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MG MGB Technical - crank case vent

I just bought a K&N air filter for a weber dgv. The original air cleaner had a fitting for the crank case vent. The k&n has no provision for the vent hose in bottom of air cleaner . I was told by moss to run it towards the ground. I always thought you need a small breather for that. Any one have this problem? Is there a breather made for that hose? thanks Doug
dd doug46chief

The vent pipe that runs to the ground (or to under the car) is from the tappet cover. This is the c/k vent exit and is powered by the suction created when the car is moving. The clean air supply enters via the valve cover and should be filtered ideally, hence the tube from the cover to the filter backplate. If your filters don't have an opening for the hose, you could make one, or attach a separate filter.
Art Pearse

I think Doug has a late car, with a lot of changes.

It needs air in at the rocker cover, through a vented cap or hose to inside the air filter - this originally would have come from the charcoal canisters through a restriction on the cover.

And it needs an air out from the tappet side cover, which would have been into the CD section of the carb originally, but he probably has no such place to connect. This can be connected to the manifold through a PCV valve, or less desirably, down as a road draft tube.

FR Millmore

The original road draught system on the MGB used a pipe hanging down from the front tappet chest cover, and a pipe from the rocker cover to the air cleaner but I can't remember at the moment which side. If it was the carb side, then in theory if there were enough suction from the road draught tube to overcome the negative pressure on the carb side of the filer, caused by the restriction of the filter, then filtered air would be drawn through the crankcase which is fine and dandy. But that would only be likely to happen at speed, when the greater air flow through the carbs is going to create a higher negative pressure on the carb side of the filter, which is more likely to overwhelm the suction from the draught tube. At a standstill there is no suction from the draught tube, but a slight negative pressure on the carb side of the filter. So it seems to me that the system will *always* suck air (and muck and moisture) through the road draught tube into the crankcase, with only the wire mesh filter in the tappet chest cover to stop it. If the vent in the air filter is on the open side of the filter, then paradoxically the filter is stopping rubbish from the crankcase getting into the engine!

If you dangle the tube from the front tappet chest on a later car with positive crankcase ventilation, then you are not going to get any ventilation at all, regardless of whether you fit a filter to it or not. You only need one of the two vents on the crankcase open to atmosphere to equalise pressure, but on cooling it will suck air and whatever is in it into the crankcase if there is an open tube from the tappet chest cover. By far the best route is to fit a PCV valve to the inlet manifold and connect the tappet chest cover to that - but it does need a tapping in the inlet manifold of course. If you still have the charcoal canister connected to the rocker cover that will filter air going in to the engine, but if not fit a small K&N filter to the rocker cover.
PaulH Solihull

The air inlet is on the carb side of the filter element, always. That is to provide only clean air to the crankcase.
Road draft tubes were used universally for many years - they do induce more negative pressure than the air filter restriction, and flow is as I stated, EXCEPT at standstill, or when air filter maintenance is neglected. Fouling of filter elements and carb internals was common on engines that spent a lot of time at standstill, and on engines that were already worn out.

Crankcase ventilation is NOT to "equalise" pressure; it is meant to provide a constant flow of a through the inside of the engine to draw off water vapour and other combustion byproducts, which causes mayhem. Without ventilation you get the same "mayonnaise" that is caused by leaky headgaskets, and acid formation, and thereby dead engines.

FR Millmore

Hi Fletcher,

"The air inlet is on the carb side of the filter element, always."

The ZB Magnette oil bath air cleaner seems to be an exception to your statement (see attached) Air connection from rocker cover is at 8 0'clock into the outer annulus of the filter casing, i.e. NOT the carb side. (Main air intake to filter at 4 0'clock in the picture) So the draft tube suction (at speed) is drawing ambient air from outside the filter albeit within the filter casing.

Still waiting for the engine to come back from the rebuilder by the way !


