rear crank leak/crankcase pressure/closed breathing

[ratrodmog]

Wow its been a long time since last posting here (although I have been using the FB MMO page regularly) Warning: this may be a long post so get a brew on ready!

This topic has more than likely been discussed at great length but I feel some clarification is needed.
A quick background on my engine. It's a 1098 that I've fitted a new crank, bearings (making sure to seal the rear scroll as well as possible), oil pump, clutch, rocker shaft (with new springs etc) and it runs an Hs4 carb, alloy inlet and Maniflow exhaust. Cold compression is 155 and all 4 cylinders pretty much the same. The oil pressure holds nicely at around 65 when moving swiftly and 40 at idle (although it's set a little fast currently). The engine was an Ivor Searle recon at some point before I got it and runs pretty well. It's definitely had a rebore and head skim at the point of being reconditioned. I ran this engine for a couple of years before the crank bearings gave out.

Now my issue / issues!
It is leaking from the rear crank into the bellhousing (a drip from the jiggle pin, clutch judder in 1st but no clutch slip when pulling) and I think burning oil. Just did a 300 mile round trip over two days and started on just under the 'max' dipstick mark and now is on just above the 'min' mark.
The current breather arrangement is as follows: vented oil filler cap, rocker cover take-off going directly into the carb body (the Hs4 has a port) and tappet chest breather venting to atmosphere (pointing down to the sump). I think I'm right in thinking this is an open system? My theory is that crank pressure is excessive (why I'm not sure) thus forcing oil out the easiest way, the rear scroll. I've been reading about the later closed breathing systems to control excess crankcase pressure and remove fumes etc and I think this could be the way to go?
So onto my idea, and please correct me if this is totally wrong, use a tappet cover from a later 1098 with the oil separator (or make my own version of), connect the outlet tube to the carb port ( and possibly run an inline PCV valve), keep the vented filler cap and block off the rocker cover take-off therefore creating a closed circuit crankcase breathing system. This should in theory allow air to be pulled in through the rear scroll (instead of pushing oil out) and in through the vented cap whilst allowing pressure and fumes into the carb and subsequently burn off.

It's pretty frustrating as I've spent a good bit of time building the engine up and it runs sweet (apart from losing oil obviously) but hopefully there's a simple fix to this problem.

Phewww! And go.......!

[pgp001]

What you have just described is exactly what I have done to my engine, it sorted out the rear scroll leak for mine OK.

Phil

[ratrodmog]

What you have just described is exactly what I have done to my engine, it sorted out the rear scroll leak for mine OK.

Phil

Thanks Phil. Did your setup include the use of an inline PCV valve ? I'm wondering if there's an advantage by using one over not? Thanks

[pgp001]

No PCV valve on mine, I think they were only fitted on US spec cars.

I just connected the tappet chest breather cannister straight onto the brass port on the carb, and blocked off the rocker cover breather connection, I also used a vented cap on the rocker cover.

With this set up you can feel a vacuum if you put the palm of your hand over the oil filler with the cap removed, also the engine revs change slightly when the cap is removed due to more air being allowed into the carb port thus weakening the mixture slightly.

Overall it was a very good upgrade and has stopped me leaving a 4" dia puddle of oil after every journey. It is not quite 100% leak free, but maybe just a couple of drips from the jiggle pin when parked up. I can live with that.

Phil

[IslipMinor]

A PCV valve is used in conjunction with an inlet manifold port to reduce the level of vacuum in the crankcase. The port on the SU is already at a low vacuum, so no need for a PCV valve as well.

The system you are proposing should work well, and is the same as the closed circuit arrangements used on later BMC cars, such as Sprite, Midget etc., with a port on the SU carburettor.

Please post the results so that we can all see the effects of the new system.

[ratrodmog]

A PCV valve is used in conjunction with an inlet manifold port to reduce the level of vacuum in the crankcase. The port on the SU is already at a low vacuum, so no need for a PCV valve as well.

The system you are proposing should work well, and is the same as the closed circuit arrangements used on later BMC cars, such as Sprite, Midget etc., with a port on the SU carburettor.

Please post the results so that we can all see the effects of the new system.

Ok thanks for that, good to know I'm not totally crazy. I've found that a standard oil breather/separator from a late model engine won't actually fit due to my LCB manifold so I've just knocked one up using some steel pipe filled with stainless pads! It''s pretty much to same idea but a DIY version. I'll get it plumbed over the weekend and let you know the results.

[ratrodmog]

IslipMinor, just a thought. Is there a difference, gain or loss, with the size of the tube coming from the tappet crancase vent into the oil separator? I've noticed on the Mini version it seems to be quite a large diameter tube (at least 1") then around a 1/2" outlet into the carb. The current crankcase vent tube is stock and looks about half inch. Would that suffice or should I go biggar/ smaller?
Cheers

[ratrodmog]

So after fitting my own version of the later crancase breather/oil separator and plumbing it into the carb body, it currently is not leaking! Granted I haven't had an opportunity for a long run but did around 25 miles with no obvious drop in oil level. The pressure is a steady 60 (ish) wiht good vac. Still need to sort the rich running but that could be a bad needle choice on my part for the Hs4 but all well at the moment. It seems the little A-series responds well to some negative crank pressure!

[mike1864]

Hi Ratrodmog

Can you post photos of what you've done. I'm not sure whether my carb has the requisite port.

[les]

I think this is what he did, if you’re still here Mike.

9585B53E-FCB3-4892-BA30-749F51BB05B4.jpeg (1.4 MiB) Viewed 1314 times