Breathing on 1275 engine

[mike.perry]

From my tests I would say a non vented cap

[bmcecosse]

I've never seen the sense of the vented cap - who wants to breathe the smelly/carcinogenic fumes that can come out of the vent?? And - don't tell me they don't........the oily mess on the rocker cover gives the game away.

[Hendy1201]

Thats great thanks, I was going to go with the non-vented for that reason, just thought I would check. Cheers.

[mike.perry]

Let us know the result

[Hendy1201]

A quick update. Finally got the engine running today (held up by Uni work). Still have the timing, mixture etc to sort out, but it runs. Started with a reasonable amount of oil leaking from the clutch housing, but this quickly reduced. The only breather is from the timing chain cover and linked to the carb. I will let you know how its going when everything is set up correctly.

Will

[bmcecosse]

Add a rocker cover breather........... If you have a cover plate over the mechanical fuel pump opening - it is possible to fabricate a breather pipe to fit on there, but a rocker cover with breather pipe is much simpler.

[Hendy1201]

I will have a go with a breather on a rocker cover aswell, but first want to get it running right. What order should I alter everything (mixture, idle, timing etc). First time I have done this so a bit unsure. Does it make much difference that I have electronic ignition?

[bmcecosse]

It's just a matter of going round the circle again and again till you get the idle nice and smooth. Then follow up with a 'not quite pinking' test - and a plug colour check!

[Hendy1201]

Thanks very much, shall give it a go tomorrow.

[Hendy1201]

Got it running a bit better yesterday before having to head back to Uni, but it is really noisy! My dad is going to get a mechanic friend to have a look at it to check I havn't done something stupid!

[IslipMinor]

With positive crankcase ventilation there must be a filtered air inlet, with a controlled restriction that creates the partial vacuum to draw out the fumes from the crankcase into the carburettor, and help to keep the oil inside the crankcase by creating a negative (inward) pressure drop across the oil seals, iffy gaskets and sump seals and rear crankshaft scroll. The vented filler cap cannot leak with a partial vacuum in the crankcase, but it can on ordinary open vented engines, particularly slightly worn ones, then you need the non-vented one. Air is being drawn down though the vented cap, any oil will go down with it!

The non-vented rocker cover filler cap on a closed ventilation system will cause all sorts of problems. How does the air get into engine to flow though the breather system? With too much vacuum being created, it could result in the oil itself being digested!

Mini transfer cases are often fitted with a separate breather that fulfils this function, but on an in-line A-Series a breather must be fitted somewhere, and the rocker cover filler cap is by far the easiest place for it, as it is a standard fitting on the later engines. Its designed to do the job exactly.

On our 1380 we have a single 10mm hose running from the timing cover to the HIF6 ventilation inlet and a standard vented rocker cover filler cap. That's it, no other breathers and the oil consumption is around 1,000 miles per litre, with the odd drip on the floor, but no more than that.

[bmcecosse]

Vented filler cap can lead to nasty smells in the car - and although the crankcase may appear to be under a good vacuum when idling - it's probably a very different case when running hard flat out! I leave the tappet chest breather open - and slightly extended with a bit of rubber hose to take it down into the under-car airflow. And there is evidence after a good hard run that some fume/oil has been expelled there. It works well for me - but I guess every engine is in different condition.

[IslipMinor]

Roy,

You are talking about a 1098 engine? The engine being discussed here is a 1275 with no side covers and using an HIF carburettor, which has a dedicated breather inlet that is subject to vacuum at all throttle openings, unlike the inlet manifold that has virtually no vacuum at wide open throttle, just when you need it most!

I would suggest keeping it very simple to start off - a single hose from the timing cover to the HIF and a standard vented rocker filler cap - see what happens and take it from there. Any fumes are drawn into the carb, and certainly with our same set up there are never any oily or petrol smells of any description emanating from the engine.

[bmcecosse]

Makes no odds which engine - timing case or tappet covers have same effect. And the inlet manifold will always at least as much vacuum as the carb - I believe the little intake pipe on the HIF is not at the venturi ?

[JOWETTJAVELIN]

If it's worth mentioning, on my old Austin Allegro 1300 (a damn good car), I couldn't see any obvious signs of engine breathing, but having looked at other photos and info regarding these engines, there were the two old rusty 'tin cans' with rubber pipes coming from somewhere, although I don't remember them being connected to the carb, or air filter.

Even by the mid-1970s they had generally done away with rocker cover and tappet chest breathers, despite using 20 year old engine designs.... And people wonder why we don't have a motor industry.

[bmcecosse]

I can never understand why the rocker breather was eliminated.

[Hendy1201]

Back home for the weekend and just picked the car up from the garage, glad to say it passed its MOT after they fiddled with the timing and carb a bit more. It is still leaking quite a lot of oil from the rear of the crankshaft but I still just have a rather leaky hose from the crank case to the carb and need to have a play with different breathing set-ups.
It's much quieter and I have managed to eliminate most of the rattles (the main one being the bumper against the wings!) The only other issue is that it produces a lot of smoke after starting which then stops after about a mile. Anybody got any ideas? Thanks.

Will

[MarkyB]

it produces a lot of smoke after starting

That would be the valve guides.

[Hendy1201]

The head was fully reconditioned by our local machining shop, what problem with the valve guides causes it to smoke?

[MarkyB]

Leaking oil into the inlet port, causing smoke on start up.
It may be that the sealing rings were not fitted but I'd be dubious about the quality of the reconditioning.