12 replies to this topic

[alpder]

Having suffered oil on the clutch, thought I'd measure the crankcase pressures to investigate whether the oil might be being blown through the primary gear. A simple water manometer stuck into the dipstick hole does it. (The bottle in the pic is partly there as a plenum to even out pulses, but mainly there to catch the water if the engine tried to suck the water out of the manometer.)

 

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My breather is the regular oil/air separators on chain-case/transfer-case, tee'd into the HIF38's breather port. No valves or regulators. The engine is 1275 A+ newly rebored and rebuilt, just run-in.

 

Results:

  - In my original set-up: about 2" of water vacuum dropping to 1" at high revs.

  - With a new replacement (vented) filler-cap: about 3" of water vacuum right across the rev range. (About 0.1psi.)

  - With the breather disconnected from the carb (i.e open to air): a bit under 1" of water pressure.

 

Vacuum increases massively - but temporarily - when the throttle is quickly opened. Unfortunately a manometer isn't much use for testing on-the-road, so under-load results can only be had either on a rolling-road or with a £proper £gauge.

 

No idea if these numbers are good or bad. But at least I'm getting some vacuum. And the replacement filler-cap has improved the vacuum a lot.

Edited by alpder, 12 April 2024 - 04:11 PM.

[ACDodd]

I think you might find this solves your problem.

Ac

[Earwax]

Some help for my slow brain this morning please.  

 

So from your results ,

 

is the vented cap letting air escape or sucking air? (which leads to a better vacuum down below)

 

off to get some caffeine in my system

[alpder]

Some help for my slow brain this morning please.  

 

So from your results ,

 

is the vented cap letting air escape or sucking air? (which leads to a better vacuum down below)

 

off to get some caffeine in my system

The original cap was also vented. The only difference between the two caps is that the new one makes a good seal in the neck, while the old one - even though it seemed tight - must have been letting air bleed in around the dried-up old rubber seal so killing the vacuum. The new one wasn't anything special - just MiniSpare's cheapie non-genuine.

 

The breather is only 5/16" pipe, so it can't be drawing a lot of air out of the crankcase. So sealing up every possible source of ingress (like dipstick and filler-cap) by fitting a fresh O-ring and a new cap respectively can clearly make a big difference. And the better the vacuum, the less oil gets out of the many, many sources of oil on a Mini... at least while it's moving.

 

to be honest, I don't fully understand why the cap is vented at all - why would the designer want to degrade the vacuum? Maybe it stops the vacuum getting excessive. And/or maybe it deals with the situation when the breather gets blocked so at least allowing the gases somewhere to escape.

Edited by alpder, 13 April 2024 - 08:13 AM.

[Spider]

Nice work.

 

As a 'by the way', the factory recommends that the vented caps be replaced every 12 000 miles

[timmy850]

The only source on intake air to the crankcase on closed breather setups should be the oil cap. 

 

On a PCV system too much intake air will disrupt how the system works.

 

On the later CCV system (with the port on the carb body) there's actually quite a lot of air drawn through the system. I often wonder why people plug the carb and add the open breathers when they had an excellent system to start with

[Spider]

,,,,,, I often wonder why people plug the carb and add the open breathers when they had an excellent system to start with

 

 

I think because they don't take the time to understand it.

For a road car, IMO, it's the only way to go and for lots of reasons.
 

[nicklouse]

 

,,,,,, I often wonder why people plug the carb and add the open breathers when they had an excellent system to start with

 

 

I think because they don't take the time to understand it.

For a road car, IMO, it's the only way to go and for lots of reasons.
 

 

And then the people who remove the wire wool and don’t replace it.

[Tornado99]

Interesting. My car only has one breather oil trap (wire wool canister thingie). I believe it is supposed to have two, one near clutch slave, other on the timing side (only that one present). Should I fit the second one?

[Spider]

Interesting. My car only has one breather oil trap (wire wool canister thingie). I believe it is supposed to have two, one near clutch slave, other on the timing side (only that one present). Should I fit the second one?

 

Depends on your engine what it would have come with. The 1275's since about 1970 had two, prior to that just one. The smaller engines only had one.

Having said that, I've found on the 1275's one is well enough. I only run with that on the Flywheel Housing. Perhaps if you do lots of short runs where the oil may not get up to temperature, the second may help with reducing the moisture in the crankcase.

[unburntfuelinthemorning]

 

to be honest, I don't fully understand why the cap is vented at all - why would the designer want to degrade the vacuum? Maybe it stops the vacuum getting excessive. And/or maybe it deals with the situation when the breather gets blocked so at least allowing the gases somewhere to escape.

 

You need air flowing in to displace the contaminated air in the crankcase hence the oil filler vent.  Also in a very worn engine under high blow-by conditions pressure can build in the crankcase so much that it overwhelms the breather system.  The pressure has to go somewhere so it is able to flow out of the filler cap vent.

[Ethel]

Drawing some fresh air through is good for removing the condensation (water is a biproduct of  burning hydrocarbons) & unburnt fuel that would contaminate the oil.

 

The critical design bit will be to control the depression - the harder air is sucked through an orifice,  the less the rate of flow increases, it's a law of diminishing returns. It's a simple & cheap way regulating the crank case pressure to 1-2 psi under atmospheric.

[68+86auto]

So many people don't understand engine breather systems.

 

I'm currently working on a MGB where the previous owner took it to a mechanic who claimed the engine needed a rebuild as it was burning oil. Looking at the invoices, it appears they then replaced the valve stem seals as a possible "cheap" fix. I'm not sure who did it but the distributor was glued in with gasket sealant, obviously someone thought that was a fix too.

 

You know what I believe the problem was? The PCV diaphragm. It costs $9 and doesn't even need tools to replace. It is absolutely rock hard and has a small hole. I haven't verified this as the fix yet as there's lots of other work I need to do to the car first. The rings could be worn too but the PCV definitely won't be helping and there was no mention of it on any invoices.

 

 

I have a friend who's a mechanic. He removed the "emissions" equipment from his car (80s Mazda). He then thought the distributor needed a rebuild as oil was getting in to it.

Edited by 68+86auto, 23 July 2024 - 02:16 AM.