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MG MGB Technical - Alternate PCV valve?

Has anyone tried replacing the stock PCV valve with a more modern alternative?

Mark. The PCV system has varied, quite significantly, over the years. Hence, knowing what year you have would be of value. I have had to replace the diaphram in the PCV valve on my 68 GT once is eight years of ownership. Not sure that I could find anything modern that would work as well, nor, last as long.

Les Bengtson

I have had good success with a PCV for a 70's dodge 440 in my 76 B. I don't recall the exact part number, but it is doing the trick nicely. Idles well and oil consumption is minimal at best, greatly reduced from before I installed the valve.

PCV valves were only used on the MGB from Feb 64 to Oct 68, after that they used the far superior carb breather system which has no moving or rubber parts. The only reason I'm aware of where someone would want to fit a PCV valve to a later car is if the carb is changed to a Weber or similar fixed jet.
Paul Hunt 2

Mine had been through several different sets or carbs before I acquired it and most of the plumbing was gone. Very semple to put in a PCV: Intake nipple-valve-tappet cover, with hoses where needed. Hardest part was getting a modern valve of correct specs, took 5 or 6 tries from walmart to get a good match.


I have found that the PVC valve listed for a 4cly carbureted Toyota pickups is a good functional replacement and readily available.

Cliff Maddox

Hello Cliff,

Do you have the PCV model number or for which toyota truck.


Jean Guy Catford


I always just look it up in the store when I buy a new one. I look in the index (Puralator?) under Toyota trucks mid 70 to 80s with the 4cly engine with carburetor. After a certain point, in later years everything is fuel injected.

Cliff Maddox

Make sure it works for your car before calling it good. I tried a toyota one and it stayed open at WOT.
They are supposed to be closed at idle & WOT, and open when cruising at part throttle.

Thanks for the good input. Paul, you hit the nail on the head. Changing to a DGV weber is the motivation. The engine is a 71 that was just rebuilt with .020 oversize pistons. Other than a BMW electrical system and redundant fuel pumps, everything is basically standard MGB.

I will experiment a bit with the suggested valves and post my findings. We have a "Pick it yourself" salvage yard nearby, so it shouldn't cost more than a couple bucks to pick up an assortment of valves. Thanks again for the input.
M Sadley

A '71 has a restricter orifice in the spigot on the rocker cover which is piped to the charcoal canister to draw a metered amount of air from it, in effect purging the canister and crankcase at a more or less constant rate. The PCV line from the lifter box is plumbed directly to the carbs, allowing any blow-by to be ingested by the engine and no PCV valve is needed or required as the carbs are calibrated for the rocker orifice and blowby is already burnt and makes no difference in the combustion.

I don't recall if the carb connection is to manifold vacuum or behind the throttle plate but the only difference that makes is that more or less vacuum will be applied to the crankcase depending on throttle position and engine operating condition, and should not effect operation, driveability or emissions. A little more vacuum in the crankcase will only help prevent oil leaks.

The simple solution is to plumb the line to manifold vacuum. No PCV valve needed or required.

Jim Blackwood

The carb breather ports are connected between the throttle plates and pistons. Because of the constant depression characteristics of the SU and Zenith carbs this port experiences an almost constant but very low level vacuum of only a couple of in. Hg. under most conditions. The exception is when initially increasing throttle opening where the piston damper holds the piston down against increased vacuum for a brief perios, which increases the vacuum between the throttle plate and piston and hence the vacuum applied to the crankcase.

The restriction (all US MGBs with a canister) has two functions - it prevents excessive weakening of the mixture, and by being between engine and canister keeps the crankcase at a partial vacuum to reduce the chances of piston pulses pressurising the crankcase and possibly blowing oil out of seals. The canister and its attached emissions plumbing is flowing air but relatively so little that it is all at atmospheric pressure. Negative pressure in the canister and its plumbing can weaken the mixture by pulling fuel out of the carb jets, which is what the anti-runon valve does when you switch off.

If you plumb the tappet chest line to manifold vacuum you will probably suck oil in significant quantities as it will be at 15 to 20 in.Hg. under most idling and cruising conditions. Even with the very low vacuum applied from the carb ports some find they burn oil if there are other things wrong with the engine and breather system, and can be quite difficult to resolve.
Paul Hunt 2

Further clarification of what I am running -
76 B
HIF6 on Marina manifold
Replacement non-vented & non-baffled fuel tank - vented cap
PCV between intake's brake booster nipple (booster disconnected)and tappet cover
Filtered oil filler cap - Rocker air port capped

I have no oil consumption problems at all - 1/2 quart per oil change ~3-4000 miles. It starts very well in all conditions, idles beautifully, and has no run-on problems. If I had all the original equipment I may have chosen to refurbish it as needed and install it, but I didn't have it. It currently runs better than anyone (non-MG owners that is) believes an MGB can run and cost very little to setup.

