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MG Midget and Sprite Technical - I think I've broke it!

This is a bit like another thread (bad news, bad news) but I thought it better to start afresh since the situation is a bit different.

I have done around 3000 miles on my Peter May 'Rally Spec' engine. I recently became a bit concerned about high oil consumption but as there was no sign of burning oil or smoke I thought it must be leaking. However, I did an MGoT track day at Donington yesterday, thrashed the engine a bit and now it is smoking quite badly (and oil consumption is high).

My immediate presumption is that I have a problem with piston rings (broken?) but is there anything else that could cause this. I did a compression check this morning (hot, plugs out, throttle open) and the readings were exactly 200 psi on all four cylinders. I thought that if some rings were damaged then that ought to show up in the compression readings.

It's the rings isn't it?
Chris H (1970 Midget 1275)

If you have 200 psi you must have well over 10/1 CR and presumably a hotter cam to go with it. Other than putting viton stem seals on the exhausts, I'd check for petrol in the oil which would lower it's vicosity. Otherwise do a leak down test.
F Pollock

I do have a high CR and a hot cam (and high lift rockers). I presume that a leak down test means take the head off and put liquid (petrol, oil?) in each bore and see if it drains away.

I did fit Viton seals on the exhaust valves when rebuilding the head. I noted at the time that the old seals had 'disappeared' (they were there originally as I fittened them).

The engine is running well so this is annoying. I did overdoing at Donington and found myself doing 7000 rpm at one time (but Peter May said the engine was good for 8000) so wasn't too worried at the time.

Chris H (1970 Midget 1275)

Leak down test...

http://www.youtube.com/watch?v=ofSiTGeLXvc
Dave O'Neill2

No, leak down uses compressed air via the plug hole and measures the loss of pressure - escaping air also gives a clue as to where the leak is,

Leak down will not help if the problem is with the oil control rings - they have a history of the expanders of certain makes of ring snapping at the ends - this will result in smoking/burning without any loss of compression.

Another possibility is detonation/pinking - at high revs this may not be heard but will cause the rings to flutter and oil will be burnt.
Chris at Octarine Services

Its not just at high revs that it smokes now. Even at tickover there is a trace and 'blipping' the throttle gives good puffs of smoke.

I did change the fuel pump recently to a low pressure high volume Facet pump and wondered if it was capabile of changing the mixture and generating a rich mixture. The reason I started the car this morning was because I thought I could smell petrol on my journey home yesterday and thought I might have a flooding carb or a leak. There was no sign of either and it would not explain the loss of oil.
Chris H (1970 Midget 1275)

Might be worth checking to see if all the seals are undamaged and still have the little spring collars on. I've had situations where the stem seals have been crushed and lost the springs. Check if oil from the dipstick smells of petrol.
F Pollock

Do you have the PCV valve fitted? If so, have you checked it for oil?
Chris at Octarine Services

Chris
I have the breather from the timing chain cover (the can shaped thing) connected to the breather at the rear of the HIF carb (and that has been like that since before the new engine and since the rebuild). Is that what you mean by PCV valve? Nothing else apart from that.
Chris H (1970 Midget 1275)

If it is valve seals, id think it would only only be on the intakes...not the exhaust

Is it definatly blue smoke comming out the exhauste pipe ???

.
Prop and the Blackhole Midget

No - a PCV valve is mushroom shaped and sits on top of the inlet manifold.

Have you been back to Peter May - surely this is a warranty issue?
Chris at Octarine Services

Chris
I've never had a PCV and I don't think 1275s in the UK had this. As for Peter May, I don't think Peter warranties his motors - after all he can't control how they are used.

I'm thinking I'll take the head off and check the valve seals. Also check the pistons for any signs of damage and bores for any marks (from broken rings etc.). If none of that helps identify the problem then it is engine out!
Chris H (1970 Midget 1275)

Are the oil scraper rings separate from the compressions rings?
In other words, could you still get normal compression with a broken oil scraper ring?
Hugh Alison

PCV valves were fitted to early 1275s - up until mid '68, apparently.
Dave O'Neill2

Up to 12CE-DA-H3201 they had PCVs fitted.

Hugh,

Yes- as I said above you can have broken oil control expander rings and compression will be unaffected.
Chris at Octarine Services

My 1275 was built in 1971 so no PCV.
Chris H (1970 Midget 1275)

check the valve stem seals.
Do you have roler rockers or normal highlift rockers?
if you have normal ones they might be side loading the valve stems causing worn guides.

Leak down test is fancy stuff, the cheap ass version is just pooring a measured amount of fluid (petrol for quick results) in each bore and time how quick it runs down.
If one is way off then it indicates trouble.
Onno K

This thread was discussed between 08/10/2013 and 09/10/2013

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