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MG MGB Technical - overdrive, cranking speed, and compression

Please see the “persistent non-starter” thread to catch up on this problem.

The car remains obstinately dead, but I just checked the cylinder compression. Kudos to M McAndrew, Lothian, United Kingdom, for suggesting this! I was horrified to find that compression ranged over the 4 cylinders from 60 to 30 psi!!! Repeated measurements show the same. About a year and 3,000 miles ago this formerly well-running car showed 150-155 psi, using the same gauge. It also had its original transmission, which still works very smoothly after a rebuild a long ago in John Twist’s old shop.

Now, the car has a newly-added 4-synchro + overdrive trans., with the required late-model back plate and fly wheel. All of this went together smoothly and fitted easily into the car.

The overdrive trans has not been rebuilt, but looks and feels like it is in good shape. Gears are unworn, synchro selectors look good, and all gears shift smoothly.

It now seems that the car won’t start because it has no compression. Why not? Could it be that the replaced trans. Is binding or dragging on the engine and preventing it from turning over fast enough? Is this alone enough to produce a non-starter? I haven't done anything to the valves or cylinders, by the way.

Has anyone ever encountered this problem with an overdrive refit?

Thanks for your advice, Mike
M.E. Whalen

The OD installation has no direct effect, the engine R&R could have several.
Is the engine cranking over at something like "normal" speed?
If not, find the ground cable that was left off. Or the nasty battery cable connections.
If so, are you checking compression with the throttle fully open, as you should be?
Do that.
If it then has compression, your no-start is likely because the throttles are just plain closed. It may well have been held at idle speed by maladjusted carb/throttle linkage, which got disturbed in the engine R&R. While that setup may now be different, don't expect it to be correct.

FRM
FR Millmore

Mike, with the compression that low I would expect the engine to turn over quite quickly on the starter. If it doesn't check as FRM suggests the earth from engine to body and body to battery. However, this won't effect the compression unless it's cranking very slowly and you have a very poorly engine!
As long as you have the gearbox in neutral and don't hold the clutch in while cranking, it won't add drag unless the input shaft or laygear bearings are seizing. But lets not worry about that until you fix the compression problem.

It's very unusual for all four cyl's to go off after a lay-up, but it appears they have and you need to look at possible causes,
1, head gasket
2, valves sticking
3, rings stuck in piston grooves
Start with 2 by checking to see if the valve clearances have opened up from their normal setting. For 3 put diesel down the plug hole and leave it to soak for a day or two.
1, is a head off and replace gasket..

best of luck

MGmike

M McAndrew

I too had experienced very low compression readings a few
years back.

It turned out that, at each shut down, oil was seeping down each
valve stem and would cook on the back of the valves. As time
progressed, deposits of coke (hard burnt oil, carbon) would prevent the valves from closing fully.

A head disassemble and parts cleaning, and new valve guides,
fixed all.
Daniel Wong

Another possible cause, of all of your cylinders reading very low on compression, is that the timing chain might be worn and it allowed the engine to jump time, causing the valves to open at the incorrect time and bleeding off some of your compression. RAY
rjm RAY

This thread was discussed between 27/04/2011 and 28/04/2011

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