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MG Midget and Sprite Technical - Crankcase breathing - again...

@ Nick and Cherry Scoop

From your photos (in this and other topics) you appear to have an oil catch can coming off the timing case cover? Where does it breathe to ?

@ Others

Just thinking out loud, and happy to be "flamed" (!?), I have 1275 with HS2 carbs, no diaphragm thingy, just a Y piece to connect to the carbs. I have an oil catch can off the timing case that breathes to the Y piece and into the carbs. Do I need to do this? Could I just breathe to the air with a filter on the end, still using the oil catch can, and cap off the two connectors on the carbs? Nick doesn't have these two connectors on the HS1 carbs.

I do remember a while back I tried just connecting the two connectors together with a tube, but engine died after about three minutes of driving...don't remember what i did with the timing case tube though...

Or should I get a diaphragm?

I have to run with the engine oil level at the min mark, otherwise the oil catch can just fills up, and I was burning oil through the carbs before fitting the catch can, so crankcase pressure from the timing case. The breather on the rocker cover is open to the air, with a filter on it.
Tim C

The Y branch system to the carbs was the last of a series of different breather systems on these engines. As engines grew in size from 803 up to 1275 the potential crankcase pressure increased, and with it the volume of oil leakage past the rear crankshaft scroll 'seal'. So the breather system design was improved in steps, the aim being not just to breathe out fumes but to actively extract from the engine's innards with the aim of achieving a negative pressure (i.e. below the external ambient pressure) to stop oil being forced out.

With the Y branch system, Tim, you have the latest and most efficient version. That is if it hasn't been modified. It must have the correct for year filtered oil filler cap. It must have the washer seal on the dip stick tube. And it should have no added extra breather ports to the open air Addition of an oil catch tank is ok so long as it is of the sealed system variety.

You will still burn excessive oil of the engine is worn, and particularly if the rocker shaft is worn. Its often ignored!

Do you have the correct dipstick? There are various lengths for different engines. Might you have acquired a shorter one?
GuyW

If your engine is sucking oil from the crankcase, then the oil mist lowers the octane of the fuel mixture, so although I had to add a catch tank due to huge oil consumption, I have left it in place.

It's know issue on the modern eco boost engines that as they wear and start to burn oil, the oil mixture pulls down the octane rating and causes detonation in the engine, clearly the eco boost engines are running high boost and very advanced timing, but even if the catch just collects a bit of oil, it is helping
Malc Gilliver

Thanks both for your comments.

@Guy

Lord knows about the oil filler cap, I have a chromed rocker cover with plastic filler cap. The rocker cover has a "vent" pipe to which i have fitted a tube to a filter, so it is open to the air.

I have recently checked and measured dipstick, following on from another topic on this, and it matches in length with the images shown for other 1275 dipsticks. Still has a rubber washer on it, that fits snugly.

I rebuilt the engine (+40 pistons) @ 45 years ago, only done @ 20,000 miles on it. Rockers not touched at that time or since. No2 cylinder seems a bit oily / black plug, worse than the others, but still working on getting the mixture right.

After engine rebuild, car sat in primer for 12 years, then back on the road for 5 years, then off again for 20 years, before I brought it back to life. I had most of the engine apart 5 years ago, but left the pistons and crank in place. Plenty of power...

Engine is from a 1972 car, but everything else is 1962 - sort of...
Tim C

The filler cap sounds ok. The correct one is plastic, and has a filter built in with an air slot under part of the overhanging top. The idea is that it draws air in, but effectively restricts the flow so that the suction from the carbs creates the negative crankcase pressure, whilst still pulling fumes out. Your setup sounds about right except for that open breather pipe.

I made a catch tank and put it in the breather pipe between the timing chain case and the carbs. Its sealed so the suction from the carbs doesn't pull in external air at that point.

I can't suggest why your engine should be prone to sucking oil through. The reason I mentioned the rocker shaft is that when worn the result is too much oil leaking past the bushes, flooding the cylinder head top and then getting drawn down the valve guides. I think the rocker shaft can get forgotten in an engine refurb as it appears to work just fine and not need renewing.
GuyW

Sorry Tim - I missed this one.
My set-up is quirky; the first owner liked little mods like this.
The breather goes to air, with a drain for oil into a catch-pot, and he probably enjoyed getting the bits made up - all because he liked the trumpets instead of filters with the breather connection on the front one.

My engine's probably only done 25,000 miles since it had a rebuilt engine, and I don't think I've ever had to empty the catch-pot. So the 948cc experience isn't going to help.
Nick and Cherry Scoop

@Nick & @Guy thanks.

I am still intrigued by the difference between the HS1 and HS2 carbs, the latter having (needing) the connectors for the Y piece?

@Guy, should I be plugging the rocker cover vent/tube then, or connecting it to something ?
Tim C

Tim, the carb Y piece connection is for the benefit if the breather/extraction system, rather the something "needed" by the carbs.

The earlier extraction system used the suction at the inlet manifold. The problem there is that suction can be much higher - too high in fact so a pressure control device was needed. The mushroom shaped thing you see on earlier cars. The other problem is that manifold suction is highest on over-run which isn't when piston blow by is at its peak. Moving the breather point to the carb body, upstream of the carb butterfly in part solves both of those issues.

Suction at the carbs is surprisingly low. I did some test measuring some while ago and was getting between just 3 and 5 psi.

