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MG Midget and Sprite Technical - 1500 Rocker Breather

Hey folks,

One of the things I have been considering doing for a while is re-instating the rocker cover breather on my engine. The original is show in the attached...

M Le Chevalier

But as I have a different carb/manifold I was considering drilling, tapping and fitting a connector to the manifold runner (there is a nice chunky boss on the manifold, as seen in attached).

I'm not really sure what the breather did/does, but it must be there for a reason, so I want to put it back. Is this sensible or stupid?

I would obviously remove the manifold to do it, to avoid any deadly swarf.


M Le Chevalier

As the breather would be after the carb it would upset the fuel/air mixture ratio so carefully metred by the carb. Earlier midgets had a PCV here to limit this, maybe you would need the same.
It would be better to breath before the carb (e.g. from the air filter box if used) so it doesn't upset the fuel/air mixture.

Rob aka MG Moneypit

"I'm not really sure what the breather did/does, but it must be there for a reason, so I want to put it back. Is this sensible or stupid?"

Answer. Sensible, as regards putting it back. Are you really being serious there, that you don't really know what it does? After all the different engine breather threads? ;)

Anyway, here's just one of zillions of explanations of why you should employ it.

But don't connect it as you've shown it. As Rob says, you'll have in effect a massive induction air leak.

Since you've gone for a weber, rather than shove the fumes in through the air box, I'd look for a pcv valve from another car. There are loads on Ebay, but I think you might have to check out the specs to make sure they will work direct to the inlet manifold correctly.

Lawrence Slater

sorry Lawrence the "I don't know what it does" was a bit of a stupid comment. I meant... I don't know if it serves a practical purpose or whether leaving it off is ok, or what the consequences of leaving it off are. I did not read the million post thread, I assumed due to the length of said thread it was a complicated subject... but maybe it was just people arguing about something inherently simple.

wrt to hook up to manifold I have not come across pcv valves, but reading up that sounds useful. I thought maybe with a straight hook up without valve it would just suck the air out of the crank case then be done and stabilise itself. but yes, on further thought that is daft on various levels!

thanks for acting as a sounding board everyonethings are making a lot more sense already!

M Le Chevalier


I assume you have no other breather, in which case let us assume the rocker cover breather is the only crankcase vent you have. The principle - loosely - is that you need to maintain a partial vaccum within the crankcase to take care of the blow-by not pressurizing the crankcase and goosing your fragile rear crankcase oil seal - at least that is what it is on the 1275 - Your Triumph lump may be different. In any case, it is undesirable due to the back pressure it creates.

Thus you need a vacuum stuck on the end of your rocker stub to withdraw the nasty gases. The trick here is not to have too much of a vacuum so it sucks up all the oil from your head, shoves it into your engine, and you create a fine display of blue smoke for all Aberdeen to admire - notwithstanding your oil consumption of course - but I don't mind that....No, what you need now that the manifold is the source of the vacuum, is some sort of a control. Hence the employment of a PCV twixt manifold and stub. However, this must be carefully sized - to prevent smoke/oil burning or too little gas withdrawal - I would imagine 1275 type would be fine.

Previously vacuum source was the carbs upstream of the butterfly - fine, but I guess now that option has gone? I note Rob's concern that it upsets the mix ratio if the manifold is now used as the vacuum source - it may do, but only very slightly. Just turn the mix screw in a wee tad at the carb to compensate.
Mark O

"... that it upsets the mix ratio if the manifold is now used as the vacuum source - it may do, but only very slightly. Just turn the mix screw in a wee tad at the carb to compensate. "

Without a PCV valve, you will apply way too much vacuum to the crankcase and suck out lots of oil, producing a 'Lawrence' cloud in the process.
Dave O'Neill 2

As Dave eludes to, the other threads were largely about oil sucking issues -- not just mine either :). I'm not so sure quite the same thing would happen to the 1500 though, as the take-off is at the top well above any continous supply of oil. But connected to the manifold without a pcv valve, it would make for a very weak mixture, and I reckon it would draw in at least droplets of oil and cause it to smoke to some extent.

