Morris Minor Owners Club

Hi
I have a 1970 with rocker cover to carb ventilation plus open breather tube from tappet chest cover. It currently has a vented oil filler cap but I’m getting quite a few oil leaks. The engine is in good condition and exhibits no characteristics of excessive wear.

Is this system negative vacuum and as such does it need a non vented cap?

If this isn’t the case, should I try to change to a pcv system and what would be required to accomplish this.

I’d appreciate any advice and thank everyone in advance for replies.

Andy Bradbury

The system that you have is not as it would have been on a 1970 car. Perhaps it is a replacement engine or somebody mixed and matched parts. For a 1970 car I think they had moved on from the PCV valve (which was only used for about a year on Minors) to a system with a breather inlet on a special type of breather HS2 carburettor and a pipe between carb breather and tappet chest oil separator.

Your system can only alleviate crankcase pressure to match atmospheric pressure but that should be perfectly adequate. There should be a solid oil filler cap and the air entry is via the tappet cover tube. Very often that tappet chest tube is totally blocked with crud. The first thing I would do is remove the tappet chest cover and the extension tube and try to blow down the tube. If it is blocked or restricted it will need poking out with wire. Then fit a solid oil filler cap and see how it goes. If that is unsatisfactory you could also try a new vented oil filler cap or clean the old one out with carburettor cleaner as they get blocked.

Stephen

Thank you Stephen

I shall change to a non vented filler cap and inspect the tappet cover pipe and clean as necessary.

Do you think that would be adequate or should I try to replicate the later system.

If anyone has a photo of what this looks like, I would be grateful.

Many thanks

Andy

This is the setup on the ‘71 pickup. The chimney from the tapper chest cover is connected to the inlet manifold. We have always used a short vented oil filler cap.

Andyeagle8 wrote: ↑Mon Aug 17, 2026 2:16 pm Thank you Stephen

I shall change to a non vented filler cap and inspect the tappet cover pipe and clean as necessary.

Do you think that would be adequate or should I try to replicate the later system.

If anyone has a photo of what this looks like, I would be grateful.

Many thanks

Andy

I think it should be adequate. This breathing system was used for a long time with success. The reason for the change to first the PCV valve system and then the breather pipe to carburettor was tighter emissions regulations and the need to move to so-called "closed-circuit" engine breathing (although it isn't absolutely "closed" because fresh air is admitted via the restricted vented oil filler cap. Engines that had closed-circuit breathing have engine numbers starting 10ME or 10V rather than 10MA.

This is the breather type HS2 carburettor. You can clearly see the brass breather stub pipe: https://www.morrisminorspares.com/fuel- ... s-p1238998

The breather carburettor was used with a special pipe that has a different diameter at either end: https://www.minispares.com/12a1735-brea ... er-to-carb

Stephen

Just for interest this is from my 1969 Morris Minor 1000 Driver's Handbook (my car is March 1969).

Stephen

Thanks Stephen and Andmurph for the photos and good advice.

I’ll clean my system and change to a non vented cap and see what difference that makes.

I will also look out for a tappet cover with a catch can to see what extra benefits it could bring.

Regards

Andy

Andyeagle8 wrote: ↑Mon Aug 17, 2026 6:38 pm Thanks Stephen and Andmurph for the photos and good advice.

I’ll clean my system and change to a non vented cap and see what difference that makes.

I will also look out for a tappet cover with a catch can to see what extra benefits it could bring.

Regards

Andy

It's not a catch can exactly, it's an oil separator. The idea is the hot oily fumes condense in the can, which is filled with steel wool, and the oil goes back in the engine. The fumes go back into the engine via either a PCV or the carburettor and are burnt.

Stephen

Yes you are right, I can see the difference. Most people wrongly call it a catch can and I got swept along with the tide!

Thanks for your help, you have added clarity to an area that I knew nothing about.

Regards

Andy