Breathing on 1275 engine

[Hendy1201]

Hi all,

After many months of gather bits and rebuilding various components I'm just about ready to put my mg midget 1275 (well 1310) engine in my Morris 1000. The only thing I want to check is the set up for the breathing.

I know there are various threads on this, but just want to confirm that I get the right bits. I have a rocker cover and timing cover with breathers on. Should both these be connected to the Carb? I have an single HIF44 carb on the engine so would need to link the two pipes together. Can anyone suggest what to use for this please. Also I read that heater hose is adequate to link the breathers to the carb, is this correct?

Thanks very much

Will (Hendy1201)

[bmcecosse]

Yes - that will be fine. Just use a 15mm T piece from the plumbing section - and yes heater hose is fine.

[Hendy1201]

Thanks very much, thats great. Oh, and one more little thing, I'm putting an electric cooling fan in to replace the engine fan. I wanted to use one of the thermostat switches where the probe inserts into the fins of the radiator so there is no need to cut any of the tubing. Can anyone suggest where the best place to get one is please.
Thanks

[mike.perry]

If you have a breather on the rocker box then block it off and just connect the timing cover breather to the carb.
I have just carried out tests on my Midget engine with the following results.
Timing cover breather connected to breather control valve, no breather pipes on carbs, and rocker box breather open, steady drip.
Timing cover breather and rocker box breather connected to BCV, a pool of oil on the driveway
Timing cover breather connected and rocker box breather blocked off, a couple of drips on the garage floor.
From this test it seems that with both breathers connected air is being dawn out of the top of the engine but not enough from the bottom which needs to be at negative pressure.
This is how a standard MG Midget is set up so maybe BMC had the same results

[win]

Mike
Do you think the rocker breather should be blanked off even if the carb has a breather, HIF 38.
Regards Win

[Hendy1201]

Thanks for the advice. I have a rocker cover with a breather take off on it and one without so I supose I could try one way and if there are any problems then try the other to see which I find best. Cheers.

[bmcecosse]

My advise - connect ALL the breathers to the carb inlet. The more the merrier.

[Hendy1201]

I think thats the set-up I will start off with.

[IslipMinor]

Heater hose is too big for the breather connection on the HIF44, you will need to get some hose that is nearer to 10mm bore, which is the tube size on the HIF, not the 1/2"/13mm that is standard heater hose. Most motor factors have different sizes of hose, and will likely have a T-piece as well.

We have just a single hose down to the timing cover connection, and that works very well, so the rocker cover has no breather connection at all. I am not totally sure that it would be suitable for a positive breather system, as there is no oil separator, as there is on the timing cover, so I would try just the timing cover straight into the HIF44 and see what the result is. The rocker cover breathing connection was usually into the air filter, which has very limited 'suck', unlike the breather connection on the HIF - the early breather was more of a prevention of pressure build-up, rather than a true crankcase positive breathing system.

[mike.perry]

My advise - connect ALL the breathers to the carb inlet. The more the merrier.

I have explained that I have tried that and the result was a pool of oil on the driveway.
The breather connections on my engine fit 1/2 in heater hose

[bmcecosse]

I'm sure the hose will squeeze down with a good Jub clip - it needs to be the larger size to fit the engine connections. On my high performance engine - I used the servo connection on the manifold - plugged it and then bored a 1/8" hole through the plug - and put the breathers on there (rocker cover and transfer case - it was a Mini) and that worked very well indeed.

[daveyl]

Just thought I'd mention. I have an early 803 rocker cover (with a vertical straight through outlet) fitted to mine.

I have noticed that the later covers (curved over horizontal) have a plate on the inside, covering the exit hole BUT has a gap either side of plate, presumedly, to deflect the oil away from the hole but still allow gases to exit.

I can only guess that the later ones are better to use.

[bmcecosse]

Probably - but I expect the early cover will be treasured by an 803 enthusiast........

[IslipMinor]

it was a Mini

Which has a 'proper' rear crankshaft seal!!

For an in-line engine to seal reasonably well, there has to be a partial vacuum in the crankcase at all times, especially at higher throttle openings, when blow-by is more like to occur. The inlet manifold at full throttle has very little vacuum, whereas the take-off on the HIF SU produces a reasonable vacuum all the time. A PCV in the inlet manifold, like the later 1098 engines had, works well if in good condition, but suffers at higher loads again.

[bmcecosse]

Well -yes and no to that crank seal - and it is extremely important to hold a Mini crankcase in negative pressure, or I can assure you the oil is all over the clutch in no time......Hence my rather extreme 'straight to the manifold' breathing system.

[Stig]

If you have a breather on the rocker box then block it off and just connect the timing cover breather to the carb.
I have just carried out tests on my Midget engine with the following results.
Timing cover breather connected to breather control valve, no breather pipes on carbs, and rocker box breather open, steady drip.
Timing cover breather and rocker box breather connected to BCV, a pool of oil on the driveway
Timing cover breather connected and rocker box breather blocked off, a couple of drips on the garage floor.
From this test it seems that with both breathers connected air is being dawn out of the top of the engine but not enough from the bottom which needs to be at negative pressure.
This is how a standard MG Midget is set up so maybe BMC had the same results

Very useful info there Mike, was that with a vented or non-vented oil filler cap?

[mike.perry]

[frame][/frame]
Engine with timing cover breather connected and rocker box breather open. Other tests were with the rocker box breather connected to the timing cover breather pipe with a short hose and T piece and with the rocker box breather replaced with a bolt and sealing washer.

[bmcecosse]

It is with the 'closed circuit' breathing system - which may have an effect.....

[Bazzalucas]

Yes, BMC- my hoses go directly (without a valve of any kind), to ports about midships on the carbs. That's the condition the engine came in, and nothing looked like it jerry-rigged.

[Hendy1201]

Right, so if I put a rocker cover with no breather and link the timing chain cover breather to the carb, would I need a vented oil filler cap or the short non-vented cap?
Cheers