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MG MGB Technical - Oil in the inlet manifold

I have a question about SU HS4s and the dashpot oil.

When I recently dismantled the carbs as part of clean up job, I noticed a small trickle of dark engine oil at the bottom of each port tube of the inlet manifold, between the head and the carbs. I started this job about 2hrs after a 1 hr drive.

I guess that this oil has come from the dashpots as there is no other way that oil can get there and the level in the dashpots was down. Is this correct?

My thoughts are that the vacuum from the engine, combined with wear in the dashpots and pistons, is drawing oil out and it's collecting at the bottom of the manifold. The carbs are original and have covered 220K+ miles. Is this a correct analysis and would this be symptomatic of having aged carbs?

(72 Roadster 18V582)

Cheers

Richard
Richard Thompson

Richard. Maybe. But, in my experience none of the worn SU carbs I have had on various cars have leaked oil into the intake manifold. On certain models of vehicles, the PCV (positive crankcase ventilation) system, connected to either the air filters, the carbs, or the intake manifold (methods vary) do allow some minor amounts of engine oil to get into the intake manifold.

I do not know enough about your UK (Home Market) vehicle to provide sufficient advise. The North American specification vehicles, about 90% of the production, used a PCV system connected to the carbs. This system, from time to time, allowed a small amount of oil vapor to collect inside the intake manifold--the oil vapor was heavier than the fuel/air vapors coming in through the carb throats.

Were it my car, I would not worry about this unless I had a very significant loss of engine oil while driving.

But, my experience is limited. I once owned a new 79 mgb roadster (Personal Export Delivery--North American Specification) and the oil use of that engine was well less than one quart per 3K miles. I also once owned a 61 MGA 1600 which used about one quart of oil per 100 miles. Learned to live with both of them and enjoy them.

You have noted something. Please write this down in a small pocket notebook and, over a period of time, your records will tell you if anything of significance is taking place. I suspect it is not.

Last month, the wife and I made a trip of 3,000+ miles in her Mini-Cooper. Used less than a cup of oil during that time. Hardly a "real British car" when you travel 3K miles and use less than a case of oil on the trip. World is going to hell in a hand basket.

Les
Les Bengtson

Dear Les,

Thanks for your comments. The PCV I had overlooked and of course you are correct. I will inspect the relevant pipes; if the oil is coming from there I'd expect to it there.

I have recently found that is has been difficult to get the tune correct and have attributed this to worn carbs; hence the clean up. I seam to recall that you mentioned in a previous thread the benefits of a new set.

I will be taking notes as you mention as I have just completed a complete rebuild on a BMH shell and this is the last item to resolve......at the moment. The idea is to keep complete record from now onwards.

Cheers,

Richard.


Richard Thompson

UK and North American cars were the same as far as the suction side of the PCV system went, all cars from Oct 68 used carb vacuum. A PCV valve on the inlet manifold was used from Feb 64 until then, and on those cars the valve can fail causing high oil consumption. The carb system is miles better being 'free' as it utilises a natural feature of constant depression carbs to provide a very consistent level of slight vacuum with little variation. However some cars do seem to suffer from high oil consumption being sucked from the breather port on the front tappet chest cover. This should have an oil separator/flame trap steel mesh filter as the base of the metal pipe, but even where that is sound some cars can still suffer from it, one chap at least only solved it by replacing the front cover with trap and pipe even though he couldn't see any difference to the old one. Funnily enough modern ZS cars can suffer from the same problem, so much so that oil runs into the air filter housing and drips on the floor, and people install an oil trap. You can do the same in the hose between the cover and the Y-splitter to the carbs.
Paul Hunt

Paul,

Thanks for your comment. I'll try to rig up a temporary catch tank with transparent plastic pipe to see if the oil is coming from the breather.

My filler cap is quite old, would this contribute to a higher crank case pressure and therefore more deposits through the breather?

Cheers

Richard
Richard Thompson

An old cap is more likely to block than anything else, which would *reduce* flow through the breather to the carb. Unless possibly you have a lot of blow-by where the cylinders are pressurising the crankcase, so a blocked cap would mean that *more* flow is being pumped into the carbs by the pistons, as none can be released via the cap. Or possibly you have a non-vented cap for some other application rather than the vented and filtered for the MGB. You can check for that to some extent by removing the cap and placing your hand over the oil filler. Firstly when the cap is removed the idle speed should change, as what you are introducing is effectively a vacuum leak weakening the mixture. Then you should feel definite slight suction with your hand, or a sheet of paper (too large to get sucked in by the way), placed over the inlet, and it shouldn't be blown off even when revving the engine.
Paul Hunt

This thread was discussed between 05/09/2009 and 11/09/2009

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