When I bought it, it had a dirty rag for a gas cap. The rusty gas tank had to go and the non-vented or baffled tank was the least expensive one to be found. I don't drive it aggressively so fuel slosh is not an issue for me. I should also mention that the SU pump was long gone before I bought the car and it has been running fine for 9 years with a low pressure square pump mounted on a simple isolation mount of my own design and a pressure regulator under the bonnet. I hear the pump only when the motor is not on.

The original design may be well thought out, but that is not to say it is the only way to meet the same goal. After all, how many systems were offered over the years?

If your HIF6s don't have breather ports then you *will* need a PCV valve for adequate crankcase ventilation i.e. removal of condensation. But they are an inferior option to using breather ports on carbs that have them.
Paul Hunt 2

On my GT I have the twin hose ports to tappet chest and carbs and when I disassembled for rebuild found the bottom end of the intake valves *encrusted* with black hardened oil, up to 1/8" thick I can only assume came from the crankcase. My machinist thought it could be cheap gas but I doubt it. I am going to make a restrictor out of a 1/2" copper plumbing end cap and drill a 1/16" hole in it and put it in the hose coming out of the tappet chest. On Doug Jackson's web site he says start at that size and increase it by sixty-fourths if necessary. We'll see.
Sam Sullivan

May well stop oil suction if that is what is happening, but the reason for the 'full bore' connection between crankcase and carbs, and the restriction being on the rocker cover i.e. the fresh air intake, is so that any blow-by is more likely to be burnt through the engine than pushed out through the intake into the atmosphere. Oil encrusation in the carbs isn't 'standard' so if it happening it must be due to some other cause. It's not an uncommon complaint (or at least oil burning isn't) but it can be difficult to cure at source, other than simply disconnecting the breather.
Paul Hunt 2

I would think that oil encrusting of internal parts comes from 3 causes: First and most likely in the majority of cases (and not saying in yours) is infrequent oil changes and/or low detergent content in the oil; second, higher than optimal crankcase temperatures; and third, excessive blow-by, which can also affect the first two. The blow-by gases are hot and carry combustion byproducts and as this heat is transferred to the internal surfaces and the oil in the crankcase some of the oil and oil mist/vapors in the crankcase will be broken down, some of the byproducts will condense out, and some of these elements will stick to the internal surfaces much the same as when you scorch food in a pan on the stove.

This layer will then act as an insulator, further retarding heat transfer out of the crankcase and exacerbating the problem, plus at some point oil consumption will go up, discouraging the owner from sticking to his oil change schedule. This makes matters even worse of course, because the above process goes on even when the engine is new but in minute quantities and therefore frequent oil changes for this cause is less critical. But as blow-by increases the oil degrades more rapidly and frequent oil changes become more critical than at any other time.

So if we look at the pattern of encrustation it can tell us things. If there is relatively little of it in the crankcase but much in the lifter box, then it is clearly being carried there by the blow-by gases, and it is more critical to evacuate the crankcase than ever before. In this case a restriction is not the answer, because you are not going to limit the amount of blow-by generated by stopping it off, but only push it out past engine seals, carrying the oil with it. Think about it, how much pressure is in the cylinder during combustion, several hundred pounds right? It's slowly leaking into the crankcase past the rings and if you let it build up there's not a seal in the engine that's going to hold it. A much better plan of action is to burn as much of it as possible, and if burning oil with it becomes a problem due to the volume of gases carrying it out of the engine then the only practical answers are two: Use an external oil trap that can drain back into the engine or; rebuild the engine. Using an orifice and backing the blowby up into the charcoal canister and past the seals is going to do more harm than good.

Sorry, that's just the nature of the beast.

Incidentally, burning blow-by is technically incorrect as it has already been burned. What has not been burned is the small quantity of air drawn in by negative pressure and any oil mist carried out of the crankcase. But this does send it through the catalytic converter thereby keeping it from mucking up the air. It is certainly possible to vent it and even fit a "road tube" to carry it below the car, but not very environmentally conscious. In the old days this practice resulted in the dark streak down the middle of the lane you see in old photos, a slick, oily and in some places thick layer of crud that could make driving treacherous.

