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MG MGB Technical - Crankcasing venting to one SU, is this sufficient?

Now that I have swapped my rear HS4 carb on my '80 B with another HS4 that has carb port vacuum for advance, I no longer have a port on the rear carb for crankcase venting. So I've reconnected the crankcase vent hose to the front carb and blocked the Y connection going to the rear carb. Is this acceptable or do I need vacuum from both carbs? I could fit the hoses to the rear of my air filters, but I hear this is messy. Opinions?

Hello Ron,

Definitely no. You must fit a PCV to center port of manifold( if not equipped with fitting a standard vacuum hose nipple will do the the job, and a vented cap. And also cap the small hole in valve cover. A 1/2 vinyl screw protector will do the job.


Jean Guy Catford


Why would you add a vented cap, and then block the vent in the valve cover? If you have a vented valve cover, wouldn't you use that for the air inlet and then insure that you have a non-vented cap?

C R Huff

Oops, I meant to address that last question to Jean rather than Ron.

C R Huff

I think venting through a single carburetor may work OK. I would try it.

Clifton Gordon


As a vented cap has some filtration device to avoid dust in engine.


Jean Guy Catford


I see your point. I guess we would need to know if Ron's car has the charcoal canister on it like the US cars. I believe the valve cover vent attached to it, and it acted as a filter.

When I got my car it had no canister, no PVC valve, no venting in the aftermarket aluminum valve cover, and the valve chest vent was open. I solved the problem by adding a mushroom type PCV valve, and drilling and tapping the oil cap. Then I brazed a small brass fitting shut, drilled it to match the vent on a stock valve cover, screwed it into the cap, and ran a hose from it to a separate filter.

C R Huff

My car does have the charcoal canisters. So far I haven't noticed any difference with the hose connected to the front carb and the other blocked or having the hose vented to the atmosphere. So I will remove the Y to the rear carb and just run a hose to the front carb port.

That must be a very old HS, or one not for an MGB, if it doesn't have a ventilation port, MGBs had then from 1964. You shouldn't have mis-matched carbs in any case, but having one with venting and one not means that if you do use the one with venting the mixture will be unbalanced across the throttle range even if you set it up correctly at idle.

If you *have* to have these unmatched carbs then to retain correct crankcase ventilation you should fit a PCV valve as per earlier cars and connect that to the front tappet chest cover in place of the carb with the port. The port on that carb *must* be sealed.

At the very least that carb port must be sealed, although that will disable the anti-runon valve, purging of the charcoal canister, and unless a filter is fitted to the front tappet chest port it will suck in dirt and moisture. Even with a PCV valve I'm not sure what the effect on the anti-runon valve would be as the factory never had this combination.
Paul Hunt2

I'm in the "getting close" stages to finishing a rebuild of a '76B that was converted by a PO who removed all of the emission controls and installed AUD135 SU's. He had an open air tube running from the front tappet cover and another open air tube from the back vent on the aluminum valve cover. Both tubes just ran below the engine to get an air movement vacuum and coated the underside quite nicely with oil.

So now with carbs rebuilt, new exhaust installed, new dizzy to replace the 25D, a set of K&N conicals to fit into the tight space with the servo (replacing a ram pipe with fine screen wiring across the float and no filters) and a rewire, I have to finally address what I do with the plumbing for crankcase ventilation. Naturally I checked the archive and this thread seemed to be the closest to what I am encountering.

The AUD135 carb only has a vacuum on the rear carb for the advance. So, do I connect the tappet cover breather and the valve cover breather into a generic PCV valve and then go directly to the manifold? I've attached a picture of the manifold on my car as it would seem I have a plethora of connection points to choose from. Is one preferred over any other? Or do I run the venting into the rear carb and look elsewhere for a source for the dizzy?


TJ Whitehouse


I have heard that some people did not have good luck with generic PCV valves, but I have never tried one myself.

