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MG MGB Technical - Oil in carb inlet manifold

I spent today removing my HS4s to get access to the tappet covers and I found a small slick of oil in both front and rear inlet manifolds. See picture.

The car has been standing un-used for several months (long and boring story). Can anyone shed any light on how this oil could have got there? There are no signs of oil on the outside of the carbs, so could it be that oil from the damper is being drawn in through the carbs/piston drillings into the engine and collecting there?

If it is being drawn in I would have expected that the oil would be drawn in to the engine and burnt up with the air/fuel mixture.

The carbs are original and have covered 220K+ miles. Any thoughts would be welcome.

Many thanks,

Richard


Richard Thompson

The oil could have come from the carburetor dashpots. If you filled the dashpots to the top, the excess oil would eventually leak into the manifold. When the engine is in use, it does get burned. But if the car is sitting for a long time, the oil will collect in the manifold. -G.
Glenn G

Hi Richard

If you still have the breathers connected to the carbs it may well be from the crankcase. If you still have the pcv flying saucer it may well be a problem with the diaphragm.

Peter
Peter Burgess Tuning

Had something similar happen on my B (HIF4s). It was definitely oil seeping from the dashpots - I have always topped up with Castrol ATF which is easily identified as it is coloured red. Hasn't happened since I reduced the oil levels in the dashpots (see the archives, there are a number of posts about dashpot oil levels).
Brian Shaw

Hi Peter,
Thanks for your reply. I'm not too sure what you are referring to when you say flying saucers. Please could you enlighten me.

I have the breathers connected to the front side plate that I have cleaned out earlier in the year before refitting. I'll check the inside of the breather tubes for oil as well.

Richard
Richard Thompson

Richard,
check out flying saucer on ebay..

http://www.ebay.co.uk/itm/MGB-1962-68-PCV-VALVE-13H5191-/230859391461?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item35c04ac1e5

fitted to early pre 69ish B"s before breather pipes moved to enter the carb's just in front of the throttle butterfly.

MGmike
M McAndrew

Hi Mike,

I see now. I have seen these on early engines and wondered what they do. I have an 18V 582 equivalent so the breathers are directly connected to the carbs.

I have learnt something new today !

If the oil is coming from the crank case this would presume too higher a crank pressure. Could a blocked filler/vented cap cause this ?

Thanks,

Richard.
Richard Thompson

Richard,
If (and it's a big IF at the moment) you have high engine back pressure it could result in oil residue in the inlet but it's by no means the only source. As you mentioned, you would expect any oil vapor to be sucked in to the combustion chamber when running which leads me to think the oil has "settled" after the engine is switched off.
A few easy checks will confirm the level of pressure generated. Disconnect the breathers at the carb's (or the y piece) and plug the holes in/to the carb's. Run the car and rev it a bit whilst watching to see what, if anything, comes out of the breather. If you have significant vapour, you need to look further. It's highly unlikely a blocked filler will cause excess pressure as it's prime flow is to allow air in to the engine to replace that which the breather has removed (always on the premise that the amount of suction is more than the amount of piston blow by..).

Best of

MGmike

M McAndrew

Hi Mike,

Many thanks for your suggestion. I will be trying this out over the weekend. I will probably change the oil filler cap in any case, as I can't recall when I changed it last. I think it is a 12K miles service item.

Regards

Richard.
Richard Thompson

I would have thought that if the oil was from the dashpots it would be clean.

That oil looks dirty, so, as others have suggested, it's more likely to be from the crankcase.
John Bilham

This thread was discussed between 07/10/2012 and 12/10/2012

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