MGB Register Forum

Hi Im a newbie on here after buying my MGB GT about a year ago. Engine was shot so had it rebuilt and its still burning oil, mostly at high revs and when on overrun....especially on overun which makes me think its breathing issue? Took it back and they said rings were faulty and fitted new ones. Still burning oil.

Block has been bored and skimmed along with head. 270 Cam fitted along with high compression pistons with rings etc. Even fitted a catch tank and original oil filler cap. Still has oil burning issue. Any advice on what to check? Only getting about 90 miles between Max and Min on oil dipstick.

Any and all advice appreciated.

Thanks,

Nik

Difficult to diagnose remotely, sounds like it could be rings. Be aware that rings may take a while to bed in, and may burn oil initially. Putting new rings in is quite a lengthy and expensive job - did they charge you for doing it?
Was it a reputable engine builder, as some may do a poor job.
Finally, I once fitted a catch tank to a TR2 engine and it interfered with the engine breathing quite a lot, so I always disconnected it when on public roads
Ian F

90 miles between max and min! Are you followed by a blue cloud? If you are burning that much oil, is it coming out of the exhaust pipe as blueish smoke? Get someone to follow you and watch your tail pipe. It could be piston rings or through the valve guides. Are valve seals fitted, at least on the inlet valves? Are the valves a good close fit in the guides?

If it's escaping into the engine bay I would have thought everything would be covered in a layer of oil. Neither my original 1800 MGB or my current V8 use any oil at all.

Thanks both, yes rebuilt by an engine builder that was recommended and smoke is out of exhaust, im thinking same, so will check valve seats and if we fitted new valve guides.

If it smokes under acceleration, it is telling you that it smokes when cylinder pressures are high, which points at the rings. If it smokes under deceleration on the overrun, it points at the valve stem seals.

This always was the guide I worked by

Ian F

+1 for valve stem seals. Have they checked/ changed these ?

Do a wet and dry compression test that would confirm rings or something else, if it was a reputable engine rebuilder member of FER then it should have a decent warranty I would not dismantle anything as that could invalidate any warranty that may be valid. take it back to them and insist they sort it.

Thanks all, so had the car back and he has changed the head and put new rings on. So went for a 10 mile run. And blue smoke on accelerating (rings will need to bed in) and white smoke on overrun!!!!! Seems to have both issues.

We had the valve seats re done but not the guides as there was minimal if any play in them.

Im going to do a dry and wet compression test at the weekend.

Other option i belive is the crankcase breathing, how do i check if this is working correctly?

Many Thanks,

Nik

You should have a rubber pipe coming off the tappet chest cover nearest the front of the engine which connects to the carbs. check this for sludge and any blockages, also remove the tappet chest cover which has a gauze filter in it clean this out with petrol and blow it dry with compressed air if you can. also try taking the oil filler cap off with engine running and put the palm of you hand over the filler feel to see if pressure builds up?

I am afraid I am going to just be an old cynic/grumpy old man here, but I am sceptical about the repair work said to have been done - in such a short time?
Have you got the better valve stem seals fitted? They are visible if you take off the rocker cover - small rubber caps fitted onto the top of the guides.
If you want to check for crankcase pressurisation, take the oil filler cap off and put a garage latex/Nitrile gloves on it. Ideally it should be sucked flat (careful - switch off if this happens, you don't want it drawn down into the rocker gear).
If it inflates into the hand shape you have pressurisation which needs to be dealt with.
Over pressurisation could be caused by gases leaking past the pistons/rings into the crankcase. Normally this will go out via the tappet chest cover via a pipe and into the carbs to be burnt. You have to make sure this pathway is clear (no catch tank system getting in the way!).
Good luck with It
Ian F