When I had to plumb my GK engine, I used the center port on the manifold (the one with the short rubber hose in your pic) and I installed the original type Smiths mushroom PCV valve.

Then, since I had a non vented valve cover and non vented cap, I drilled and tapped the cap, installed a brass fitting that I had brazed shut and drilled to the size of the vent tube in the vented valve covers, and put a fuel filter on a hose connected to my brass fitting.

You can get some of it from the attached pic.

Are you running with no heat shield?


C R Huff

Most people have the best luck with the Smith's PCV. The typical US type PCV's seem to not work as well with our engine setups.

I think the SU's with the crankcase vent were set up so that the carburetor connection went into the chamber behind the piston but before the throttle valve. Increasing flow and throttle opening would draw progressively more flow through the engine. I'm guessing the metering needle was designed to compensate for this flow.

So, I guess with the vent connection at the carb body plugged you may want to change the needle.

Robert McCoy

Charley and Robert,

Thanks for the information and advice. I also had noted in other threads in the archive that there were some disappointing results on generic PCV's so I guess I will have to bite the bullet and buy the Smith's version.

The AUD135's are one of the non-vented SU carb versions - I'm guessing pre-emission control days but Paul H will correct me if I'm wrong - and the only vacuum point provided for is on the rear carb and that is a very small ID line which can you see on my earlier picture. The front carb doesn't have this point.

The heat shield doesn't show well on the picture but it is there and I didn't cut down the phenolic spacers so the carbs should be at the maximum distance. This of course meant the conical K&N's which also seem, like generic PCV valves, to get a bad rap from the very technical amongst us. But I wanted to try them as opposed to installing an air box and a pipe.

TJ Whitehouse

AUD135s were used from Feb 64 for the remainder of Mk1 production, with the separate PCV valve.

Without purpose-designed PCV ports on the carbs I'd go with a modern piston-type generic PCV valve plumbed between the front tappet chest cover and the inlet manifold. These are better than the original diaphragm-type MGB PCVs which are larger and prone to failure, and were only used for three years. Observe flow direction, which is from the crankcase to the manifold. You must also let air in to the system somehow, if your rocker cover has the restricted breather then it is better to put a small K&N filter on that so it is obvious what has been done, rather than block that off and fit the earlier vented and filtered oil filler cap as someone may subsequently replace that with a solid cap and stop the ventilation.

But you seem to have an alloy rocker cover, I didn't think they came with the correct restricted port. If someone has added a port to that cover then it must be restricted, to about 1/32", or it will seriously unbalance the mixture through the throttle range.

You can't use the single vacuum advance port on the rear carb for crankcase ventilation as that doesn't have the correct signal, and it will unbalance the carbs. Use that for the distributor. The reason both carbs in later incarnations have the PCV ports is to keep the carbs balanced, not because the engine needs suction from both carbs to correctly ventilate the crankcase.

But if you were getting oil from the existing ports which were just open to atmosphere then you may well find that with a PCV you will be burning oil through it and high oil consumption at least, if not smoking and plug fouling. This may be because of excessive blow-by, or by a faulty oil separator/flame trap in the front tappet chest.

What's the right-angle port at the centre of the manifold? Is that capped off? If so then that might be preferable being central. There also seems to be an open port to the rear of the small capped off port, that must also be capped off before running. However if you use that for the distributor, then the rear carb port must be capped off.
Paul Hunt


Thanks for your comment and you may have saved me some trouble as my fingers were already scrolling through the Moss catalogue for a mushroom PCV.

The alloy rocker cover has a pin-sized vent hole in the cap as well as a 5/16" ID brass fitting on the rear of the cover. This fitting was attached to a rubber hose which dangled down to the gearbox, open to the atmosphere. I've attached a picture. I'm guessing from your comment on intake restrictions that I should be removing the fitting and blocking the resulting hole in the cover.

