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MG Midget and Sprite Technical - Compression Test

Total newbie here! I just purchased a 1978 MG midget. The body is in pretty good shape and it runs but blows white smoke out the tailpipe. I haven’t driven it passed second gear because it’s not registered and my driveway isn’t that long. So I don’t really know how well it drives but the engine runs and that has to be a good start, right? Also, all the emissions had been removed from the engine. I don’t have to worry too much about that here in MA but eventually will probably try to reinstall it. My goal is to get the engine in great running condition and then start on everything else. I’m not looking to do a full restore but I do want to get it looking and running great so I plan on speeding a lot of time/years on this project. So the first thing I did was do a compression test. I turned the engine 10 times for each cylinder for a dry and wet test. I did both the dry and wet test twice just to make sure.

Here are my results:

Dry
Cylinder 1 – 95 psi
Cylinder 2 – 180 psi
Cylinder 3 – 170 psi
Cylinder 4 – 170 psi

The second time I ran the dry test, I got “zero” on the first cylinder and the same readings as before for the remaining cylinders. I may have not fitted the compression hoses correctly because every time I tested the first cylinder from that point forward, I got something like a “zero” reading.

Wet
Cylinder 1 – 0 psi
Cylinder 2 – 215 psi
Cylinder 3 – 210 psi
Cylinder 4 – 210 psi

I probably should have just turned the engine 6-7 times but that shouldn’t be that big of a deal right? I’ve included an image of the spark plugs which I pulled just before conducting the compression test. The first spark was wet and looks like it’s never been used. The second plug was a little black and the rest of them I guess look fairly normal? I’m not a mechanic by any means, but I’m enjoying the learning process. My question is simple, what am I dealing with here in terms of the block/gasket and or head? I’m also wondering if I’ll need to have the engine rebuilt? I’ll run the test again on the first cylinder when the weather is better to make certain I was seating the compression tool correctly in the block. I never got anything above “zero” after the first try. So any help would be greatly appreciated!

DW


DLW W

It is quite difficult to get zero, doing a compression test. Even if there are major problems, the piston going up and down usually gets some sort of reading! Sounds like something wasn't connected properly for the test.

But that aside, 95 on #1, together with a plug that looks like it hasn't been firing and white "smoke" would suggest a blown head gasket, or possibly, but much less likely, a cracked cylinder head. The gasket is easy to deal with. Cracked cylinder head not so easy, so assume it is NOT that for now!

The other 3 are good readings, so the gasket failure could be just by #1. And the good readings would suggest that the pistons, rings and bores are good on those 3 and should be OK on #1 as well, unless there has been some other sort of major mishap.

Guy
(white smoke is usually water vapour)
Guy Weller

There could be a number of things causing the results and symptoms you are seeing, although Guy may well be correct in his assessment. You could try other tests to find the problem but with it being so easy to remove the cylinder head, and likely as not even when you know what is wrong it will have to be removed anyway, emove the cylinder head and find out for sure what is wrong.
Bob (Robert) Midget Turbo

Bob,
I did consider some other possibilities, like a dropped valve or even a broken con-rod (no compression after the first upwards stroke?) but the engine would make such a clatter.... Dropped valve, - maybe, but then why the white smoke unless they have been voting in the Vatican again.
My next move would be to a) poke a screwdriver down #1 plug hole and rotate the engine slowly by hand to check that the piston is moving. Then b) take the rocker cover off and inspect the rockers / valves. then C) remove the head for a proper look.

Guy
Guy Weller

I know that guy and you are most probably correct, but whatever the head will need to come off so with readings like those it is straight forward to remove the head. Any other course of action I think is just a waste of effort.
Bob (Robert) Midget Turbo

This thread was discussed on 27/09/2008

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