Here's a better link explaining why --- apart from possible oil leaks due to crankcase pressure, and polluting the atmosphere --- , you should use PCV rather than just leaving the vent open.

It potects your engine too.

"The PCV system helps remove moisture, a major contaminant, from the oil. When an engine is run, it generates a good deal of heat. When the engine cools, condensation forms. Engine oil additives absorb this moisture and hold it in suspension. In time the moisture content exceeds the capacity of the additives. When this occurs, moisture attacks the metal parts of the engine causing damage. ------ "

Better to suck it out all the time.

If you google adjustable pcv valves, there are adjustable PCV valves on sale. Sadly they seem to be only on the USA market. But maybe a regular one could be made 'adjustable' in some way.

Here's one. USA.
Lawrence Slater

hmm... suppose this might help cure my wee oil leak too. will certainly do this before pulling various bits apart to replace gaskets etc.

M Le Chevalier

As the 1500 uses a proper rear crank oil seal I don't believe it needs to maintain a negative crankcase pressure in the way that a 1275 does. On the latter it needs this negative pressure to literally "suck" the oil back into the sump as it tries to work its way out along the rear of the crank.

On the 1500, negative pressure isn't needed, but it does need to be able to "breathe" excess pressure to the outside. At its simplest, this can be an open vent on the rocker cover but this does tend to make for a smelly and oil vapour-filled engine compartment. Better is to vent this air into the air filter box, but not direct to the manifold as the suction there is too great.

Guy W


Check out Paul Hunt's website for a little more reading...

Early B's have a PCV in the manifold almost exactly where you suggest. My 1300 also attaches to the inlet manifold.

I can probably find an old PCV in the garage for you to try (and you now what a challenge that might be :-D). The big thing to watch for is that the amount of air in via the vented rocker cover cap, excess crank case pressure or any other sources is not so large as to prevent the valve closing when vacuum is high i.e. at idle. this becomes obvious when you can't get a decent idle mixture/speed or the thing stalls when you pull up at a junction (just like my 1300 :-( ). I've temporarily removed it and the oil leak from the front of the crankcase is defo worse but it's a Mini and they all do that :-D.

Best of....
M McAndrew

Guy don't you think the oil vapour will tend to gum up the weber, if it goes in through the air box? I thought that was in part why breather pipes were moved closer to, on actually into the inlet manifold.
Lawrence Slater

It appears you're not alone in asking about crankcase venting with a weber on a 1500.

Do a google search "triumph 1500 crankcase ventilation with a weber"

And here's a good discussion about exactly you're situation with a Midget 1500, in which some have resorted to going back to a draft tube down the side of the engine, because the vent into the airbox doesn't pull enough vacuum.
"Positive crankcase pressure and Weber carb in 1500",2340974
Lawrence Slater

the memories of oil sucking flood back ;-D
Where to vent and why!

Interesting read! It's nice to know we are not alone.....

IIRC we discussed and noted that unless you have a very restrictive air filter, the pressure in the filter housing was at best marginally below atmos and hence useless for creating -ve crankcase pressure.

Next is the very old draft tube down and exiting under the engine. This relies on the "vacuum" or pull generated by the speed of the air over the tube end to create a depression and therefore draw the crankcase over pressure out. A nice underbody rust prevention solution!

And last... The PCV (or HS's but we'll discount that one!).

If Malc can put up with the after effects of the draft tube I would stick with that. Why? because I've yet to find a reliable PCV (mind you I haven't looked too hard for one). Anyone?

Best of.....
M McAndrew

I still think that the point is that the 1500 doesn't really need to produce a negative crankcase pressure because it has a proper crank seal. It only needs to balance the pressure with the outside ambient air pressure so an open vent works well enough. A little assistance as with the original tube down the side of the engine into the air stream is plenty. The only reason that some connect to the intake tract (air box) was to dispose of fumes from the engine bay. An early pollution concern. Not seeking a suction as such.