I don't know what the right-angled fitting in the centre of the manifold was for. It is capped with a rubber "finger". Nor do I know from whenst the manifold came nor how it came to be matched to those AUD135's. Suffice it to say that a PO somewhere back in time pulled the emission controls and the ZS carburetor off.

My history with this car is very short having only acquired it sight unseen about ten months ago (I know, I know - "A fool and his money are always welcome.") But it did survive the eighty-mile drive up from the border at 60mph speed with overdrive working and fairly smooth running, albeit lacklustre pulling power, with me at the wheel trying to overlook the scarey brakes, the hole in the exhaust, the lack of working electrics and a difficulty in smoothly finding second gear. But no sooner in the garage chez moi where the car was stripped and the rebuild commenced. It is a long time since I rebuilt a car and that was a TR3A back in the 70's and I had my Dad, an ex-pat Brit, to lean over my shoulder and tell me what I was doing wrong. Thank God for the Internet.


TJ Whitehouse

I have been underwhelmed with the "mushroom" PCV. I have the original and a replacement sitting on a shelf in the garage, neither worked. Both have a diaphagm in one piece and no obvious defects. A generic one is an intersting idea.
Stan Best


If you find a modern generic PCV valve that works well, please post back with some part numbers. I used the Smiths mushroom valve because I already had it, but if it failed I would not like to pay for a replacement.

Regarding the pin hole in your valve cover, and the fitting with the rubber hose, I think you only want one, not both. That way, you can get the proper size hole for the air intake to the ventilation system.

Paul said about 1/32 inch, and he is probably right. When I brazed shut and drilled my fitting, I just stuck drill bits into a stock valve cover vent until I found the right size, and didn't check to see what size it was.

Ah yes, the TR3. I used to have a couple of those. Wish I still did.

C R Huff

Right angle fitting is where the "gulp" valve hooked in. Remove it by twist/pull and plug manifold with a 1/2" aluminum plug/loctite - looks a lot better! One of the straight fittings is for the manifold/dist vacuum, the other one is for OD vacuum on cars that have that, or possibly also gulp valve. The extra is probably 5/16 - 24 UNF, use a short bolt & copper washer to plug.

Get the correct bracket for the choke cable, it is also the nuts for one (front)air filter. While you're at it, the matching nut bracket for the rear one. The cable and the driver will be much happier; besides, it hurts me to see a cable attachment like that!

FR Millmore

Strange, I posted a reply to this and it vanished, but since I forgot some things, here we go again. (Seems I was off-line! Edit is cool!)

The right angle port is where the gulp valve hooked in; remove by pull/twist and plug with a 1/2" aluminum plug, plain bar (unthreaded) and Loctite - looks far better.
The straight ports are for gulp valve control, dist vacuum, and OD vacuum; usually they are 5/16-24 UNF; plug extras with short bolt and copper washer.

The rocker cover vent hole is bigger than Paul states - 3/32" by memory. I think these carbs would have had the filtered cap; the hole for that is 9/64". These calibrated restrictions are included in the needle profiles, so willy-nilly mix ups are not on.

Get the correct front carb bracket and trunnion connector for the choke cable anchor; it hurts to see a cable attached like that, and it eats the cable and is hard to pull. The bracket and its mate for the rear carb are the nuts for the air filter mountings.

FR Millmore

FRM - I'm sure you can see both now!

Trevor - you do need one port on the rocker cover, I'd recommend doing what Charley did i.e. brazing shut then drilling a very small hole, then fitting a small K&N filter, then using a non-vented cap.

However there is an even cheaper and fault-free way, which is how it seems to be done on some modern cars like my 2004 ZS 180. That has a tube (1/4" inch bore or so) coming off effectively the inlet manifold i.e. after the throttle butterfly, i.e. high vacuum unless the throttle is wide open, going via what looks like a flame trap but is actually a restriction (may include a flame trap) to one side of the engine, then the other side of the engine uses another 1/4" tube, unrestricted this time, going direct to the air-filter side of the butterfly, i.e. atmospheric but filtered. No PCV valve as such.