It is at this point that I somewhat agree and somewhat disagree with Paul. More vacuum in the crankcase is only detrimental if it results in enough additional flow to carry oil mist out with the vapors. With the system designed to use ported vacuum the crankcase will see less vacuum and the oil separator was designed with that kind of volume in mind, but if blow-by is within specs the extra vacuum will only result in a minor increase in air passing through the inlet orifice and not much difference in the overall flow. For the system to work exactly the same as it did with ported vacuum is simply not possible using manifold vacuum, but that is not to say it can't work adequately well. The ideal solution of course would be if you had a large ported vacuum tap on the new carb but that isn't likely so the thing to do is to look at the operating conditions.

As engine load goes up blow-by increases, but as load increases manifold vacuum decreases. The SU being a constant velocity carb, the pressure drop at the vacuum port remains constant except at very low speed and idle where it drops off. What this means is that up to the maximum flow rate of the port a steady decrease of vacuum will be seen by the crankcase as load increases and flow through the port increases. With manifold vacuum maximum vacuum will be at idle and will drop as load increases until at some point there may be enough blow-by to pressurize the crankcase slightly. This can also happen with the stock system but requires more blow-by to do it due to the pressure drop at the port.

In practical terms then, use of a PCV valve with manifold vacuum is not comparable to the stock system in terms of performance, and only becomes comparable once you plumb an unrestricted breather into the air filter housing instead of the charcoal canister so that all blowby coming out of both ends can be ingested. This is because a PCV valve is not simply a check valve or an orifice. In fact it is much more sophisticated than both combined. The major function of a PCV valve is to restrict flow under full engine load. It does this with a weighted plunger which allows full flow and full vacuum to the crankcase until a predetermined volume is reached and then closes against a seat at which point flow is reduced to a predetermined level. This accomplishes several things at once. Oil consumption at WOT is reduced because it is not carried into the carb. Blow-by is redirected to the inlet breather where it is vented into the air filter housing and ingested, and these inert gases cause enrichment of the air/fuel mixture because fuel is metered to it just the same as with air. Since the breather tube is usually 5/8 or 3/4" in diameter, flow through this tube is slow enough to allow oil mist to separate out and drain back to the engine as well.

This leaves us with the problem of purging the canister, and as this is a rather large volume passage a simple vacuum port on the carb may not be adequate and gas fumes may escape, however if there is a 1/8" ported connection on the carb this may do the trick as it is a near constant vacuum source and will behave very similar to the 1/16" orifice in the original system.

Jim Blackwood

Thanks Paul and Jim for the info, lots of good insights. The buildup must have been from blow-by because the oil was always clean and the engine ran at normal temps. Strange that I never saw *any* smoke at the tailpipe.
Now that the engine has been rebuilt I expect that the problem should go away. I also washed out the wire mesh in the tappet cover to help keep the oil in the crankcase. I was thinking of hooking up a generic clear plastic fuel filter in-line, in the tappet cover 1/2" hose to see if any oil *is* being drawn out to the carbs, but that will have to wait until driving weather returns..
Sam Sullivan

One disagreement with Jim's comments, with an SU carb if the engine is running there is (with one exception) always the same amount of vacuum between the throttle plate and the air piston and hence at the breather port, and this includes idle. You can confirm this is the case by removing the oil filler cap, and the revs will change. This is because the greater flow because of the removal of the restriction from the rocker cover weakens the mixture. In normal use, increase the vacuum by opening the throttle plate and the piston rises to reduce it again, likewise closing the throttle weakens the vacuum and the piston falls to restore it to the 'constant depression' value again. The only time it varies from this is when first opening the throttle, the piston damping does result in a larger pulse of vacuum at the breather port. By contrast, a working PCV valve delivers an even more constant level of vacuum as there is no damping on its diaphragm and valve. But this tiny difference is insignificant and is more than outweighed by the failure rate of the valves.
Paul Hunt 2

I defer to Paul's expertise concerning the SU. Also note that the discussion has not so far differentiated between the british diaphragm type PCV valve and the american plunger type. My writings have addressed the plunger type exclusively and I am quite confident that the two have entirely different modes and methods of operation.

Jim Blackwood

This thread was discussed between 04/01/2007 and 16/01/2007

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