is the thing supposed to breathe in? Am I missing something? Seems to me most american v8s just have a breather filter on the opposite valve cover. Do they make oil filler caps with a vent and a screen mesh like you can get for regular MGBs?
Ted Haskell

I had the same question and noticed on my Rover valve covers that there is a small pipe. I think this originally went to a charcoal canister and air is deawn as part of the emissions system. My canister is long gone. I used the smallest K&N breather. I don't remember the number but it is mentioned in their web site as an engine breather filter. Hope this helps.
By the way I took my pcv valve off of the big screw on fitting with the wire mesh. It screws into the hole opposite the oil filer.
Dick Porter
Richard Porter

On the factory V8 there is a breather filter right at the back of the engine behind the air plenum and slightly to the RHS of the centre-line of the engine. Not sure if its hose does down to the head or the block - probably the latter. More towards the centre of each cover on the inner side there is a vent that has a short hose to a flame trap and from there to its respective carb.

Paul Hunt

The two covers have screw thead holes - one has the oil filler cap the other takes a separator can which connects to the carbs.

One cover also has a small pipe that should be fitted with a small filter can - I use one off a late SU carb.
Chris at Octarine Services

Hi Paul,

That breather at the rear goes into the valley. (On my conversion, at least.)


Nick Wilson

The breather at the back of the block was only used on pre 1976 engines. Later models have the breathers on the rocker covers only, one for in & one for out into the air cleaner or carby depending on the set up. I did try to modify an early system once ,by blocking off the rear inlet & using only the rocker cover inlet/outlet set-up. the result was blown valley oil seals.HTHs Barrie E

with my setup i have 2 holes one on each rocker cover.
One of them has a pipe leading back into the the plenum chamber and the other is left completely open.
My question is do i leave it open or plug the hole?
I can see oil vapour on that rocker cover coming from that hole,but ive been told if i plug the hole i will stuff up my plugs?
is this right?

Steve Berno

Steve - with the engine idling block up the open hole with your thumb. Seems to me that if you can feel suction or the revs alter then it should either be piped back to the carb or fitted with a breather - the former if you have the valley breather as described above (via a Y-piece if you only have one carb breather pipe) and the latter if you don't. The breather limits the air flow through the system, without which it mucks up the mixture between idling and running.

Paul Hunt

I have a pre-76 engine (3.5) which has the breathers on each valve cover, plus the breather on the back of the block, high on the passenger side (LHD). I plugged the back breather and one of the valve cover breathers. The remaining one was attached to the constant vac port on the carb. I ran this one through a PCV valve for a 327 chevy, and it works great. Without the PCV or a with one for a larger block, I ended up with too much vac being drained from the system. If you don't allow enough vac to be drained (by either plugging all of the breathers or using too small a PVC), you will end up blowing oil out of your dipstick neck, or possibly blowing a seal somewhere else.
Mike Akin

This thread was discussed between 21/10/2000 and 27/10/2000

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