Under most throttle conditions the restriction limits how much flow there is through the crankcase. If there were no inlet then you would still get a very high vacuum in the crankcase which would be bad for the seals and oil consumption, but because the other side of the crankcase is at atmospheric there is no actual depression inside the crankcase, just a small flow through it. The MGB system always has a depression inside the crankcase because the restriction is on the inlet side, hence you need either a PCV valve to control the depression, or use the low-depression ports on the later carbs.

In theory this should reduce the oil pulled through the breather, but in fact ZSs have always tended to do this, which runs doen the throttle body and into the air filter. It also gets sucked into the inlet manifold and the Varible Intake System, which has caused problems with that. No risk there on an MGB, but it would be interesting to try that approach i.e. the restriction on the suction side rather than the intake, which could have a simple small external filter if you didn't want to plumb it into the carb side of one of the air filters, which is ironically how the first MGBs were plumbed.
Paul Hunt

FRM et al,

A lot to play with here and while I'm whiling away the time getting a new master cylinder on and getting a sway bar fitted, I'll think on how I'm going to go about this - PCV or Paul's theoretical musings on

FRM - If you can steer me on to a part number for the choke cable attachment that you mention, I would thank you for that. In all of the SU information that I have been able to glean I haven't found anything that shows the choke cable connection or mentions a choke cable connection piece of hardware. Once I figured out - thanks to the archive on this site - that the cable works in reverse of what I had expected, I was sort of stumped as to where to connect it and there was that empty hole on the front carb mounting just sitting there. When I got the car the cable was just tied to the front carb choke linkage and there was no interconnection linkage to the rear carb. I had the carbs rebuilt and purchased the interconnecting linkage and thought that this was all that was needed. This hasn't been used yet, other than just to see if movement is free, so no harm, no foul and I can put it to right with some guidance.

Paul - you mentioned a "piston-type" PCV valve and no amount of searching on line through automotive parts sites gives any information as to how the internals are arranged in the generic valves. Most appear to be right-angled types that suggest some sort of diaphram arrangement but I have seen a couple of straight line types - is that what you meant by "piston"? Do you happen to have a particular model in mind?


TJ Whitehouse

Yep, really helps to connect the computer to the line!

You say "there is an even cheaper and fault-free way", and then go on to enumerate all the things wrong with said system! But it is a good way to do it, IF you can keep the air filters from getting wet, which is not a problem with K&Ns, but is for paper. The throttle body etc. problem is not so bad on a carburetted engine, since the fuel washes the system down internally. My Mazdas all have such a system, and I periodically clean the inside of the intake pipe where the oil fumes go, to keep it from building up enough to run down into the moving vane MAF unit. Do have to keep cleaning the throttle body though.

The main point to watch in sorting this is that flow should be IN (filtered air) to the rocker cover, OUT from the tappet chest to manifold. Trevor should have an oil separator there, if he has the late tappet cover. Big problem is when flow is OUT from the rocker box, and aftermarket covers are known for not having oil splash baffles as OEM covers do.

Note that oil consumption is not caused by crankcase vacuum per se, but by flow resulting from vacuum + air inlet, intentional or otherwise. Excessive vacuum does load the oil seals excessively and will wear them out.

A good possibility here is to use a vented cap or a filter on the rocker cover, in conjunction with the Smith's valve from a Triumph Spitfire or TR6, which presses nicely into that center manifold hole, a much neater method than MG used. The Roadster Factory is super on maintaining availability of these, and I think they have the diaphragms available as spares.


Moss #s: (P23, HS4 Manifold/Air cleaner)
#50, 372-340 bracket, front air cleaner
#51, 372-330 bracket, rear air cleaner

Also (P18, Engine Controls)
#40, 372-340 Bracket, choke control = same part
#44, 375-128 Cable stop (trunnion)

"PCV valves" is a bad catch-all term, Some are true modulated control valves, like the "mushroom" Smith's one, some are just restrictions = little hole, some are just check valves (a ball bearing or a disc and a hole, some are both a restrictor and a check valve, some have flame traps, etc.

