Engine breather definitive setup?

[M25VAN]

I've seen lots of mention of the benefits of the rocker cover with the breather take off feeding into the carb. I'm just about to go over the breathing on my engine and want to check what the other parts of the system should be.
I've got the following parts to go on:-
Rocker cover with breather take off feeding into
late carb with the brass tube on the side
plain timing cover
'short' type oil filler cap
What should the two tappet cover plates on the side be? Both plain or one with an open pipe coming off it?
I know this has been gone over quite a few times but never seen a complete setup mentioned.

[aupickup]

One os the tappet covers should be an open end one

[M25VAN]

Thanks aupickup, I'll get those bits swapped over.
I currently have plain rocker cover and timing cover with one of the tappet covers feeding in to the carb and a short filler cap. Not sure what it was doing but it wasn't 'right'.

[aupickup]

on that set up you want the long or vented oil filler cap

[M25VAN]

Yes, I think that is what I have. It has what looks like wire wool inside it. Sounds like the setup I have is breathing 'upside down' to the desired setup.

[billlobban]

After rebuilding my 1098 engine and reading lots of posts I decided to retain the original rocker cover pipe and fit the tappet cover vent pipe from the later model engine I used for the rebuild. Connected both via an MG 'Y' piece and then into the carb - seems to work well but obviously hard to tell since the engine is 'new'.
I had the original set up with the downwards pointing pipe open to the atmosphere which appeared to do precisely nothing. I know this is one of BMC's pet subjects so maybe he will venture an opinion

[Stig]

I can't see that it'll make any difference if you have a pipe from the rocker cover or the tappet cover - they're both connected via the pushrod holes.

[M25VAN]

After rebuilding my 1098 engine and reading lots of posts I decided to retain the original rocker cover pipe and fit the tappet cover vent pipe from the later model engine I used for the rebuild. Connected both via an MG 'Y' piece and then into the carb - seems to work well but obviously hard to tell since the engine is 'new'.

Have you got the breather type filler cap? If not isn't it a 'sealed' system with no air entering? This is where I start to get confused.

I can't see that it'll make any difference if you have a pipe from the rocker cover or the tappet cover - they're both connected via the pushrod holes.

So does it matter if the system draws air in from the side and up through the rocker cover then to the carb or from the top via the filler cap and down through the tappet chest and then into the carb.
I know that engine breathing is important, just want to get the correct setup. C'mon BMC....

[bmcecosse]

Only use the 'breather' oil filler cap if you like breathing carcinogenic oil fumes - and having an oily mess on the rocker cover. I've always had great success with rocker cover to carb inlet (simply because it's a nice short pipe) - and cam follower cover pipe open to atmosphere. This allows air to enter the crankcase - and be drawn through the carb and burned in the engine. No nasty fumes and no mayo. It works.

[Stig]

I'm confused. Why doesn't the pipe on the cam cover emit fumes in the same way that a 'breather' oil filler cap would?

Surely as long as there's never any positive pressure (because the carb sucks it out of somewhere) then there won't be any fumes.

[bmcecosse]

At times the pipe may very well emit some fumes - but they are down below and into the airstream. So you don't breathe them, and they help to rustproof the underside!

[M25VAN]

I've always had great success with rocker cover to carb inlet (simply because it's a nice short pipe) - and cam follower cover pipe open to atmosphere. This allows air to enter the crankcase - and be drawn through the carb and burned in the engine. No nasty fumes and no mayo. It works.

Thanks BMC.

[mike.perry]

The function of the breather system is to relieve the build up of air pressure in the sump which i caused by bits of engine pumping up and down and going round and round etc. As long as the air pressure can escape from the sump then oil should not be forced past the crankshaft oil scroll.
The side valve engine achieved this simply by having a vent pipe attatched to the tappet chest cover which allowed air pressure to escape under the car. Early A series engines achieved the same thing by ventilating through the rocker box but piped the air into the air cleaner having decided that breathing the oily air mix was not such a good idea.
The 1098 seems to have gone back to the original system of venting from the tappet chest into the atmosphere but (I may be wrong here) also vents from the rocker box to the air filter.
When we come to the 1275 engine this does not have tappet covers and also has less clearance past the push rods to allow air to escape from the sump to the rocker box. This can be seen when topping up with oil which is slow to seep out of the rocker box into the sump. Therefore the air has to be physically sucked out of the sump and this is achieved by fitting a crankcase breather on to the timing chain cover and piping it directly into the carb.It is important for the 1275 engine to be ventilated correctly or there will be problems with oil leaks at the rear of the engine.

[billlobban]

[quote="bmcecosse"] and cam follower cover pipe open to atmosphere. This allows air to enter the crankcase - and be drawn through the carb and burned in the engine. quote]

This is the only bit I am slightly unsure about. The crankcase is under pressure from compression leakage so why does the cam follower breather draw air in, unless that is the carb is so efficient at sucking fumes and therefore excess pressure that it also sucks in air from the cam breather. Why then would you get blow by at the vented cap if you use a sealed follower cover? surely it would just draw in air

[bmcecosse]

When the engine is working lightly air will be drawn in through the vent pipe, sucked through the crankcase and taking any moisture with it - into the carb and burned up in the engine. But when the engine is working hard - there will likely be excessive pressure that the carb inlet can't cope with - and oily fumes will be blown out of the open vent pipe, which is fine if going below, not so fine if being blown over the rocker cover and into the car and ultimately into your lungs!

[billlobban]

Point taken - think I'll go back to the rocker cover pipe, open ended tappet chest cover and 'sealed' filler cap - thanks for the explanation

[bmcecosse]

It works.

[silloyd]

I have a rocker cover with no breather hole in it, so I can't connect it to the carb, but the tappet chest cover breather pipe is connected to the carb.

Is that good / bad / indifferent? (delete as applicable) What's the correct type of filler cap for this set up?

[M25VAN]

This is the setup I currently have, it has the long type filler cap which can let air in to the system.

[bmcecosse]

And unfortunately will also let fumes out! The rocker cover with breather connection is the one to get!