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MG MGB GT V8 Factory Originals Technical - Breather pipes

Hi All,

Second question of the night!

I have a one way PCV valve on one of my rocker covers - it originally had the flame trap on it.

On my other rocker cover I have a much smaller vent pipe - only about 6-8 mm diameter. I assumed that I needed to connect this into the pipework that feedback back into the carb.

However, trawling the archives I saw a post from 2001 from Geoff K north of the border, Geoff said :

" Air needs to get in to the crankcase and fumes need to escape, usually air will enter the rear of one valve cover and vent from the front of the opposite cover through a flame trap to down stream of the air filter. "

Hmm, should I just leave this smaller pipe vented to fresh air?

The engine is from a 1985 SD1.



Liam H

Thats what I've done. Or rather, left in place. On mine it is a small tube with a penny sized metal disk on the top that has a hole in the middle. Onto this a small black plastic filter clips. The filter has a bit of foam sponge rubber in it. I put in a new bit of foam from a cleaning sponge.
Its the crackcase breather. You can buy some quite nice looking chromed/stainless, small breather filters at auto shops if yours has dissapeared.
On my EFI the flame trap on the other cover needs to go, via some ,approx' half inch, thick walled tubing, to one side of the air intake near, just opposite, the throttle mechanism.
Sounds like you're doing finishing touches Liam

On many engines there is a filter atached to the rockerbox to allow air in to replace that removed from the engine by whatever vacuum method is employed.

If there is no other obvious inlet I would be inclined to leave that smaller one vented to the atmosphere via some sort of filter to remove the chance of dust and grit getting in there.

Cheers , Pete.
Peter Thomas


Sounds like you have it about right, on mine the left hand valve cover has the vent/filter and the right valve cover has the flame trap going to the PCV valve then to the centre large pot on the carb.

Iv'e always been curious as to whether this arrangement has the effect of weakening the mixture, my new filter 14" x 2" (still too small) has the facility to plumb the crankcase ventilation into it, and if done would not effect the mixture as it is above the carb, would be interested to here other opinions on this.

Pic of my layout with my temporary filters prior to fitting the RV8 bonnet.

Kevin Jackson


That's centre PORT on the carb (Edelbrock/Weber 500)

Kevin Jackson


That's identical to my set up now.

I see you have your distributor advance connected to left port on the carb - what are people's thoughts on this? I have read that it should be connected to both left and right. think RPI say right in the docs, but I think Weber say left?

I also have a vacuum gauge - which port should this be connected to?


Liam H


Vacuum advance retard connects to the left hand port on the Edelbrock/Weber.

Not sure about connection for vacuum guage.If its the same port then you would have to make up a T piece.

Kevin Jackson

On the 1967 Mini Cooper I had (all those years ago !) there was a small metal tube coming off the top of the rocker cover. This was virtually closed off apart froma very small hole in the end of about 1/8 th. inch diameter.

There was attached to this metal tube a short section of rubber hose feeding to the air filter housing up stream of the carbs so the mixture would have been unaffected.

The rubber hose had a slight dip in it which had the tendency to fill with condensed oil vapours until it filled completly whereupon the residue was sucked into the engine in one go causing a small volume of smoke to pass out the exhaust.

I had the car for abut seven years and never found out if this was a "design intention" or something which slipped through the R&D dept's best efforts to provide an alternative solution to this problem.

Cheers , Pete.
Peter Thomas

The 85 SD1 (non Efi) would originally have had the small pipe at the rear of the LH rocker cover connected through a filter to the air box, alternatively a standard Efi filter works just fine or a K&N or similar if you want to spend more money.
Geoff King

One cover should be connected to the PCV port on the front of the Edelbrock carb through a PCV valve. The other cover can either be left open to air, but through a breather filter to avoid dirt being drawn into the engine (open breather system) or it can be connected to the base of the air filter (closed loop system)
The vacuum advance for the distributor should be taken from the higher of the two small ports (left if looking at the carb fronm the front) This is a timed vac port and will be controlled by the butterfly in the carb. The other port is below the butterfly and is FULL vac all the time. This one should be blocked off.

J Robinson

This thread was discussed between 06/04/2007 and 15/04/2007

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