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MG Midget and Sprite Technical - Smoking Gun

Ok here's one for you all.
Fully reconditioned 1275, bored block, cylinder head, valves, guides seals, pistons, you name it, it's been done. Everything new. No expense spared.The works. Compression great.
Done about 400 - 500 miles.
Starts fine on half choke.
Once warm, runs fine.
After descent of moderate hill, 10 mins from starting out, blows huge clouds of smoke that you would think could only come from a stuffed motor. Once cleared, does'nt do it again.
This smoking has started only after flushing running in oil out, and putting in normal super grade oil.
The only thing that hase'nt been rebuilt on this car are the SU's. Having said that, they appear to be ok. No apparent flooding and are tuned well.
Have had the guy remove the cyl head. Obvious blow by at no's two and three cyl's. He'd used a NOS head set. Valve seals appeared ok but hard compared to the new ones in the new set just purchased. Suspect he may not have had c/head torqued down correctly also.
Any other suggestions ?

Best

Geoff. I did tell him way back i'd stick an A15 in it for him, but he likewise told me where he'd stick one as well !
g r mattar

Until the point you said there was blow by on the head gasket - I'd have said it was possible (though unlikely) to get a slug of oil through the breathers int the carb and manifold which would create a large cloud for a few moments until all burnt throurgh.

Does sound like the headgasket was either incorrect torque or incorrect technique. You need to start in the middle and work out with the torquing to ensure its clamped as tight as possible in the middle.
Dean Smith ('73 RWA)

Did the head get retorqued a week / 500km of use. Also what sort of hills are we talking about. I use to live in NZ and them hills could be pretty rough. Also what head gasket was used and was the deck plained.
Allan
Allan Jacks

Classic sign of oil being sucked into the combustion chamber on over-run (e.g. down valve stems) and then burned when throttle next applied...

A
Anthony Cutler

stem seals

and put 'em on exhausts as well as inlets.

running downhill the front valve is sitting in a puddle of pooled oil on top of the head isnt it

we had this with an Astra many years ago, fitted new valve stem seals all over, job done!
Bill

Stem seals are a real possibility, but you say these are new! and on a full rebuild they should be. As Bill states it is wise to have them fitted to both inlet and exhaust valves on a road car.
However there is another classic cause of the symptoms that you describe and that is the "popit valve" (or whatever it is called) fitted to the inlet manifold of a number of Spridgets.
This valve when failing can open when the engine is under hard suction (going down hill)and the pipe from the front engine breather (timing cover) can be exhausted into the inlet manifold. This can allow oil mist straght into the engine induction system. This can be made far worse when going down a steep hill, because oil is thrown forward and saturates the oil seperator on the iming cover which is then sucked straight into the inlet manifold if the valve has failed.

If you do not have this valve fitted then ignore all I have said. If your car is fitted with it then blank off the pipe from the front timing cover and drive down the hill to test the theory. :-)
Robert (Bob) Midget Turbo

Bob - PCV valve (positive crankcase ventilation)
David Smith

If its a popit valve thats "STILL" functioning, Its time for a carb rebuild......let the dinosours die.

What color was smoke that came out the tail pipe....that would tell alot

Blue, Black, or white???

Prop
Prop***The End in 2012

Thanks guys. The hill is a fairly moderate one, but quite normal to coast down.
White smoke and ton's of it. Only does it once though which is a bit odd if it's valve seals. Don't believe it has a pop valve. I'll check. Just one hose off the timing cover to the Y junction of the SU's.
I've checked all the guides for the guy ( it's not my car ) and the head has definately been rebuilt alright.
The gasket set he used was new old stock. Not sure of age but the valve seals are definately hard compared to the new one's purchased.
The gasket itself appeared to be a normal old type and painted with some sort of silver paint. It took an eternity to get it off. ( the head that is )
I'm fairly confident the head was bolted up without a torque wrench as the owner has bought a new one. Very definately blow by evidence on two middle cyls.
The thing that puzzles me is why it only does the smoking routine once. There are several bigger hills in the area on his way home and it does'nt ever do it again. He was not aware of it doing it at all when he was running it in with the lighter grade oil.
I'll let you know how it all works out. Thanks again.
Geoff.
g r mattar

Geoff

This happened to my midget for years... I really didn't see it as a problem. It only happened if I drove from home cold and went down a particular series of roads; then you could say 80% it would happen pulling away from a set of lights.

It disappeared when I rebuilt the engine (found a broken ring) as it had done 105K and I wanted to rebore and re-cam for competition.

I originally though it was worn guides (really should have been; or at least poor valve stem seal)... but I put the head back on unchanged.

A
Anthony Cutler

I have just been through the same excersize as my Midge does exactly the same.

I have just replaced the head with an unleaded metro head with no bypass and put valve stem seals on both the inlet and exhaust and now the problem has got worse. With super stealth mode.

This happens only once and then the car appears fine so I have assumed it must be the rings but will try removing the breather and taking it for a spin.

Shaun
Shaun

White smoke indicates burning anti-freese, if you have the head off have it rechecked (magnifluxed) for cracks, most likely the head gasket went bad, lots of cheapos out there, or you didnt get the head retorqued in time (500-miles max), or the head wasnt torqued properly....start at the middle work out and torque a third of the spec. at a time....exp. if the spec is 60ft/lbs...do 20/then 40/then the 60. Also go with a payne composite HG they are the best.


White smoke = coolent burning

Blue smoke = oil burning

Black smoke = to rich a fuel/air mixture

Prop
Prop***The End in 2012

Geoff and Shaun, I would always remove the breather pipe from the carbs/inlet manifold when I had smoke as you describe, it is simple to do and instantly illiminates one of the things that can cause such a problem.
Robert (Bob) Midget Turbo

Ok it's all back together and running great thanks. Bit smokey on start up and early running probably due to residual oil on top of pistons etc. I think the carbs need new needles and jets but appears to have fixed the problem. I'd say it was the gasket being old and head maybe not torqued down correctly. Thanks all for your comments. Best. Geoff.
g r mattar

This thread was discussed between 22/01/2009 and 25/01/2009

MG Midget and Sprite Technical index

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