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MG Midget and Sprite Technical - How do i best vent my engine?

We have reduced several bad rear main scroll oil leaks in XPG engines to at most a mere finger-nail sized drop by the installation of a closed circuit system and nothing else.
We use a simple PCV valve available over the counter locally, Tpyota application IIRC. Sizing of the valve is important, we got supplied a Chevvy one by mistake and it caused running problems. Fitted the correct one and the problem went away.
Paul Walbran

er, make that XPAG ...
Paul Walbran

I've been back through the archive, trying to find out what a PCV is, and I have.
But I see that it isn't relevant to me with a 948.
Before I took her to pieces, Cherry did leak a fair bit from the rear of the engine, and I now assume one reason was no vacuum under the rocker cover. This is how the breather was terminated, to atmosphere with a dinky little oil catch tank.
I haven't decided what to install upstream of the carbs yet, but I guess a bit of vacuum might help.

Nick and Cherry Scoop

I have the rear crankshaft oil seal kit but there is still a leak. I fitted the standard 1098 closed circuit breather, with additional pipe from the redundant fuel pump take-off on the block. This seems to help a bit, but if there's a better PCV I might try that.
L B Rose

How does the closed circuit breather work, Les? That take-off position sounds perfect - right in the crankcase.

'Scuse my ignorance but, on the 948, what's the route from rocker cover to crankcase, for the vacuum to be passed along?
Nick and Cherry Scoop

The early A-series uses a 'draft tube' that hangs down from the front tappet cover into the air stream below the car, and was intended to create a partial vacuum (maybe more hope than reality?), in the crankcase. The air comes INTO the crankcase via the hose connection between the rocker cover and the air filter - it draws filtered air IN from the air filter. The oil filler cap is NON-VENTED, and must be, as it plays no part in the ventilation process.

At low/no speed there is no ventilation taking place, and quite often oil drips can be seen coming out of the draft tube.

The second version is a 'closed circuit design', and uses a PCV valve in the inlet manifold to create a controlled partial vacuum in the crankcase, via the connection to the front tappet cover. It uses a VENTED and filtered oil filler cap, and there is no connection to either the air filter or carburettor.

At full throttle, there is very little, if any, ventilation taking place.

The final version is also a 'closed circuit design', and instead of using a PCV to create the partial vacuum, uses a connection from the timing cover to a port on the side of the carburettor to achieve the same effect. Again it uses the same VENTED and filtered oil filler cap, and there is no connection to the air filter.

The carburettor port sees some vacuum at all throttle opening, so provides the most consistent ventilation design.

With a Weber, the options are basically a PCV valve in the inlet manifold and a connection to either the front tappet cover (not 1275) or the timing cover, or an 'OPEN' system with breathers and a catch tank, but this will NOT create any vacuum and very likely will result in oil drips from the engine.
Richard Wale

Thank you, Richard; that turns my thinking on its head. I have been puzzling over types of air filter, believing that the spigot on the front Cooper SUCKS. But if it's simply a filtering ventilator, then the present (illustrated) set-up works, and my choice of filters expands.

I always wondered why there was never any oil in the catch-pot. I expect it was on the garage floor instead. I find it best not to worry too much about that.
Nick and Cherry Scoop

In fact - I don't see why I shouldn't use an air filter that fits an HS2 (loads more options) and just modify things slightly to fit. Here's what the H1 carb outer face looks like, with the H2 holes superimposed. The top right opening is just the compensating air passage to allow the piston to rise, and I reckon half the area should be ample.

Easy-peasy - no?

Nick and Cherry Scoop

Sorry Nick for late reply. I have the standard 1098 front side cover with oil separator, plus a second separator that I welded up. That connects to the fuel pump point with rubber hose. Both separators then connect via a Y-piece to the standard 1098 breather valve. With the 1098 system you have to use the vented oil filler cap - which also has a valve in it. As I say, not perfect and I'm open to suggestions!

L B Rose

Thanks, Les. That sounds good, and I'm surprised you're still leaking.
Nick and Cherry Scoop

So am I Nick, but in the 40 years I have had this car it has always leaked, even after changing the engine and rebuilding that twice! I console myself with the automatic rustproofing underneath. When installing the crankshaft lip seal kit I always wondered how effective it would be at such a huge diameter. I did get a Speedy Sleeve put on the flange and there isn't any more I can do to that. The closed circuit system probably helps, but I still have problems with getting a stable idle speed. Right now I'm running without the closed circuit system, just a catch tank, and the idling is better. Might try a PCV one day.
L B Rose

Vented and filtered oil filler cap photo for reference, as I'm sorting photos -

Nigel Atkins

I've been doing some further reading around this subject, and it occurs to me that there are major limitations of the standard 1098 breather system. The valve is designed to prevent excessive crankcase vacuum under closed throttle conditions, but when cruising fast and the valve is open, the manifold vacuum is nearly zero. So just when you are getting maximum blow-by, there is hardly any vacuum to deal with it. On reflection, I suppose the idea might be that at open throttle all you really want is breathing to atmosphere or thereabouts, but there must be considerable resistance from the plumbing and especially the oil separators. So a bit of suck would not go amiss.

