Dear All
This is the Morris Minor "other" forum, so maybe the best place to sort my dilema...
I have researched this via the web ,Utube experts? ,Wikied it , etc.etc.etc.
Have fitted a Positive Crankcase Ventilation valve fitted atop the oil seperator/breather , fitted to the fuel pump opening.
My dilema is where to plumb it and why?
To the air-side of the filter ( low vacuum?)
Between the air filter and the throttle plate ( higher vacuum?)
To the inlet manifold ( Higher varying vacuum? )
All offerings considered...
Thank you
John
Not very good pictures but I mounted mine to a port on the alloy inlet manifold I fitted. the other end went to the crankcase breather. I've since pinched the port for a servo but will probably drill the manifold for the PCV some time in the future.
Panky has shown a Smiths PCV valve. This was used for a short period of time on 1098cc Minors but was used on a lot of other British cars too.
My car has the factory standard set up for 1969 with an engine number 10ME (closed-circuit breathing). In this set up the oil separator chimney is on the tappet chest cover nearest the front of the engine. The pipe then goes to the inlet of the PCV. The outlet of the PCV goes to the inlet manifold. A special manifold is used that has a threaded inlet for a fitting that the pipe connects to and a blind threaded hole for the bracket that supports the Smiths PCV.
Yes the vacuum varies in the inlet manifold but the internals of the Smiths PCV are designed to keep the vacuum on the crankcase side near constant regardless of the varying inlet manifold vacuum.
It does not matter whether the Smiths PCV is mounted vertically or horizontally but it should not be mounted upside down. There are various other configurations that could be used. A timing chain cover with breather oil separator could be used e.g with a 1275cc engine that does not have tappet chest covers. The inlet could be one of the carburettor spacers with an inlet spigot like one of these: https://www.morrisminorspares.com/brake ... rs-p829337
Stephen
1969 1098cc Convertible “Xavier” which I have owned since 1989.
You may find that the carb (which appears to be non standard) has a vacuum pipe fitted to which the vent pipe can be fitted. That pipe may be currently blanked off.
If connected to the manifold you do need to have a PCV fitted (it does not have to be the Smiths type as there are several types available).
As Phil says, there may be a vacuum inlet on the carburettor. To simplify (and make cheaper) the crankcase ventilation the very last Minors made had a variant of the HS2 carburettor that has a vacuum stub pipe. The pipe from oil separator was connected directly to the carburettor vacuum stub pipe without a PCV. Many of other SU carbs of the era also have such a stub pipe.
Stephen
1969 1098cc Convertible “Xavier” which I have owned since 1989.
philthehill wrote: ↑Sun Jan 18, 2026 10:47 am
You may find that the carb (which appears to be non standard) has a vacuum pipe fitted to which the vent pipe can be fitted. That pipe may be currently blanked off.
If connected to the manifold you do need to have a PCV fitted (it does not have to be the Smiths type as there are several types available).
Yes I have now connected the crankcase vent to the carb vacuum port, no real need for the PCV. I fitted it as the engine was breathing a bit more than it should, and burned a bit of, oil but since I re-built it is much better.
A Positive Crankcase Ventilation (PCV) valve controls airflow by acting as a variable, vacuum-operated check valve that restricts or allows the flow of "blow-by" gases (combustion by-products) from the engine crankcase back into the intake manifold. It modulates this flow based on the intake manifold's vacuum level, which changes with engine speed (RPM) and load, to prevent excessive oil consumption, engine sludge, and oil seal failures.
How the PCV Valve Controls Airflow:
• Idling/Deceleration (High Vacuum, Low Airflow): At idle, the intake manifold vacuum is very high. The PCV valve uses this high vacuum to pull a tapered plunger (or pintle) against a calibrated spring, seating it to restrict the flow to a minimum. This prevents the engine from running too lean (too much air, not enough fuel).
• Cruising/Partial Load (Medium Vacuum, Moderate Airflow): As the engine accelerates and intake vacuum drops, the spring pushes the plunger back, allowing more blow-by gases to pass from the crankcase to the intake.
• Full Acceleration/High Load (Low Vacuum, High Airflow): Under heavy load, manifold vacuum is at its lowest. The spring pushes the plunger to its widest opening position, allowing maximum airflow to remove the high volume of blow-by gases generated.
• Backfire/Turbo Boost (Positive Pressure, Zero Airflow): In the event of a backfire or in forced-induction engines where the manifold becomes pressurized, the PCV valve slams completely shut, acting as a one-way check valve to prevent high-pressure intake air from entering the crankcase and damaging seals.
Components of Control:
• Internal Plunger/Pintle: A bullet-shaped, tapered piece that restricts the opening.
• Calibrated Spring: Determines how much vacuum is required to move the plunger.
• Intake Vacuum: The driving force that pulls the vapours out.
Failure Modes:
• Stuck Open: Causes too much air to enter the intake, resulting in a lean mixture, rough idle, and high oil consumption.
• Stuck Closed: Causes excessive pressure build-up, leading to oil leaks, damaged seals, and engine-damaging sludge.
The above description is all too complicated for me.
All I want to know - does it work or does it not work.
The biggest problem with the Smiths PCV is the rubber/neoprene diaphragm (though I have seen a few instances of the spring rusting away).
If the diaphragm is ok the valve works.
If the diaphragm is not ok the valve does not work.
There is no complicated engineering with the Smiths PCV.
There may be other types of PCV out there but the Smiths valve was the one fitted to the 'A' Series so my comment only reference the Smiths PCV.
The early Smiths PCV have the valve pin vulcanised to the diaphragm. That type of pin/diaphragm arrangement is no longer available so be vigilant if you pick up a used or NOS Smiths PCV because you cannot accurately tell if it is a early or late type unless you disassemble the valve.
