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MG MGB Technical - Sooty spark plugs

I am just putting my car up for the winter, and part of my routine involves changing the spark plugs. The insulators on plugs 1 and 4 were very clean, and kind of tan in colour. Plugs 2 and 3 were black and sooty. I initially thought the mixture was too rich but because it is a one piece manifold, I thought it should be the same throughout.
The car runs well, has compression near 150 in all cylinders, and doesn't smoke. An oil change is also part of my fall routine, and I am always surprised how black the oil is after maybe 1500 miles. The car is going up as is, but is there something I should be thinking about for next spring?
Glen
G Nicholas

What year - single carb?

Shouldn't be that different, just like 1 shouldn't be that different from 2 or 3 from 4 on twin-carb setups. Perhaps oil is getting in on 1 and 4. You mention smoke but not conditions - any sign revving it after idling some time? Or accelerating after a long downhill run on the overrun?

The B-series is known as a 'dirty' engine, my roadster dirties the oil much quicker than the V8, for example. Particularly if you don't use flushing oil repeatedly until that comes out clean (not that I'm recommending it or would ever bother), as the engine and cooler are 'full' of old oil if you just drain it hot and refill. Even then mine starts off clean, but soon dirties up.
paulh4

B series engines tend to idle on cylinders 1 & 4 so the plug colour may be after a period of idling.

You really need to check colour after a good run with little idling afterwards.

Having said that, since 1 & 2 share a head port (ditto 3 & 4) there should be little difference in the mixture between them. 1 & 4 being clean and 2 & 3 being sooty suggests there is an ignition problem on 2 & 3.

Oil burning doesn't usually result in sooty plugs, they will be black & glossy.

So from what you have said, the plug colours indicate either nothing (if all four are clean after a run) or that the plugs / leads / cap need renewing.
Chris at Octarine Services

I suppose if Glen's car was single carb it might be that the two inner cylinders were slightly richer than numbers 1 and 4, if carb set rich. However the outer pots sound correctly coloured. Compression sounds good, so no blown head gasket or burnt valves. Clutching at straws: timing/points gap right? valves adjusted correctly?

Funnily enough the oil stays relatively clean on my B Roadster, despite many of the journies being relatively short.

However, my Riley RM's oil gets black very quickly which I'd put down to excess fuel getting into the oil, as all four plugs on that are always very sooty and yet the car runs well.
Peter Allen

If its 1500 miles a year and a lot of short journeys that might add to oil colour and a few good blow-out runs might help with running subject to all parts, components and settings being good.

Nigel Atkins

Sorry, I should have mentioned my car is a 71 roadster with twin SUs. I haven't touched the engine in a few years, so my first item for next spring will be a complete tuneup and maybe a good long run. Thank you for your comments.
G Nicholas

This thread was discussed on 04/10/2018

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