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MG Midget and Sprite Technical - Crankcase oil breather valve

It was a nice evening yesterday to take a mate down for a pub supper as a (pre) Fathers' day thing with our 2 families coming along in a normal car.

Starting up afterwards there was a sound like a soup tin lid falling out of the engine bay onto the car park floor. Then the engine revved up to valve bounce territory and repeated each time I started.

Parking up safely and looking in the engine bay, the breather valve top had disappeared. My friend is into 2 stroke Lambrettas where air leaks are cause revving at idle reckoned the valve was allowing air into the manifold but it wasn't that at all. Looking in the car park we found the 'lid' but not the retaining spring. The pub kindly found us some bulldog clips and while they didn't blow off, the over-revving continued.

The throttle cable wasn't jammed or stuck open, it was pitch black in the car park and we couldn't see anything out of the ordinary with small torches. Being tired it must be something to do with the lid blowing off right?

Okay, before I explain what happened any ideas what caused the over-revving?

-----

Almost out of sight the clip (that held on the 'lid') had dropped under the carbs and acted as a keep the left carb open spring. The AA guy who found it after some looking couldn't believe it had fallen into the position as it looked like it had been fitted there and wasn't coming out with out good pull with pliars.

Anyhow, looking at previous threads, it seems different engines had different solutions to crank fumes. I think on my Morris Minors for example there was a pipe from the tappet cover into the air filter box without any cannisters.

Two questions: What is the likely cause of the lid blowing off? Oil pressure is 60 when cold, just above 40 when warmed up

Is there a better system for dealing with the oily crank fumes that could be fitted from a later A series or even aftermarket that doesn't involve things flying apart?

Thanks























P Peters

That system is pretty good for managing the breather system. I would guess that the spring clip had either not been clipped in fully, or had sprung loose and just needed refitting securely again. Just keep an eye on it checking to make sure it isn't working lose again.
There is a variety of breather arrangements that were designed for different versions of the engine. They are fairly sensitive to attempts at altering from the design and it is easy to cause collateral issues if changes are made. I would be cautious. It's quite a wide topic!
GuyW

Thanks Guy

Can I take it then that there isn't something blocking and causing massive pressure to blast the lid off? The design looks like it is a pressure relief system - albeit a once only release.

P Peters

The pressure, in the sump/crankcase, comes from piston blowby. That's ONE reason for the pcv valve. But that is extremely unlikely to have been enough to dislodge the lid, unless as Guy said, it wasn't clipped on properly.

The other possible pressure that *might* have blown the lid off, is if there was a misfire/spitback into the inlet manifold, *AND* the clip wasn't fitted properly. But since you don't say the car was running badly, it seems to me that Guy has it. The clip was loose enough to be dislodged by the normal amount of pressure in the crankcase.

Also, the valve is spring loaded. If the clip was loose, along with crankcase pressure, engine vibration would shake off the clip, and the valve spring would help the lid on its lift off.

There is a later system, but you'd have to swap your carbs and the timing chain cover, AND you wouldn't gain anything much, if at all.

Hence, if you haven't had trouble up to now, just make sure the clip is in place, don't change anything, and leave well alone.

I'll nick and repeat Guy's suggestion. 😏

"I would be cautious. It's quite a wide topic!"

If you're really worried it may happen again, keep the bull dog clips on there too, as belt and braces. They look quite neat. πŸ˜….

Fortunately, I assume you didn't lose any of the internal bits. Did you?


anamnesis

The operation, as I understand it isn't really as a pressure relief valve anyway. If anything it is the opposite. Its a pressure control valve.
In normal operation the incoming breather pipe (presumed #14 on the drawing, though its off screen on my image of it), connects freely to #15 at the manifold.
Suction is "moderate to fair" drawing crankcase fumes away into the manifold and adding to the combustion stream. But when manifold suction is too high, it drags the diaphragm #18 down against the spring, closing off the connecting pathway (14 to 15) to prevent excessive suction of oil via the breather.
(this excessive manifold suction is typically on deceleration from speed when carb butterfly is closed, restricting air in, but engine is still spinning fast and sucking in volumes)

On this basis, the lid top shouldn't really be under much pressure anyway. Its suction really unless as Annam says, there was blow back into the manifold and carb at some point.
GuyW

Ya see wot ya done now P Peters? 🀣🀣🀣.

You gone and done started our all time 'favourite' discussion off again.

Look/search in the archives. Follow that rabbit. Key words and phrases being oil sucking and engine breather, along with mushrooms ( not magic, but hey, go ask alice anyway). Feed your head. πŸ˜‰.

https://youtu.be/7dF2PqEq8d0?feature=shared

But seriously, lol.

Pressure in the sump is not wanted or intended. It's not about controlling the pressure per se, it's about releasing it, lest it finds its way out via oil seals, or in our case, the rear scroll.

But since you can't prevent the pressure without big open vents, or elliminating blowby (even then you may get pumped up pressure), you can call it control or release, as it amounts to the same thing. And I pretty much agree with your description of how the mushroom works.

On the later valveless system, that function is performed by the so called constant depression area in the SU carb body. But as your rubber glove demonstrated, under the right conditions, pressure remains.

But pressure is only one aspect. Ventilation is the other. Get the harmful gasses and contaminents out of the sump, not least, to help preserve the oil.

PCV. Positive Crankcase Ventilation.

Wanna know about rubber gloves P Peters?

Welcome to the funny farm.

Welcome to the nightmare.

https://youtu.be/_xRCbdFrSSc?feature=shared

https://youtu.be/aOeP4p1fjMs?feature=shared



anamnesis

@anamnesis no I have all the internals.

I'll put back together with perhaps a wire on the clip and disc (jbweld a wire on he disc, not drill) so if they ever come off at least they won't get lost or go somewhere harmful.

Anyhow thanks all.

P Peters

You were just unlucky P.

I personally wouldn't jb weld it or anything else. If the clip is in good condition and fitted correctly, they don't come adrift.
anamnesis

thanks.

I'm not going to ask about the rubber glove - it seems suspiciously related to the golden rivet in the Navy or the soap in the prison shower block where one regrets curiousity :)
P Peters

P

It’s more like a high five.
Philip Sellen

Oh, very good Philip πŸ‘‹
Bill B

Inflated 5. I'm sure Guy will oblige. He has the original visuals. πŸ˜….

anamnesis

This thread was discussed between 15/06/2025 and 18/06/2025

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