Morris Minor Owners Club

The 1275 Midget engine works on a negative crankase pressure system, that is the pressure has to be sucked out of the sump. If this system is not correct you will get excessive leaking from the rear main bearing. There should be a pipe connecting the oil separator on the front of the timing chain cover to the carburretor. This will draw the pressure from the sump. If the carb has not got a suitable connection then you will need an exhaust emission control valve (p.99 Haynes Midget workshop manual). This is a mushroom shaped device that connects the breather pipe to a suitable location in the inlet manifold. Contact Morris Minor Centre Birmingham.
Check your geabox oil level to make sure there is no exessive wear on the front oil throw.

The engine is from a midget 1275cc.
If it is oil leaking from the rear crank seal, then what does that signifiy and how big a job is it to put right

Don't assume it's the costly and difficult rear crank seal. On my 1275 Ital there was a bad leak from the rear of the block and I feared the worst. When the engine was removed I found the oil was escaping from the edge of the cover plate on the back of the oil pump.

If the rocker cap is vented - there should be no change in engine revs when the cap is removed - it will make no difference!
The non vented caps are far better - no carcinogenic fumes/smells and the engine block will have slight negative pressure which contains the oil leaks. There is absolutely no need for any air to enter the block via a 'vented' cap.

I've been advised to unblock the sealed off pipe on top of the rocker cover. To put a long peice of rubber tube on there so it can vent harmlessly to the bottom of the engine bay. I'll need to have the carb set up though as when I unblock that pipe the car runs badly and stalls when you put the clutch in
Is there a how to on setting up SU carbs?

BMC you are missing my point. Mugwamps car is fitted with a sealed cap at the moment so removing it WILL have an effect on his mixture, it is when a vented cap is removed that there should be minimal difference
once the Carb has been set up correctly.

I've been advised to unblock the sealed off pipe on top of the rocker cover.

As standard the Midget 1275cc cover does not have an outlet pipe from the rocker cover that's why its been blanked off.

As standard the Midget 1275cc cover does not have an outlet pipe from the rocker cover that's why its been blanked off.

ok, but would that be with a vented oil filler cap?

My 1275 Sprite has:

a vented filler cap
no pipe on the rocker cover
a breather from the timing chain cover to the carbs


Mugwump, I don't agree with the advice to open up the rocker cover pipe - it'll suck in dirt. A vented cap has a gauze filter in it to keep the air clean. You'll still need to adjust the carb if you fit a vented cap - it's just an adjustment screw towards the back between those pipes and the air filter.

Another thought, do you know what grade oil the engine's got? Thinner oil (e.g. 10W40) will get out more easily than the standard grade (20W50).

Thanks for that advice Stig28. I'm going to change the oil (for 20w50, find a vented oil filler cap and adjust the carb. Then we'll see.

Does anybody know where I can get a vented cap. ESM just seem to have the same one I already have.

Isn't this one the vented type? Drop ESM an email or give them a call to check.

I bought a vented cap for mine from Halfords for about £2.50

Willie - read your post again - that's NOT what you say there!! As I said - vented cap when removed will make no difference to engine idling. Sealed cap when removed will cause poor idle. It sounds like this engine IS correct at the moment - with a sealed acp and a negative pressure in the block.
My advice - connect the rocker pipe to the carb (certainly DO NOT leave it open) and DO NOT use a vented a cap! You want suction on the block to prevent fumes and oil escaping - there is absolutely NO need for a vented cap!!

bmcecosse wrote:there is absolutely NO need for a vented cap!!

I agree. Given that the best breathing is the system that uses the pipe on the carb, a vented cap will introduce a huge vacuum leak causing over lean mixture. This is often felt as uneven running on over-run. You should also get slightly improved economy using a sealed cap, as the mixture can be set correctly. The other benefit, is of course reduced oil leaks, unless the engine is shot.

Are we just talking modified engines here or does the sealed cap apply to standard 948/1098 engines also?

Not conclusive, why didn't they put vented? Both types are listed in the
Minor Developments Ltd catalogue so you could ring 01562 747718 if
you have further trouble locating one. I always understood that the solid ones are hard to locate not the vented.

the general rule is that you need a SOLID oil filler cap if your car has a breather tube which is connected into the AIR FILTER. If your system is the type which feeds directly into the CARB you should have the VENTED
type.

Hmmmm, so I have the wrong one then... Pants again

The joys of Minor ownership!

Indeed.... Runs fine though..... Apart from the knocking big ends

OMG I'm really confused. So how would I conect the pipe from the rocker cover to the carb as there is already a pipe from the carb to the canister thingy on the front right hand side of the block. Should I join the pipe from the rocker cover into that pipe with a T piece?

I know that one of the great and interesting qualities of forums are for us all air our opinions but I think I may have opened a can of worms on this one.
Some people on here say that the system will only work well with a air inlet to the block via the vented type of cap. Others say there should be no inlet and the set up is correct and producing negative pressure.

I've ordered a vented cap anyway and have got a pice of rubber hose so I'll have a play around with different ideas. I'm also going to change the oil for 20w40 to see if it leaks less.

By the way, do keep the suggestions coming