Andrew Dear

I have a canister adsorption system still hooked up to my 79b. The valve cover pipe goes to one of them. I have the weber and no provision for the crankcase

pipe. I have the pipe running down to the bottom of engine bay and its open. There are no pollution controls at all other than the canisters. I am great full for the help but a little confused about what to do. I will put a filter on the crankcase pipe coming out of the side cover . I will change the charcoal filters so I can get the gas smell out of the trunk . Will it work and not ruin the engine?
dd doug46chief

ps, in addition to the above , the valve cover cap is none vented. Doug
dd doug46chief

"Crankcase ventilation is NOT to "equalise" pressure"

I didn't intend to imply that is what crankcase ventilation is for, merely that without the suction provided by the carb(s) or a PCV valve you only need one vent to prevent crankcase pressurisation and possibly blowing seals, not two.

There is probably no point putting a filter on a tube dangling down from the tappet cover, it's only likely to prevent what little through-flow that system might give anyway. But because the charcoal canister is on the other end, that may well prevent any circulation anyway, and without positive ventilation you will almost certainly get internal condensation and corrosion - fit a PCV.

If there is a fuel smell in the boot/trunk it will be because there is a leak in the vent plumbing, filler or possibly the tank. The connection between the tank and the charcoal cannister is merely to allow the pressure in the tank to equalise when you are using fuel, and as it expands and contracts with temperature changes, via the charcoal canister which traps the vapours on expansion. The positive ventilation provided by a PCV valve is needed to purge the canister of those vapours. Without that you are likely to get fuel smells from the canister as well.

A non-vented oil filler cap is normal for a system with a charcoal canister, and the early draught tube system as well for that matter.
PaulH Solihull

Hope ok to come in on this thread dd - I picked up on the mayonnaise! I'm getting this stuff through the hole on the oil cap (new) on the valve cover. I have a late model (77 roadster) USA spec with all the emission stuff removed. I have a small KN filter on the tube from the tappet chest cover.

No blown head gasket - Have checked the oil and coolant and all clean. So am wondering how to prevent this horrible gunk. Reading the comments on this thread I assume I have a crank-case ventilation problem. It is I who has attached the KN filter - before that there was nothing on the tappet chest tube - just open. But I was getting an oily mist in the engine bay and fumes in the cabin....


Thanks Paul. So the pcv would go on the tube coming out of the side cover instead of a crankcase filter. I would guess any pcv from a auto store would do. If this is correct Paul let me know,thanks , Doug
dd doug46chief


Summary system works like this:

The system sucks clean air from the front carb filter in through the oil cap, and sucks oil vapor out of the engine through the pcv valve and flame trap into the manifold. This keeps the engine under a very light vacuum at all times, and that helps the seals not leak, and makes your oil last longer by purging any fuel vapor that blows by the rings.
MGB with HS4 were feeding air by vented cap and had those primitive diaphragm type PCV on top of intake manifold.

I fitted a Ford type PCV on a B near front oil separator cover, when diapragm were not available, and it worked fine.

For pcv systems evolution have a look:


Jean G.

P.S. Ford PCV must be in vertical position to operate adequately...
Jean Guy Catford

Doug - that's it, a generic PCV like you describe would be better than an original MGB PCV which had a diaphragm that used to split.

"MGB with HS4 were feeding air by vented cap and had those primitive diaphragm type PCV on top of intake manifold"

Not strictly correct. The MGB PCV valve was only used until 1967, in 1968 suction came from the carbs. The vented cap was used until the charcoal canister was fitted, which was 1970 for California and 1971 for the rest of the USA, but the HS carbs were used until 1972. The UK kept the vented cap until the end, with both HS and HIF carbs, as it didn't have the charcoal canister.
PaulH Solihull

"The MGB PCV valve was only used until 1967, in 1968 suction came from the carbs. "
>>Smith's PCV was used through 69, carb CD vac came in 70.

