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MG MGB Technical - Oil and breathers

I have a UK-spec 1973 1800 BGT, extensively rebuilt 3-4 years ago and owned by me for 2 yrs, has run v well until recently, when it is now using a lot of oil - smoking not leaking, but seems to be sporadic. I suspect that this may be due to my having used fully synthetic 5w/40 - I have today flushed and put good old fashioned 15w/50 mineral oil, so will see, but I suspect that damage has been done. Plugs all OK but no 2 is a bit sooty, but not oily. I wondered if the breather could be blocked - it's the simple type that goes from the from tappet chest cover to both carbs. On pulling it off and covering the pipes into the carbs the engine ran much more smoothly - any ideas why? I'll investigate in more detail next weekend but I'd appreciate any thoughts!

By the way, many thanks for everyone's comments on the wire wheels - I did pen a detailed reply which doesn't seem to have posted. In essence I'm about to replace the discs and hubs anyway so will try to sort the wheels after that
JN Davies

Hi..

A compression test would help with the diagnosis.
Was the oil pressure OK with the 5W/40 ?.

Don
Don

I have heard from several sources that full synthetic oil should not be used until an engine has at least 10,000 miles on it. This is allegedly because the synthetic oil is too slippery to allow proper break in of the engine. I don't know if this is accurate or just an old wives tale but it may be something to consider. Does your breather tube go into the air filters or directly to the intake manifold. If it goes to the manifold without a PCV valve of some sort your improved running was because you sealed the vacuum leak caused by that setup.
Bill Boorse

Thanks, Don and Bill.
Don - yes, OP was fine. Having changed the oil it does seem better, with no evident smoke, but it's early days so I won't count my chickens yet
Bill - thanks for your suggestions. What's a PCV valve? Should I just try to use a jointing compound?

Thanks again

Jonathan
Jonathan Davies

No PCV on a UK 73. The carbs suck on the front tappet chest, and the vacuum is slowly filled from the vented oil filler cap on the rocker cover. Removing the oil filler cap should cause a noticeable change in idle revs, and you should be able to feel the vacuum when you place the palm of a hand over the filler hole. There should not be any significant difference between the normal arrangement and sealing the breather ports at the carbs. If there is it implies a vacuum leak somewhere in the breather system which is upsetting the mixture. The carbs should be setup with the normal breathing system in place so as to take account of it.
Paul Hunt

I know I've read that it's not a good idea to change to a synthetic oil, and I know JN that you probably agree now, but I just wanted to confirm, so had a look around and came up with the following:

>>If you've been driving around with mineral oil in your engine for years, don't switch to synthetic oil without preparation. Synthetic oils have been known to dislodge the baked-on deposits from mineral oils and leave them floating around your engine - not good. I learned this lesson the hard way! It's wise to use a flushing oil first.
If you do decide to change, only go up the scale. If you've been running around on synthetic, don't change down to a mineral-based oil - your engine might not be able to cope with the degradation in lubrication. Consequently, if you've been using mineral oil, try a semi or a full synthetic oil. By degradation, I'm speaking of the wear tolerances that an engine develops based on the oil that it's using. Thicker mineral oils mean thicker layers of oil coating the moving parts (by microns though). Switching to a thinner synthetic oil can cause piston rings to leak and in some very rare cases, piston slap or crank vibration.
Gaskets and seals! With the makeup of sythetic oils being different from mineral oils, mineral-oil-soaked gaskets and seals have been known to leak when exposed to synthetic oils. Perhaps not that common an occurance, but worth bearing in mind nevertheless. <<

Mike
Michael Watkins

Should point out that the previous came from:
http://www.chris-longhurst.com/carbibles

Take a look, some one has worked damn hard on the site.
Mike
Michael Watkins

This thread was discussed between 26/08/2003 and 28/08/2003

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