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MG MGB Technical - Engine Breather

Hi all,

I'm hoping someone with a bit more knowledge than me can help with a tricky engine problem I've got.

I have a Riley 1.5 with an MGB engine which used to run perfectly. Last year the engine breather started pushing out oily smoke - far more than was healthy for my lungs!!

I should point out here that my breather does not recirculate, as it has a webber carb, so a little bit of oil mist will always be expelled here.

The problem is that it is now big plumes of smoke and bubbling oil. I took the engine to pieces and replaced the pistons/rings etc. The engine ran fine for 600 miles before doing exactly the same and spewing out clouds of smoke. It has also started being a PIG to start although there is no discernable loss in power once driving. I'm confused!!!

Has anyone had a similar problem or have any tips for sorting this?

Many thanks
Ian
i osborne

Did you bore or hone the cylinders while you had the engine apart? If not, the rings can't seat properly and will cause the symptoms that you have described. Also, make certain that the oil separator, on the front tappet chest, is clean and filled with brass or steel wool. RAY
rjm RAY

I Osbourne, So I guess what Ray is saying is that crankcase pressure has gone up so expelling fumes and oil into the oil breather. Yes, I can see if piston rings are not seating, or sealing properly, then gas pressure will blow past the pistons into the crank gallery so raising its pressure? You might see oil trying to seep past the rocker cover gasket also?( take the oil filler cap off while engine is running, more fumes here as well? )So whats to do? Check rings, oil ring, was there a right side up? binding, etc? Mike
J.M. Doust

i had the same thing with my engine. I had it breathing to atmosphere and while on motorway it coughed out a huge amount of oil into the engine bay. It then did this whilst i was on the nurburgring which was quite scary. I then changed to having the breather connected to the inlet manifold, via an oil catch tank. it used to fill the oil catch tank, so i then changed the breather plate on the side of the block. I havent had it reoccur since.
A P New

So, Have we come up with an answer? I have one of these little breathers on the end of the tube but have none of the problems that have been discribed, so what is happening? And, along the lines of power, oil fumes that are drawn back into the engine via the intake, are stricly speaking reducing the air/fuel density of the incoming charge. Or in other words robbing power. Mike
J.M. Doust

Large amounts of oil being blown out of the breather would be an issue with whatever crankcase ventilation system is used, it's what is pushing it out of the crankcase that is the issue and that will vary from car to car, not what you do with the breathers. I've known people not find a solution until they replaced the front cam-follower cover, even though the flame-trap/oil separator mesh was present and in apparently good condition. There were at least two types of cover for the MGB, as shown in the attached. The one on the left has a number of features that would limit oil egress.

PaulH Solihull

The cover on the LHS as viewed in your pic Paul is the early version. My car has one fitted and I have tried to buy a replacement. When it arrived it was the later one as seen on the right. These only work with the later crankcase vemtilation systems so it had to go back. I did put some pics on here at the time. Mine still has the steel wool filling and you can see staining marks where it does trap and return oil.
I would agree with ealier posting re this car, a bit more BMEP should have been applied while running in. A simple engine like the B series can use running in oil, just watch the levels, it gets thin when warm.
Stan Best

>oil fumes that are drawn back into the engine via the intake, are strictly speaking reducing the air/fuel density of the incoming charge. Or in other words robbing power. Mike

Oil has a higher BTU content than petrol so it may in fact be /increasing/ power. ;-) Realistically, though, even if it is "robbing power", the amount will be insignificant* and the benefits of reduced emissions by recirculating crankcase vapours will far outweigh any power loss.

*assuming everything is in good nick -- if not, the problems need to be rectified rather than trying to apply a band-aid by altering the breather system....
Rob Edwards

Ian,
What oil are you using? Particularly when you had done the ring work............hopefully after "busting" the glaze in the bores.
Don't use any of this modern, thin, oil.
Al
Allan Reeling

Firstly - thanks to everyone for their comments and suggestions. Had to do a alittle bit of googling to understand BMEP but I'm with you now :)

Now to clarify a few of these points....

I followed the advice on switching the breather unit as a first step. New one in and same problems - as you say this is only a band aid and does not solve the problem of smoke out via the crankcase, particularly as there is so much of it. On that point there seems to be a back pressure on the oil cap that is forcing a small amount of oil out - however this is not as a fume but a fine spray of oil. Pressure definatly in the wrong place.

So to temporarily make the car drivable for it's 80 miles journey back home I have attached 4 metres of garden hose on the breather all the way to the back of the car, via a different route than the exhaust as it would most likely melt it. Temporary I know. Effective definately. In fact whilst driving I realised this is actually quite an effective James Bond gadget for spraying oil on the road.....

The other thing to consider here is that the starting on the car became atrocious the moment this engine problem occurred so recirculating the fumes would not resolve this either as I would still have starter issues.

So it's off the road now for a closer inspection. Looks like next steps are to have the block apart again to inspect wear, although when this was done previously there was no sign of wear, so we shall see what they look like now. In terms of whether they were bored in properly - I believe they were although this was left to an expert - I think I will have a crack myself now and takeon board some of the suggestions here. The oil I run in here is the standard classic motor oil from Halfords (possibly 20/50 from memory).

It'll be a while before I get my hands dirty on this so I'll come back in a couple of months and let you know how things go.

Many thanks everyone
Ian

i osborne

Stan - thanks, just to clarify, 'original' and 'replacement' in the photo refers to the order the person who solved his oil burning problem used them, 'replacement' solving his problem. The covers appear to have changed once for the 3-bearing, and twice for the 5-bearing. The middle one, 12H 3684 is supposed to be compatible with the later 12H 4395, although I don't know what the innards of each look like.
PaulH Solihull

This thread was discussed between 31/10/2011 and 03/11/2011

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