Sounds like Doug and Moss have the same setup/problem.
Need a vented oil cap and a PCV valve in the tappet chest to manifold hose. As Doug still has charcoal canisters, he can use those, but he still has leaks in the boot.

Knew some smart a** would bring up primitive systems, was even going to go look for a Magnette filter to check air flow paths - not convenient, and I'm not even sure I have one. I have examined these in the past and found the flow paths to be much less obvious than it appears, but I don't remember exactly what was going on in the labyrinth. It's still a cleaner source than just an open hole to atmosphere. Once upon a time, you didn't even get valve covers, and had to oil the rockers with a squirt can!

(The great engine builders seem a bit slow for a "no problem" situation!)

FR Millmore


That site explains all you should know on crankcase vent evolution:

And I will be a bit skeptical on strictness of evaporative evolution, as mentioned Paul, my '68 was unmodified Calif. type bought new in England by a friend. It has that diaphragm pcv with Su without vent.
I do not mentioned that air pump prone to cause some "BANG" when stopping engine...


Jean Guy Catford

"Smith's PCV was used through 69, carb CD vac came in 70"

Not according to Clausager, page 140, I'm not in a position to argue the point from personal knowledge.
PaulH Solihull

Paul I installed a pcv on the crankcase side cover and drove it, works good. It puffs wisps of light white smoke as the engine idles. I think it should but wonder as the engine is warmed up should it stop? thanks Doug
dd doug46chief

Don't have Clausager. US Moss shows emissions stuff, and it matches my memory of carbs, of which I have very many both in memory and piles!

Doug - the PCV valve connects one end to that pipe and the other into the inlet manifold. You can't possibly see any puffs if it is connected correctly.

FR Millmore

>Not according to Clausager, page 140, I'm not in a position to argue the point from personal knowledge.

I am -- my very original '69 did not have a PCV valve but rather the the tappet cover vented directly to the carbs.
Rob Edwards

The carb is a weber dgv and the manifold is also . The old air cleaner had a provision at its base for the pipe. The k and n has no provision for the pipe . Doug
dd doug46chief

After the pcv valve a tube runs it to bottom of car ,hence i can see the smoke. there is no provision for the pcv to go to except the ground unless i tap the manifold and it already has a hose going to the booster for vacum. Doug
dd doug46chief

According to Moss, change point from PCV was 187210 rdst, 187840 GT. If true, that is exactly at the change point to 70 model cars, 187211 rdst, 187841 GT.
The change to bias needles was for 69, and the venting in 70, also as I recall. My thought at the time was that they had somewhat redeemed the stupid needle design with the improved breathers.

As for Doug, what comes on aftermarket stuff is not necessarily what you (or the engine) need or want. If you do not want, or are not able, to hook it up right, that's your choice or predicament, but do not expect things to operate as they should under those rules.
Right now you have some ventilation, but it is not
(P)ositive (C)rankcase (V)entilation, nor was it with a hose connected to the filter.
Your engine will live, especially if you change the oil per old school.

FR Millmore

>According to Moss, change point from PCV was 187210 rdst,

FWIW, my roadster is 182608. Out of curiosity I checked the BL parts manual and it shows the PCV valve used up through the end of 18GF, and 18GG and forward using the direct-connect breather. GH started in 1969.

Fletcher, are you looking perhaps at an older catalog? The web page shows the PCV valve ending and the direct-connect breather starting at RD from (c)158371, GT from (c)158233 on

Rob Edwards

I dont mean to be rude but I grew up in brooklyn new york so forgive my abrupt disposition ok, thanks to all for the help. Now, is there anyone in mgb land that has put a k&n air cleaner (with no provision for the vent pipe )on a weber dgv? If u have this set up please tell me how u routed the vent pipe and what you put on the end of it or if u just ran it to the ground. thanks , Doug
dd doug46chief

Also FWIW Clausager states 158231 rdstr and 158231 GT as the change point from PCV to carb vacuum.

