MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - pcv valve installation

I have re arranged my crankcase ventilation system and have a question about the orientation of the pcv valve. I have closed off the tappet covers with one of Doug Jackson's plates, drilled a hole and fitted a tube to an oil separator, and then a tube to the pcv valve which will then be connected to the HIF-4 carb, the vacum source. I would like to hook the base of the pcv valve directly to the carb intake but that puts the pcv valve at a 45 degree angle. I can't see any reason why this should effect it's performance, but thought I'd see if anyone had any thoughts. Many thanks.
Chris
Chris Kasten

The PCV valve needs to connect directly to the manifold - not the carb.
Chris at Octarine Services

Hi Chris.

I am interested in the question of optimum crankcase ventilation, please keep us posted on how you get on.

I have a strong suspicion that the funnel type PCV valves are mounted 'mouth up' for a reason, the reason being that oil will tend to accumulate between the diaphragm and the 'funnel' if it is otherwise mounted.

With the outlet port pointing downwards, any oil will gravitate downwards and be sucked into the manifold and cleared out of the valve.


HTH.. Don
Don

What kind of PCV valve are we talking about here? The old Smiths "mushroom" or a US made PCV from the rotating carousel at the parts store (one of maybe hundreds available)?
Furthermore with the HIF carbs you do not need a valve per se.
From the description of your setup I would simply run a hose from your oil separator, use the stock "Y" fitting and run hoses to each of carbs, as per the stock setup.
THe Smiths PCV is mounted via a short tube to the intake manifold, and vacuum pulls a rubber diaphram against spring tension to shut the valve.
The US made PCVs should be avoided. They are calibrated for specific applications and I doubt you can find one that works by trial and error.
You will also need a vented oil cap to allow new air to be drawn into the engine. Or if you have an 18V valve cover there is a tube on the top that is normally connected to the charcoal cannister, which provides filtered air. An alternative to the cannister connection is to attach a small air filter to this breather tube.
Paul Slice

The posting attributed to Mr. Slice above was made by Andrew Blackley. The BBS is playing tricks today...
Paul Slice

I use an american pcv valve on my B. I chose one from a comparable 4cyl engine with carb (I don't remember which one anymore) and it runs MUCH better and idles smoother than without. Paul, I guess I don't understand the difference between Smiths or ant other brand of pcv valve. It seems to me that most of them have a rubber diaphragm that seals with manifold vacuum, vents under engine pressure, and closes via spring under neutral pressure. Am I mistaken? Some leak and are of poor quality, but that's another issue. Its always a good idea to buy parts that are designed to last a while, but to state across the board that all american parts are not worthy is a bold statement. The use of synthetic rubber seals versus natural seems an improvement to me.

"I doubt if you can find one that works by trial and error." You should have more faith in MGB owners. While some of us shouldn't be working on our own cars, the majority of us have developed acute diagnnostic skills which brought up this thread in the first place.

Chris, while sitting at a 45 degree angle isn't optimum, it will work. As the pcv valve is subjected to engine heat the spring will fatigue and be less likely to seal via gravity as well as potential problems from oil build-up that could prevent the diaphragm from sealing. Regular maintenance will keep potential problems in check.

Jeff Schlemmer

Jeff: My point is that the Smiths was engineered to work on the MG, the others are not.
Believe me, I do not have any lack of faith in the ingenuity of my fellow MG owners (or any other car enthusiast for that matter). Based on my own experience, having tried several US type PCVs, I could not find one that I thought did the job as well, and as there are many types available I decided a new diaphram in an old Smiths valve was a cheaper alternative. I give you credit for finding one. When you replace it perhaps you could post the part number.
A. K. Blackley

Hi all.

Some older petrol Land-Rovers were fitted with Smiths PCV's, they may be availible from LR specialists... whether they have the right characteristics is another matter.

Don
Don

thanks for all your helpful comments. I should have mentioned that this is a supercharged setup and so there is no intake manifold with a vacum as the intake manifold is after the blower and will maintain positive pressure instead of being a vacum source. I have a Smith's PCV valve and an oil separator between the tappet cover and the PCV valve. Hopefully the oil separator and distance from the PCV valve will keep the oil out of the single HIF 4 carb. I have a small K & N filter hanging directly off the valve cover with a fuel metering valve so I can adjust the degree of vacum that runs through the crankcase by opening or closing off the valve. I'll give it a try and thanks for everyone's thoughts.
Chris
Chris Kasten

Let us get back to Andrew's original "Paul Slice" posting. He is perfectly correct that the use of later HS series carbs or the use of HIF carbs does not require a PCV valve.

Early engines used a "road draft tube", simply a metal pipe hanging down below engine level, for the crankcase ventilation. It is interesting to note that, inspite of limited budgets, MG replaced this inadequate system significantly before US cars did.

MG then went to some form of PCV value, known as the "Smith's Mushroom Valve" to enthusiasts. This system was used through 68 or 69 and, on my 68, is very effective.

After that, MG went to using a direct take off from the carbs themselves without using a PCV valve plumbed into the intake manifold. (When the rubber diaphram of the Mushroom valve fails, oil usage rises dramatically.) By going to the carb sourced vacuum, MG developed a workable system that did not "go off" over time. I agree with Andrew/Paul that the factory system worked well and, if possible, that is what should be used.

Chris. You do not mention what year your car is, nor what year the carbs are. I would assume that you have a 72-74 1/2 model. If so, using the carbs as a vacuum source makes sense. If you have an earlier intake manifold, you should be able to plumb a Smith's valve into one of the drilled and tapped openings on top of the intake. My 68 is running an HIF intake manifold with HS carbs and a Smith's valve with no problems noted in many years of ownership. Les
Les Bengtson

The early cars with the non-positive or open-circuit breathing systems also had a hose from a port at the front of the rocker cover to the front air cleaner, as well as the draft tube Les mentions. The PCV valve was a way of limiting the suction on the crankcase which prevented excess oil being drawn up, which failed over time as Les describes. From 1969 the SU carbs each had a port which had a restrictor and was plumbed direct to the crankcase without using a PCV valve, this type was much more reliable, and if your carbs have this port it is a much better option than fitting a PCV valve. Whether a vented oil filler cap is used or not depends on whether it is a North American spec car with carbon canister or not. If it has the canister connected to the restriced port on the back of the rocker cover it should have a non-vented cap. If it doesn't have that port or it is sealed off (it shouldn't be left open to the atmosphere or it sucks in dirt) then it needs the vented cap.
Paul Hunt

This thread was discussed between 27/12/2003 and 29/12/2003

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.