The conical K&N have a serious design deficiency. The big pan head screws that hold it together hit the vacuum chamber, which can damage it irretrievably. As supplied, they come with a 3/16" thick cork gasket to give clearance here, but it is impossible to load this such that it will stay in position and sealed and keep the bolts tight. I've seen them sucked into the carb throats, backplates bent, bolts loose. Air/dirt leaks and the possibility of ingested bolts scare me! I have in the past made and sold 3/16 aluminum spacers to correct this in conjunction with two standard gaskets. You can make these out of aluminum or phenolic plastic; or Contact me at email above if you are interested, but I will be out of touch from 7 Aug for a couple of weeks. Think I have some already made, if we can get them over the line to you.

The conical K&N are dependent on correct motor mounts to clear the booster, another subject.

Looks like you are doing a splendid job - I had trouble deciphering the picture at first due to excessive reflections!

FR Millmore

Trevor - this is an example. 'Piston' is my own terminology, 'plunger' is used in I reckon most current generic valves are of this type as they have a much smaller diameter than the original MGB diaphragm type. Should be more robust than the diaphragm type. It's not something any of my cars use so can't comment on brands you might get in Canada.

The problem with the ZS was to do with use of synthetic oils in new engines failing to allow the rings to bed in and pootling about, cured by using mineral oil for a while and giving it some hard use. I have put a clear fuel filter in my ZS breather but it has only collected about a tyre valve caps worth in several hundred miles.
Paul Hunt

Just to report back to all and sundry that I have everything plumbed up and all appears to be working tickety-boo.

I went with a generic piston-type PVC between the tappet chest cover and the centre port on the manifold. This was obtained from our national general parts supplier in Canada (Canadian Tire) so I'm not sure their part number would be of any use to anyone outside of this country. It is though the appropriate-sized valve for a 4-cyl Nissan.

There does seem to be a fair vacuum pull though at idle on the valve and this has me concerned that the internal spring arrangement in the PCV isn't strong enough. As I understand it, vacuum should be minimal at idle and increase as the throttle is opened. I'll keep an eye on this and may experiment with other valves and if I find one that works better I'll report back.

As FRM surmised, there is an oil seperator internal in the tappet cover that was fairly gunged-up when I had it off and cleaned it out.

On the conical filters - FRM is right in that they are tight to the vacuum chambers and perhaps could benefit from increased gasket width, but I don't see any possibility of ingestion of parts on the conicals mounted to HS4's and there is a good filter to carb mouth seal. But I'll keep an eye on this.

On a closing note, a fellow Ottawa MG Club member came over and set up the carbs with a Colour-Tune which has mightily impressed me.

All being equal, this garage queen should be back on the road in all of her new finery by the end of next week.

Thanks all.
TJ Whitehouse

Your setup is operating properly. The maximum vacuum is obtained at idle, with the throttle plates shut. When the throttle is opened vacuum falls off dramatically allowing the mixture to richen. RAY

Inlet manifold vacuum *is* at a maximum at idle, with the throttle plates nearly shut, and does *progressively* fall off as the throttle is opened. But whilst that causes the *volume* of mixture to increase, it doesn't richen it per se. What change in mixture richness that does occur comes from the shape of the needle as it rises in the jet. As the butterfly is opened that causes the high vacuum to be applied to the piston. That causes it to rise, and it is only then that the vacuum in the inlet manifold falls. The vacuum between butterfly and piston is relatively constant (once any initial mixture enrichment caused by the piston damper has dissipated), hence the description 'Constant Depression' that is applied to these carbs, and it is the source of the PCV vacuum after October 1968 in place of the mechanical and troublesome PCV valves.
Paul Hunt

This thread was discussed between 09/05/2008 and 07/09/2009

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