Also, on my single carb system the valve is connected to the rear branch of the manifold, albeit a fair way up stream. I never see any evidence of excessive carbon in the rear 2 cylinders (head off the other week as you might remember) so there is very little oil getting into the intake. But I wonder if bleeding air into the manifold asymmetrically makes sense, however minimal. Obviously air comes in via the little hole in the filler cap.

The point of these ramblings is that I would like to get some more crankcase vacuum so as to reduce the leaks. As an experiment I am running with just a catch tank at present (never see anything in it), but the timing cover is now leaking - despite a sleeve on the pulley and a new lip seal. With the closed system it didn't leak. The persistent leak is from the rear crankshaft, and I would love to deal with that.

In David Vizard's book he talks about a constant vacuum system. This needs a tube inserted into the exhaust manifold that points down stream, so that the gas flow sucks on it (there is a blowback valve for obvious safety reasons). This augments the inlet manifold suction, so there is vacuum at both closed and open throttle conditions. The filler cap is sealed. Has anyone tried this? What about an engine driven suction pump? That might generate vacuum proportional to engine revs, and roughly to throttle opening.

L B Rose

>>air comes in via the little hole in the filler cap<< if that means you have a metal oil filler cap rather than the plastic one in the photo I put up then a change might help. I was told many years ago that the plastic type was more efficient than the metal type.

At 3.25 (Oil filler cap- Vented GFE6003 MGOC Spares) it's worth a try.
Nigel Atkins


I have the correct plastic cap. I haven't changed it for years so maybe I should.

BTW The timing cover isn't leaking. The wet patch on the cross member was water from the cooling shenanigans.

I just looked inside the breather valve again. It is very badly worn and can't be working correctly - hence the idling problems. But a new one is nearly 60! Maybe one from a modern car will work, but which?
L B Rose

You could connect your vacuum take off to the carb rather than the manifold. The problems with the manifold connection are twofold. Firstly it provides the maximum suction at the wrong times when you are feathering the accelerator, or on the overrun, as you have identified. The other problem is the suction is too great, and hence the need for a Pressure Control Valve.

If you modified to the 1275 engine set up then the suction point on the carb is upstream of the butterfly and is directly proportional to the volume of air being drawn through the carb, and hence the revs of the engine. Just what is needed! The suction at the carb is much less than at the manifold so it doesn't need a PCV , just a direct connection - another bonus. Your HIF44 should have a suction pick up point already, at least mine does.

You can still connect via an oil separator if you wish, you just plumb it in between the chaincase connecting pipe and the carb. I also have one, but it doesn't collect a great deal of oil.

I've done the visard modification you describe

Guy ... your correct

but in this case the vent pipe les is describing he is using the exhaust manifold not the intake manifold to create the negitive pressure.

My experisnce with the vizard system was half completed and I never followed thru with it as my engine once broke in after the rebuild has not leaked at the rear scroll seal. So I never went th ur with it to completion but the parts are there in place should I need to

As is described in vizaards book will NOT work or work very well as laid out for a street car with a everyday muffler / silencer... The exhauste HAS to be non restrictive unobstructive pass thur... like straight pipes with little or no bends... And really the exhaust pipe would benefit from being sectioned into 1foot sections and reassembled so the each pipe starts wider then narrows into the next section in order to form a anti back wash for lack of a better word

The big problem is for a street application there is to much back wash of the exhausts and it pushes against the anti pressure valve shutting it off and thus creating a sudden positive spike of pressure in the crank case.

That said... I do have an untested solution that has a decent chance doc working

What I did was attach the vent to onto the raight part of the exhausts pipe it's self instead of the exhaust manifold to take out the bends in the manifold

I had a tube about 3 inches long attached to the exhaust pipe with the vent blow back valve attached to the end if it, then the vent to attached to the end of that blow back valve ... then a very small muffler (haven't sourced yet) for like a small lawn mower tractor 8-10 HP, at the end of that to quite the exhaust sound

This will (maybe ???) Provide an unobstructed exhaust gas flow that will create the negative crank case pressure.

I hope that made scence

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****What I did was attach the vent to onto the raight part of the exhausts pipe it's self instead of the exhaust manifold to take out the bends in the manifold ***

EDITED & Should read....

What I did was attach the vent pipe onto the straight And level part of the exhausts pipe well past the exhausts manifold to take out all the bends and flow obstructions

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Yes Prop, I had seen that Les was asking if anyone has experience of the exhaust extraction system as described in Vizard. But I didn't think that is the system he was using at the moment.
I was suggesting an alternative solution as works on the 1275.

