13 replies to this topic

[cheesebadger]

I currently have a small bore engine fitted with twin HS2s. I have a hose connecting the rocker cover to the air filter box, with an additional open-to-atmosphere breather on the tappet chest cover (without cannister). This is how the car was when i purchased it. After reading all the FAQs and other posts i would like to enable positive ventilation. I have just purchased a second hand ITG filter that does not have a breather input. What i think is the correct layout is to attach the tappet chest cover breather to a PCV valve on the inlet manifold. Here are my questions...

Is this a sensible layout? Should i change the tappet chest breather to include a cannister? And what should i do about the connection on the rocker, should i change my rocker to one without a breather or plumb it into a PCV valve?

Sorry for the long post,

Rich

[Sherlock]

It depends on the age of your SU's, later ones will have a brass connector tube on each pointing up at about 45 degrees near to the throttle spindle area, if yours has these, they will no doubt be blanked off with your current set up. If it does have them, all you have to do is to connect the tappet cover breather, (a canister is preferable) via a 'Y' - piece connector to the breather connections on your carbs, as mentioned above. Then you need to fit a breatherless rocker cover with the correct oil filler cap which has a filter in it. If you have older SU's there will be no breather connections & you will need the valve on the inlet manifold as was standard on Coopers & Cooper 'S's in the 60's. You can connect that to the tappet cover breather & as before use the breatherless rocker cover & appropriate oil filler cap.
No probs with the long post, a good description is an advantage.

[bmcecosse]

KEEP the rocker cover with breather! Connect it to one of the carbs - or if no carb connections - I have in the past tapped a hole in the inlet manifold (if you don't have a servo connection ?) - and then connected the rocker breather to the inlet manifold via a 1/8" restriction hole. The idle needs re-setting - but it makes a huge difference to the oil consumption and eliminates oil leaks! The tappet chest breather should be left open - to flush soem air through the crankcase.

[Sherlock]

He was asking about positive crankcase ventilation, which it cannot be if the tappet cover breather is left open, the oil filler cap allows a metered amount of air through. If you connect a breather to only one carb it will make it very difficult if not impossible to get the mixture settings equal.

[cheesebadger]

Thanks for all the replies. I don't have the brass connectors on the carbs, does this mean they are older ones? and the new air filter doesnt have a connector either, which is why i was thinking about the PCV valve on the inlet manifold.

So to confirm, for positive crankcase ventilation, do not connect the rocker to the PCV valve?
I am now unsure whether to leave the rocker open-to-atmosphere or fit a vented filler cap?

[bmcecosse]

Is 'positive' in the sense that it's being ventilated - rather than left to breathe by itself. It is best to leave the tappet breather clear and pull some air through the crankcase - this helps to keep down moisture content and reduce 'mayo'.

[liirge]

you forget that the Oil cap is the air breather Bmcecosse, tap everything you can into your inlet manifold, or carburettor, BMC got this one right.

[cheesebadger]

Thanks for all the guidance i think i am nearly there, one last question...

So can you get different types of oil caps? some that can breathe and some that can't? Sorry if that sounds like a silly question.

[Sherlock]

There is now only one standard cap generally available, which is the breather type cap.
http://www.minispare.....0GENUINE VE...

[bmcecosse]

Better with a solid NON breather oil cap! Avoids nasty mess on the rocker - and horrible smells in the car. The oil fumes by the way are carcinogenic - best not to be breathing them!

[Sherlock]

To repeat what I said before, the gentleman asked how to install a positive crankcase ventilation system, (or as I normally refer to it, constant depression breathing). For the record, my 998, with this breather system has done 103,000 miles, I have never had to clean the inside of the rocker cover & the rocker arms are not even black from oil fumes, I think that speaks for itself. A 1275 Healey Sprite I owned some years ago with a similar breather was exactly the same.
One of the reasons for constant depression breathers was to avoid fumes getting into the car.

Edited by Sherlock, 10 January 2010 - 07:48 PM.

[cheesebadger]

Once again, thanks for replies. Is there anyway i can tell what type of oil cap i currently have fitted? (breather or non breather)

Also, Sherlock, thanks for the idea, I might see if i can 'borrow' the breather from my dads frogeye restoration, he wont notice for a while, lol!

[Ethel]

Both systems could be considered "positive" because they are using manifold pressure to suck the fumes out of the crankcase. I'd say Sherlock's method is preferable because, owing to the restriction in the filler cap, it will also reduce the crankcase pressure below atmospheric stopping oil being blown out past the seals. If you're going to hook up the plumbing to the manifold there'll be way too much vacuum on overrun. This is where the gulp valve comes in, or Bmcecosse's 1/8th restrictor. The factory system lets air in at the top of the engine and sucks it out at the bottom to help draw the oil back to the sump.

[Sherlock]

Once again, thanks for replies. Is there anyway i can tell what type of oil cap i currently have fitted? (breather or non breather)

Also, Sherlock, thanks for the idea, I might see if i can 'borrow' the breather from my dads frogeye restoration, he wont notice for a while, lol!

The non- breather cap is quite flat, only about 1/2 an inch deep & very plain looking, the breather type has a barrel-like extended portion which extends below the bayonet fitting pins in the filler neck & you will see a hole in the bottom of it of approx 3/16 inch diameter.
If your Dad's frogeye has the PCV, it will be the same breather system as fitted to the Mk1 998 Cooper & 1275 'S'. Just don't get caught 'borrowing' it! lol!

Edited by Sherlock, 12 January 2010 - 12:39 AM.