As all items appear good and meet all the guidance from several members, should the engine stall (shut off) when oil cap is removed when the engine is idling?

According to several sources of original service documentation, the idle speed should increase slightly when the oil cap is removed when this breather system is fitted. If everything about the breather system is OK, then it would seem you have an air leak somewhere else: throttle butterflys worn, manifold gasket leak, brake booster leak, etc. I am sure everyone would be interested to know what you find eventually.

Getting back to the core of the issue, if all is well, then opening the filler cap shouldn't stall the engine. I can only be sure that you understand the PCV to be perfectly serviceable. If you pop the clip off the PCV, this disc will come out and beneath that there should be a diaphragm (check for cracks or holes as they age), a spring and another little device that looks like a pin welded to a disc. The PCV body should have a hole in the center with a little spring-loaded disc on its underside. Check that it opens and shuts with a push of a small pen or similar.

The PCV will draw air through the manifold when the car is idling. The vented cap and a little blow-by allow it to do so. It will close once the butterflies are opened until cruise is reached (This is why the PCV is blocked by the mechanic when tuning the engine, as not all the air is drawn from the carburetor/s and will throw out their tuning tests).

But this small amount of air allowed in when pulling the cap will demonstrate the relative tuning of your engine, much like lifting the carb damper piston 1/8th" does. Opening the cap allows a little more air than usual at idle into the manifold. This will lean the inlet fuel-air ratio a little. If it speeds up, then the mixture is on the slightly rich side of a stoichiometric mixture. If it slows down, it is on the slightly lean side.

So, to tune out the problem, your mixture might be a little on the lean side or the revs a little low at idle. Richen the mixture or increase the idle to compensate. If you're sure it's not one, then it will be the other.



This is my perception of the problem, and I will always defer to a day-to-day mini-specialist if they chime in.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

Thanks Mick.
I will now try to tune the twin SU's without the PVC in the equation.

Having just looked at the pv valve I have here there should not be enough air able to pass through it to cause any major change in revs. As said above the revs should rise by 200rpm when the filler cap is removed. If the valve is letting too much through then it may cause your symptoms.
A neat check would be to use a clamp to clamp down on the hose from the valve to the manifold blocking it off. Then removing the filler cap should have no effect. Then slowly undo the clamp and the revs should rise. If the valve is passing too much you should not get very much from clamped close before the revs change. And as you unclamp more the revs should change dramatically.

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Cheers Dave.

Have you tested the fuel mixture using some kind of analyser or gauge?

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1986 Mayfair (998 auto), 1974 S, 1974 Van, 1972 PMG Van

Too many minis

For ease of format, I've written up an account of how this valve works and the significance on idle speed and included it on my "articles" web page if anyone is interested in more detail. https://leylandaustralia.com.au/articles.htm

68+86auto I have used the 2 pins on the rear of carbies to test mixture.

I have found more info on the subject "Crankcase Closed-Circuit Breathing".
When I renewed my hoses, the reason was because when I remove the oil filler cap the engine stops but it has not fixed the engine stopping problem.
Any thoughts?

Assuming the breather valve is OK and not missing any springs and has no diaphragm leaks and the plunger isn't stuck, I'd still be looking for a crankcase despression leak. Fuel pump blanking plate, speedo drive, dipstick, oil separator connections both hose and mechanical fitment to cover, drive yoke seals. Perhaps post a couple of photos of the engine as a whole so we can see the situation.

Just a thought , block the hose from the breather to the PVC valve , start engine , if running remove oil cap , if engine still runs unblock the breather hose , if engine stalls , could be a problem with your PCV valve , or remove the PVC valve and block the inlet in the manifold and run the engine , remove the oil cap and see what happens

eightfifty Hope these picture help

slacko Which ever way round I do this, the engine still stalls.

PCV top is on upside down . Is the top flat or has it a recess ?
not that it might make a difference
just saying
Both of the 2 PCV ' s I have , have flat tops , yours looks raised in the photo

Top lid is on upside down. The crossed pressings should be down, not up. This makes a difference as to where the plunger sits.