I do have a separate idea along the same line of the vizard solution but does not involve the exhaust manifold to create the vacume... but it's only half developed/ backed idea and have not presumed it beyond basic therory

My thought is to...

Use a funnel on the front section of the car behind the grill in one of the large 6 inch Diameter holes ... maybe 6-9 inches at the funnel widest opening and 1 inch Diameter at the funnels exit with a hose attached at the exit point leading out the back of the car as straight as possible and have a one way valve then have the crankcase vent hose connected just just before the one way valve then a bit more hose angled at the exit into the wind under the car to help create a bit of suction

The idea is as your driving Air would pass thur the funnel into the hose and create a vacume on the vent hose from the crank case then exit past the one way valve with a bit more hose attached to it into the wind stream under the car

The idea is to...

have forced air pressure at the funnel plus sucking air pressure at the exit of the system ... air would constantly be flowing until the car is at idel and or stopped moving forward at which time the one way valve would activate shutting down the pipe thus not allowing a vacume leak back into the engine while no air pressure is being sucked out of the crank case

That's the theory anyway, but I have nothing in practice but your welcome to it

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Sorry guy

I wasn't sure what direction you were presuing... I'm a little off today. I'm on the back side of the flu epidemic going around

Luckily I'm not in to bad a shape like others

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I'd be curious if guns are better then one for creating suction has any one tested the notion

My thinking is if you have only one carb delivering the same amount of CFM of air into the combustion chamber via the carbs as 2 seprate carbs by way of a splitter

Then I'd think the single carb has the advantage due to just effiancey alone as you would not have the constant back and forth sharing of of carb pressure due to a siemise head that is forced to share one exhauste port in cly 2/3

Maybe that's an easier solution for engines with worn piston rings and are more reliant on crank case negative pressure
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Maybe the real solution ...

to the rear crank shaft seal leaking... is to just fix the problem instead of extensive creative work arounds

The whole problem surrounding the rear scroll seal leaking is two fold and covers a good 80 to 90% of the problem

1..external air leaks into the crankcase
2... worn piston rings causing blow by that pressurize the crankcase

Really those 2 conditions are the heart and soul of the leaky rear crank shaft debacle

I'd say 1st try sealisealing the crankcase system up, valve cover, sump pan gasket dip stick, front engine cover area ECT ...

If that didn't work then a weekend minor engine in place rebuild ... not extensive but just basic maintance type of rebuild with the engine in place which is great if you don't have the space, tools, experiance time and ... CASH for a proper full on rebuild

All you would need is new piston rings (gapless would be interesting) , a cly hone, a head gasket, big end bearing shells for the connecting rods and new connecting rod hardware and a sump gasket, cly head studs /nuts and probably 3 free days as im sure once your into it there will be other " while I'm there might as well projects"

But if you fixes those 2 issues and thinking about it how hard is that to do what I outlined above

Then if its still leaks ... it may need a more in-depth extensive rebuild so what have you really lost... maybe $200- $300 USA money and a few hours ,compared to what all the time and expense of doing an end run around ?

The good news is... you got a chance to clean out the sludge and replace the bearing shells and hardware adding alot of new life to a tired engine ..its not perfect, but certainly with in the scope of the avg person with an avg tool collection and avg skill level.

Its Just a thought...

God I should do more anti flu drugs on a daily basis... that almost sounded like I knew what I was talking about... haha

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Actually the biggest favour you can do is to ensure the rear scroll is assembled to the correct tolerance. If the diametrical clearance is kept at .002" and you don't have excessive crankshaft whip, you should be able to keep the rear of the sump pretty much dry - either running a properly set up closed circuit system, or one that's fully open to atmosphere if you suffer excessive crankcase pressurisation. Although sump gaskets and their cork seals often get blamed - if oil is coming out of the rattle pin, it almost certainly points to a worn scroll or incorrectly set up half moon. Unfortunately with engines as old as these new rings and honing will make little difference if the scroll is worn outside of tolerance. You are left either with line boring, adding a rear seal kit, or doing a JB Weld squeeze cast as described by John Twist.
f pollock

And that's wear the next level comes in of a full on complete engine out proper rebuild with a machine shop and lots of massaging

I just think if if your going to go though the task of creating a work around then at least try what I'm calling a maintenance rebuild

And your right about the half moon mine had to be massaged as well during the rebuild and it still leaked until the new rings finally bed into the cly at about 1500 miles and the rear scroll has been dry ever scence

It's an odd line... do you do a work around, do you do a maintenance rebuild or do you do a full blown rebuild... those are your 3 option

Ideally #3 is the best of the 3 options but it will take cash, time skills space and tools how many of us have all 5 of those lying around

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Guy, yes the HIF44 has a vacuum point upstream of the throttle, but it's tiny. The breather valve fits on a " fitting on the manifold. So do I adapt my " hoses from the oil separators down to this tiny connection? I assume the valve won't be needed. Will there be enough breathing through this little orifice?