Doug - I initially assumed you put the PCV on the inlet manifold and connected its port to the side cover, which would be correct. But then it appears you have connected the PCV to the side cover, and have a pipe dangling. This won't do anything useful. You do need to screw the PCV into the inlet manifold. It uses the vacuum there, effectively reduces it's level from up to 20 in Hg to just 2 or 3 in Hg, and applies that to the crankcase i.e. the side cover. That draws air from through the charcoal canister is provided, or a small filter on the port on the back of the rocker cover if not, to continually scavenge the crankcase of fumes and moisture.
PaulH Solihull

Paul ,when u say inlet manifold u are talking about the carb manifold correct? If so. do I tap into the weber carb manifold and put a fitting in to connect the pcv valve and then run it to the side cover? thanks Doug
dd doug46chief

I am just looking at the weber maniflod and there is a hose running from the manifold to the brake booster. Would I be able to get the vac for the pcv off that if i t into it? Doug
dd doug46chief

Indeed I am looking at an older real paper catalogue.
And I don't necessarily believe either Moss or Clausager, especially when they don't agree with each other or themselves.
And there are US/UK/ECE differences which are poorly documented, varying by where the info source is/was or originated.
And my memory may be clouded by years and fumes and most especially the fact that so many parts have been swapped around.
I am inclined to believe your report of your car, moreso if you bought it new, but your memory could be addled too! And having worked for a couple of dealers, it was not rare that things were retrofitted, especially when we had all manner of emissions issues. FI, I personally threw a complete very early TR4 inlet manifold complete with the first iteration of ZS carbs in the trash can, replacing the set with the manifold plus SU complete supplied by Triumph factory.
I just look at the car in front of me and figure out what bits it needs to make one of the numerous combos that works.

For Doug-
Your options with that carb are
1) the original primitive road draft tube, requiring a pipe from the side cover down to the bottom of the engine, secured such that air flows across the end of the tube when the car is moving. Requires air inlet to the rocker cover, from a stand alone filter, or vented cap, or inside the air filter, or from the charcoal cannisters. Works but is not very good and has various issues.
2) a similar source of air, and a PCV valve plumbed between the side cover and manifold vacuum.
Much more effective once it is correct.

You can Tee into the vac booster line, but the connection MUST be on the manifold side of the vacuum check valve to the booster. Check valves are either screwed direct into the manifold, in the line, or at the hose connection to the booster. Problem is, I think you probably have the direct to manifold check valve, and it can be tricky to find a way to hook the
PCV line in.
Are there any cast bosses anywhere on the manifold which could be drilled for a connection? Could be anyplace from the bottom of the carb to the head, even in the carb mount flange if it is thick enough.
Depending on your facilities, it might be worth having a boss welded on the manifold if there isn't one - which there should be and very commonly is/are.

FR Millmore

Thanks guys ,I really appreciate the help on this and now have I think, a better understanding of this. I do have a boss on the manifold before the vent for the brake booster. I returned the old air cleaner with the connection for the side vent. I will run this not so good set up for now. OVER the winter I will remove the manifold and tap it ,put a hose on it and connect it to the pcv valve ,then i will run the hose to the side cover vent. Thanks to all for your patience . Doug
dd doug46chief

I concur with FRM on the inlet manifold and the booster check valve.

Ironically the recommended installation for the after-market anti-runon valve available from some sources is to Tee into the pipe between that valve and the booster. But the drawback with that is that if your ignition supply - that closes the valve for running - should fail, the valve will open and 'empty' the booster of vacuum. The check valve is there to prevent that ordinarily, which gives you two or three assisted pumps before you are on your own. It's not too bad with the remote servo as that only gives light assistance, but I understand the later master with integral servo gives significant assistance, so loss of vacuum could cause problems.
PaulH Solihull

This thread was discussed between 12/06/2012 and 20/06/2012

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