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MG MGB Technical - SU HS4 Question

I have a 3 bearing MGB engine in my MGA with its original H4 carbs. I would like to add better crankcase breathing, i.e. better than the basic pipe as on MGAs and the very first MGBs.

I understand there were 2 approaches. The first with a diaphragm valve and the second with piping directly to the carbs on the manifold side of the needle. Clearly the second approach is simpler but I do not have the fitting pipes connectors on H4 carbs, but what I do have are special versions of the blocks that sit between the carbs and the heatshield that do have vacuum pipe fittings.

My question is simple. While I can see from photos what the pipe connector looks like externally, i.e. brass pipes about 8 mm diameter, I don't know what happens at the internal end of this connector. I presume the hole is a lot smaller than the external diameter of the pipe but can someone with an HS4 of the car tell me what diameter it actually is?

Paul
Paul Dean

You do have another option, depending on your air filters, i.e., feed into the filter casing. Down side of this approach is oiling the filter elements.
To answer your question, looking at my HIF 4 carbs the hole dia is the same as the bore of the brass tube. No reason why HS 4 should be any different. I presume you are breathing through the front valve chest cover. Don't forget you will need the filtered oil filler cap as well.
Allan Reeling

The hose that went into the front air cleaner can was always part of the original MGB system, together with the road draught tube that hung down from the front tappet chest, but was always hit and miss as to which direction the air travelled and how much. When you get oil mist on the filter that means unfiltered air has been sucked into the crankcase from just above the road surface - not ideal.

MGB carbs from October 68 had the breather ports which apply an almost constant low level of suction of just a couple of in. Hg to the crankcase. You could fit ports to the carbs by drilling into the space between the butterfly and the piston - if you are brave.

You can't use just blocks that sit between the carbs and the manifold as that would apply full manifold vacuum to the crankcase. You would need a PCV valve with those, you could use the MGB items if you have a suitable tapping on the manifold, but otherwise you could use a modern PCV valve with hose fittings each side and plumb it into both blocks. The other end of the PCV valve has to connect to the engine somewhere - usually a tappet chest cover.

That's not the end of the story, there also has to be a filtered and restricted fresh air inlet elsewhere. On UK MGBs this was achieved with a breathing oil filler cap, which may or may not fit your rocker cover. If not you could attache a port to the rocker cover and fit a small K&N filter, but the port would need to be restricted down to 1/8" or less. Or you might be able to fit a rocker cover from a 1971 or later North American engine which already has the restricted port. Using a port such as that the oil filler cap must be the sealed type.
Paul Hunt

Thanks for the input. I will order an MGB PCV and oil filler cap. I have an MGB rocker cover. In time I will fit a takeoff to the manifold but in the meantime I suppose I can use the fiitings in the carb to manifold blocks.

Paul
Paul Dean

This thread was discussed between 24/05/2015 and 25/05/2015

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