I would plug that extra rocker cover breather. At least as an easy test to see what difference it makes. You can always unplug it again!I

My oil catch tank consists of a sealed can (illy coffee tin) with the incoming pipe from the chaincase extended inside right to the bottom of the can. Above this is a perforated baffle plate with a stainless steel pan scraper sitting on top. The theory is that this catches oil vapour which then drips down to the bottom. The outlet pipe to the carb Y piece is close to the top so it sucks the fumes through without picking up any oil from the bottom.

That said, it doesn't accumulate a lot of oil. Maybe an eggcup full every 1000 miles? Its definately not pulling oil through, just fumes and oil vapour.
GuyW

A few things to try out then. Will advise on how it goes.
Tim C

I'm surprised than Anamn hasnt pitched in with lengthy discussion on why he thinks some cars suck in copious amounts of oil through the breather systems whilst many don't. But maybe, Tim, you have already trawled the archives for that, and survived!
GuyW

Lol Guy. I don't need to. You've explained everything Tim needs to know.

I'll only add a picture and a comment.

This is the correct oil filler cap for Tim's needs, with the rocker vent capped off. Arrows show Guy's description of vents. Note the small size of the hole.

Also, the oil does not get sucked up from the sump/crankcase. It gets sucked out of the timing chain cover, when/if excess oil is in there. Why that happens is the other subject; again as Guy says covered copiously in the archives. 🤣🤣.



anamnesis

Yes as I recall, many hundreds of posts about the oil issue eventually came down to oil accumulating to too great a depth in the chaincase and not draining down back into the crankcase fast enough via the front crank bearing or drain holes. I never quite grasped why this should happen on some engines and not others.
GuyW

Very interesting. It's 20 years since I rebuilt a 1275 engine - coincidentally also into a 1962 car - and I recall the tube filter on the front of the timing cover. The hose must have led to a spigot on the HIF44, I guess. The rocker cover certainly had a ventilating tube, but I don't remember what I attached to it.

It's hard to imagine much negative pressure achieved on a 948cc engine. The rocker cover to front air filter set-up looks unconvincing. But there's also the drain pipe from the tappet chest, which ends under the car. Is the rush of air across the pipe end supposed to suck?
Nick and Cherry Scoop

Just re-read that last post. Utterly useless to Tim. Sorry.
Nick and Cherry Scoop

And it remains a mystery Guy. The only common denominator seems to be that the engines that do 'suck', have been rebuilt at some point. But, not all engines that been rebuilt, suck oil. A goldilocks factor at play.

Speculate Mr.Spock. 🤣.

"Vulcanians do not speculate. I speak from pure logic. If I let go of a hammer on a planet that has a positive gravity, I need not see it fall to know that it has in fact fallen."

If an engine sucks oil, there must be a perfectly logical and fascinating explanation.

Yeah right. 🤣🤣.

https://youtu.be/cFods1KSWsQ?si=5uAHobSO-YPw3kIT


Nick. It's not a drain tube. It's a draught tube, that does indeed cause suction from the engine block.

Research crankcase ventilation; both passive, and so called Positive(PCV).

Careful chaps. I see a rabbit's hole entrance ahead. 🤣



anamnesis

@Nick

"Utterly useless"

Not at all, everything has a place.

@Guy

"archives"

Yes, I read the really long one awhile ago, what did it get to, +500 posts and counting ?
Tim C

Just been out to check things over. My cap on the rocker cover is just a simple cap, no hole or vent on the side, so guess I do need to use the rocker cover vent pipe.

Need next to test with Y piece disconnected from the carbs and the spigots left open / blocked off. This will have to wait until next weekend.
Tim C

Tim,

You said.

"I have 1275 with HS2 carbs, no diaphragm thingy, just a Y piece to connect to the carbs. "

In that case in order for the pcv to work, you MUST block off the rocker cover vent, and get a vented oil filler cap with the restricted vent shown in my picture earlier with the red arrows.

Or I suppose you *might* be able to continue with the sealed cap, and create a FILTERED SMALL vent hole in the existing large rocker cover vent. But I suspect that would block up constantly.

The carbs suck air from the timing chain cover, through the engine block, down from the rocker cover. The vent in the rocker MUST be small enough to allow negative pressure to develope in the sump.

If you don't want pcv, then you have to leave make it all as open as possible.

But pcv isn't only about reducing crankcase(sump) prrssure, from the effects of blow by.

It's equally about removing corrosive gasses, protecting the oil, and minimising condensation; -- all of which protects the engine.



anamnesis

pcv seems to cause some confusion. I've seen it as short for Pressure Control Valve (n) - i.e. the mushroom thing on early 1275's extracting via the manifold.

But pcv also stands for Positive Crankcase Ventilation. - i.e. a system where crankcase ventilation is positively assisted by an extraction process.

Take your choice!
GuyW

Whoops. Posted this in the wrong thread.

----

Maybe whoever named it should have called it Active cv vs Passive cv.

anamnesis

Bought a new vented rocker cap, to fit, and search boxes for suitable plugs for the holes!

Then test.
Tim C

Personally i think you're better off with everything sealed off except the hose to the carbs, any leak anywhere just increases the flow up the hose.
William Revit

That's what I have William. No vent from the rocker cover, 1098 side cover with oil separator plumbed to the inlet manifold, sealed oil filler cap.
Les Rose

This thread was discussed between 03/09/2026 and 19/09/2026

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