I don't know anything about Webers and how they behave with oil fumes going through them but I doubt it would gum them up. But adding oil fumes to the petrol does have the effect of lowering the octane rating which is probably a bad thing.
Guy W

I know it's the A series engine but the Maestro A series also had a proper crank oil seal but Rover saw fit to ensure it had a breather behind it connected to a Y pipe (with rocker breather) connected to the carb to provide -ve crankcase pressure. Just because it has a "proper" crank seal doesn't mean you can get away with not having proper breathing.

Rob aka MG Moneypit

It's quite a while since I've had a car with the engine breather connected to the air filter box. I can't remember which one it was, but I can remember the air box often used to be oily inside. The vapours condense on the inside surface. Messy. But I suppose it also depends on the condition of the engine to, as to how much oil vapours would be coming off the top of the engine.

Rather than spend money and time playing around with a PCV valve then, it's probably worth dropping a suitable angled end tube over the side to create a draft tube, and see how that does. If it isn't sufficient, you can try a valve.

These are the joys of fitting non-standard bits and pieces.

What was wrong with the SUs/Strombergs anyway anyway?
Lawrence Slater

Air boxes often usd to be oily inside - yes they did. Largely because the old style air filters as fitted to things like Austin A40 Devons, used a large cylindrical air cleaner containing a wire filter that you had to regularly washout with petrol and then dip in clean engine oil again!

I used one of them once as a silencer on my motorbike.
Guy W

Quite right. I'd forgottten the pleasures of oiling the mesh filter. My memory must be failing faster than I thought. lol.
Lawrence Slater

Thanks for all the input folks. I ordered a PCV valve of some description on Friday night and started taking off the manifold today. Managed as far as the stupid bottom two nuts that are a massive ar*e to get at before giving up!

To answer your Q Lawrence, I needed a new manifold and to completely overhaul the SUs anyway, so when I was offered/found a DCOE and manifold at about the same price as for fixing the SU setup I thought I would go for it just for bling factor! :-D

M Le Chevalier

To follow up on Guys thinking...

Id be tempted to vent the crank case to the exhaust pipe the way vizard outlined in his bible

You can buy a similar kit on ebay fairly cheap

All you would need to do is run the hose off the valve cover, to a pvc valve thats made for the coupling thats welded into the exhaust pipe.... (I think vizard employes a catch tank before the ex pipe)

The flow of exhaust will create the 1-3 psi of vacume ( not nessary ie. Guy, but wont harm ither ) and the nasty gets burnt and vented out the tail pipe

Personally... I think a small amount of vacume is always a good thing for old classic engines, regardless of oil seal design

Prop and the Blackhole Midget

Here is the valve i was talking about... my guess its just a pipe fitting standard that the pcv screws onto... you can probably use the product code number on the ebay site and purchase it at a local auto parts store

Also just remembering... you may want to install/place this on AFTER the muffler/cat because the back pressure of the exhaust can close the pcv ... I recall vizard saying this needs to be on a fairly free exhaust system

Prop and the Blackhole Midget

I would accept that if no pathetically fragile crankcase seal is present aka 1275, then the necessity to provide such a strong vacuum as per 1275 c/w PCV is much reduced. However, I would try and provide at least as much vacuum as the original set up - via the carbs as per pic.

If it can be provided via a downtube - fine, but Mr Bernoulli seems to suggest this will not be much - especially if airflow is impeded. If there is a stub on the new carb - upstream of the butterfly - that would be another option. Note it may be blanked off. Airbox would present a possible further option - provided it is well sealed. I seem to recall several old Golfs were piped up that way from the rocker cover.

My take...suck it and see - no pun intended - using the 3 options above which are pretty simple to effect. I would thus leave the manifold alone, only employing a PCV and threaded take off if the other 3 options fail.

Mark O

Sorry last note

DO NOT, DONT !!!... if you do, please for the love of god video the consequences of what happens....

Hook up the hose from the valve cover directly to the exhaust without this little pcv made for this application

Shooting a back fire explosion out the tail pipe is cool, but detonating your engine into a million flying shrapnel pieces like an Israeli sucide bomber would be just EPIC !

Prop and the Blackhole Midget

This thread was discussed between 07/11/2014 and 08/11/2014

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