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MG MGB Technical - Su conversion and increased oil consumption?

Through the help of the fine folks on this board, I did an SU conversion on my '79 B a while back. It now has rebuilt HIFs and K&N filters.

I have noticed, though, that my car now seems to use a little oil. Granted it does have almost 110k miles with no rebuild and it's not a lot of oil, enough to be noticed when I check the oil level.


I have it plumbed exactly like a '72 - '74 B and it runs great.

I'm guessing it's ring blow-by coming through the side cover vent which Y's into the base of both carbs.

Why the increase? It did not use much oil with the original (and working) setup.
Is there supposed to be some 'restriction' I'm missing?

Cheers to all,
Dave
David Steverson

Simple enough to understand once you think about it. More power means higher pressures inside the combustion chambers, thus a greater chance of blow-by.
Steve S.

Good answer Steve. I had not thought of that. It's a small trade-off for more power.
Thanks, Dave
David Steverson

What rate of consumption? My twin SU equipped cars would take about 3k miles to get from max to min although I only usually let it get half-way before topping-up. MGB SUs from Oct 68 have a restricted breathing arrangement instead of the earlier troublesome PCV valve, but they also need either a restricted breathing oil filler cap on cars without emissions plumbing, or a restricted vent on the rear of the rocker cover connected to the canister on emissions equipped cars. As well as preventing an unbalanced mixture as the throttle is opened this also prevents too much by way of oil droplets being dragged through. Having said that there have been more than a few comments recently about high oil consumption, like down in the low hundreds per pint, which is something to do with the crankcase breathing but very dificult to resolve.
Paul Hunt

Paul,
I would estimate a quart in 1000 miles +- . Before, I could go about 2000 without adding any.
On the carbs plumbing, I used some photos from another MG enthuiast and the Moss catalog to try and duplicate what came with the HIF equipped cars.
I still retained the original charcoal canistor and it's connections.
Dave

PS My wife and I are going to spend 2 weeks near you (Shropshire) in February. I only wish the pound vs. dollar was better..
David Steverson

Ah, Shropshire, my favourite county. Totally unspoiled until the relatively recent blot on the landscape that is Telford. If you have any interest in industrial archeology (and can con the wife) you must visit Ironbridge and its ... iron bridge ... and the Ironbridge Gorge Museums, enough there for several days. For pretty bits you have The Wrekin, Wenlock Edge, The Longmynd, Clee Hills and others. Ludlow and its castle are a fine example of an unspoilt Shropshire market town, paradoxically with more Michelin-starred restaurants anywhere outside London.

Your current consumption is a little on the high side, in my experience. To see if it is being drawn through the breather you could fit a small bottle in the pipe 'tween front tappet cover and Y-pipe and see if that starts gathering oil.
Paul Hunt

David
I recently found that fitting K&Ns on my 1980 GT did not do a lot for the oil consumption! It's a stage 2 head with 715 cam and during the laid back running in period it was using very little oil, but as I started to take the revs up, it would consume as much as a pint in a 100miles. Friends following me said that under acceleration and after a long period of trailing throttle, I would get a puff of smoke from the exhaust.

I found that by fitting a small K&N breather on the tappet chest stack, and then just passing the hose between the carbs (Too lazy to properly block off the carbs) the oil consumption went to virtually nil. In fact, even on Mobil1 0W40, it has hardly used a drop in 300 miles, if any.

Apart from the extra pressures created in the cylinders, my thoughts were that the vacuum created in the intake manifold might have increased causing more suction. As I didn't do the rebuild myself I don't know for certain that the wire mesh strainer is still fitted in the chest to capture the oil droplets/mist. But the mention above of the cap is worth investigating as I have a standard black plastic which may not be properly restricted.

I would only suggest this option if you regularly do long trips so as to allow the engine to get properly warmed up so that contaminants in the oil can get properly burnt off, otherwise without vacuum from the carbs, these may stay in the sump during short runs to the detriment of the oil and the engine.

Martin
Martin ZT

As Martin implies the problem with disconnecting the carb suction is that not only do you not scavenge fumes from the crankcase, but you are likely to get a build-up of condensation which will cause internal corrosion, usually visible as a milky scum on the bottom of the oil filler cap and elsewhere in the rocker cover. Several hundred thousand MGBs had positive crankcase ventilation *without* high oil consumption, there is another cause of the high oil consumption which would be better fixed, IMHO. I can't really see why just fitting K&N air filters to the carbs (if that is what was meant) caused a sudden increase in oil consumption via the breather.
Paul Hunt

My car has almost 110k with no rebuild. I happened to park on an incline yesterday and not set the handbrake. I found the car did not want to hold itself = poor compression.

So, it's proably getting due for a rebuild, the money for which I'm spending for that UK holiday.
Dave

PS. Sadly, Paul my mate lives in Stirchly, very near Telford. I agree, it's not the most scenic area around. But, as you say very close to some lovely places. I've done that walk up the Wrekin, quite nice.
David Steverson

David
I guess this lot answers your original post pretty well - if you were asking 'should I be concerned' the answer is no; the engine was originally designed to burn oil at a rate of about 1% of fuel consumption - allowing American measures, you are close enough to make no difference but when consumption increases, you might want to play with the breathing, you might also want to think about your choice of oil; the ambient temp. in Georgia probably deserves some thought but in Northern temperate climes, Oil Mineral Detergent 20 - 50 works well - avoid synthetics and the standard 10 - 40 used by so many modern engines - both burn faster.

Happy New Year

Roger
RMW

This thread was discussed between 28/12/2004 and 31/12/2004

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