L B Rose

Les this has been discussed many times before. Some years ago now I measured the vacuum draw on my carbs to see how much was needed to work the 1275 positive cranckase ventilation system. I assumed it would be quite a lot but as I recall, it was surprisingly little, measuring about 2"Hg at 3000 rpm.

There will be lengthy discussions of this somewhere in the Archives but I "don't do" archives on the BBS as I have never yet manage to get the search engine to pull out the information I am seeking.

As I recall, I had started out confirming that with the breather hose from the timing chain case disconnected - i.e. open to atmosphere, but no positive extraction, at 3000rpm my engine built up a positive pressure in the rocker cover (which is of course the same interconnected void as the crankcase).
Found these photos:
1. This is demonstrated as it inflated a rubber glove attached over the oil filler hole. You can also see in the bacground the vacuum guage fitted to the carb vacuum take-of point


2. The vacuum guage reads 2"Hg. at 3000rpm

If I then reconnected the oil breather pipe in place of the vacuum guage, the glove, far from inflating, was in danger of being dragged inside the rocker cover as the revs increased.

In this photo, you can also see my DIY in-line oil catch tank, bottom left of picture.


The old trick was to put a beer mat on the open filler hole. If you could rev the engine and it stayed in place, the PCV was considered correctly matched versus pressurisation.
f pollock

My vac gauge is in psi not bars but I measured this at around 2 psi IIRC


why the huge air cleaner for a single carb... left over parts from something else? An interesting test would be to install a smaller pancake air cleaner and see how that effects the performance as I'd guess you would have more construction on the carb venturi hole creating more air suction into the carb ... (less air space = more forced air

What did you make your catch tank out of... like it, small and compact ... but I'd fear the smaller diameter hose going in and out would create more suction inside the crank case which is why the oil cap it trying to eat the glove so readily

Then agian... it's a single carb so you would need to double the suction of the carb to equal a duel HS2 set up

Even tho it didn't appear to be catching anything measurable... I think it's a great idea just the same as its catching really fine oil particals... over time that crap adds up, and the secret to trouble free carbs is keeping them spotless clean... cleaner the better as you already know.

As just a general thought I've wondered about a air compressor butterfly valve after the catch tank to better regulate the amount of air pressure that the carbs are sucking out of the crank case in addition to a PVC valve

Thanks for the photos of your set up

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F P.

My dad back in the 70s would hold a dollar bill over the same hole...

if the dollar bill flapped in the wind ... then he had a dollar to buy a new pvc valve, if it didn't flap and got sucked in while holding it... then he had a dollar bill to test agian 6 months later


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Ahh, questions Prop !
The catch tank was made from an Illy coffeee can. They are metal with a screw lid. I fitted two connectors on the lid with the inlet pipe going about 2/3rds of the way down and the fumes are then drawn up through a SS pan scourer and through the outlet that is connected to the carb vacuum. This system doesn't need a PCV as the vacuum is drawn off upstream of the carb butterfly so the suction is self regulating and responds directly to engine revs.

The large air filter is because I adapted it from the K&N filter formerly fitted to my twin SUs. I made a new backplate to fit the HIF44 carb. Inside there is a size matched bellmouth so it doesn't create turbulance problems but the large size provides ample air flow. It works just fine, and gives a nice induction roar too!


Put an O in front of the logo and it would be perfect.

MG Moneypit

Cool Coffee cans, we don't have any thing like that

I followed the same path and used an aftermarket rad catch tank

I was wondering if you had a bellhousing in there, good idea

Just clarification I was thinking of the pvc system for drawing suction from the manifold side... I sort of forgot to mention that...haha
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My engine has a vent pipe the like of which I have not seen before. Can anybody identify it?
It also has the timing case vent but this is not currently plumbed into anything. Is there a way this and/or the vent off the block could be piped to the Weber manifold?

Clive Berry

Image of vent for post above!

Clive Berry

Did similar with an HIF 44 setup. Vent from fuel pump mounting (got a nice billet aluminium adapter from one of the mini suppliers) to the air filter housing. Did a great job of reducing the leaks to almost nothing.
T Dafforn

I now have both breathers connected to the HIF44 upstream of the throttle, and the leak has been reduced a fair bit. Good result so far.

L B Rose

Clive thats the mechanical fuel pump take off correct thats been vented to atmosphere?
DJ Paul

Correct dj, just to atmosphere. It needs to be piped somewhere!
Clive Berry

I'd pipe it to the air cleaners in a way that you can get suction from the carbs venture hole

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This thread was discussed between 06/01/2018 and 03/03/2018

MG Midget and Sprite Technical index

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