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MG Midget and Sprite Technical - compression test attempted/need help

1969 M.G. midget mark 111...Trying to perform a compression test of the motor. Battery reads 12.5 volts prior to cranking. The engine will only turn over 2-3 times before it won't crank anymore. I connected the M.G. to another car with a "good" battery and tried again with the same results. This indicates to me the batt. is ok, unless I'm wrong. The car runs well with no obvious signs or sounds of binding from within the engine which might inhibit cranking. I drove 60 miles yesterday with no difficulty. Help will be appreciated.

Thanks,

Keith
KTM Moore

what happens if you forget the compression tester and just crank it over with all spark plugs oout?
David Smith

Have you removed all 4 spark plugs?

Why do you think you need a compression test when it goes so well?
Onno K

My car runs well for a car with 100,000 miles. I noticed on my drive yesterday that as I went up short hills the car would maintain speed but would not accelerate. Driving up longer hills the car seemed to accelerate ok. I have felt for a while that I wanted to do a compression test as the car does not have all the pep it used to have, as expected I know. I am basically trying to determine the condition of the engine so I can determine what the engine needs to bring it back to an optimum operating condition: e.g cylinder rebore with new pistons and rings, cylinder honing with new rings, new or rebuilt cylinder head, etc. As I could not crank the engine sufficiently to perform a compression test, I decided to perform a leak down test (for the first time I might add.)The results of the leak down test were:
Cyl. #1 20% air loss from dipstick tube.
Cyl. #2 25% air loss from dipstick tube.
Cyl. #3 22% air loss from dipstick tube.
Cyl. #4 55% air loss from #3 spark plug hole, both
carbs @ jet bridge and exhaust tail pipe

RE; the battery, I noticed as I cranked the engine the voltage dropped to about 10.5 volts. I placed a charger on the battery and it showed it needed to be charged. I am charging the battery and will try the compression test later.

I guess my question has now changed from compression test help to: are there any thoughts as to what my engine may require to bring it back to optmum operating condition?

BTW, I had removed all four plugs when I attempted the compression test.

Thanks,

Keith


KTM Moore

Jumper cables are a poor way to check cranking problems. Frequently the jumpers just don't make good enough contact. Better to put a good battery in the car. Batt voltage drop to 10.5 on cranking is pretty usual. Question is, is the delivered voltage at the starter 10.5 or less? Less indicates bad cables or connections. Bad cables might also cause poorly charged battery. Measure from starter terminal to block while cranking.

Your #4 leaking through #3 means bad head gasket. You might expect it would leak the other way too, but they often do not. It would certainly account for a lazy engine on hard pulls! (and will do further damage to head & block if run that way)

You did check/set valve clearances before leak test?

At 100,000, you can guarantee it needs valve work, cam, tappets, chain, and probably everything else, assuming that stuff has not been done before. Very well maintained engine might get by with rings and bearings, but that is rare; expect bore/pistons/crank grind.

You probably can get by with just a head gasket and valve job if you want to put off the major work now, like so you can drive this year, but it is just a delay.

FRM
FR Millmore

FRM, Thanks for your input. I will check the voltage at the starter terminal. I did replace a head gasket about 300 miles ago, torqued and retorqued the head to spec. after lapping the valves as there was some pitting on the valve mating surfaces. I set the valve clearance to .012 cold per the stated spec. To check the efficacy of the lapping I inverted the head and filled the combustion chambers with water and left it overnight. The next day I found there had been NO leakage so I felt the lapping had resolved any valve issues that may have existed. I also replaced the valve springs with new ones.

Keith
KTM Moore

Keith-
If you could lap the valves at that mileage, then it has certainly had work done. What do you know of history?
Are the pistons marked for oversize? Measure bores this time you have the head off, especially checking for wear ridge at top. If you don't have the tools and knowledge to do this accurately, find help.
Did you check head & block for flatness? Especially in narrow sections between bores. Why did you change the last gasket? What head gasket did you use - black composite or copper faced? How long since the retorque?

FRM
FR Millmore

FRM,

I am the original owner of the car and it has been in my possession the entire time except for about five years when I sold it to my Father as I needed a family car. I believe he had the head rebuilt during that time. I changed the head gasket as the car severely overheated on one occasion and it started to blow steam from the tail pipe. Upon removal of the head I found water in all four cylinders. I checked the block for level as best as I could, and replaced the gasket with a Payden black composite gasket. It has probably been 300 miles since last retorque.
My intention is to have this car as long as I live, then my oldest son will take possession. I think to enjoy this car to the fullest, even though it drives ok now, I will remove the engine, inspect it and do what is necessary to restore it to like new condition.

Keith
KTM Moore

Keith-
At that mileage and history, I think you will be best off and happiest with a complete rebuild. In the process, you can easily gain a very useful few HP (and/or mileage, depending on the right foot), at little to no additional cost. Or a lot at some increased $$$!

"I checked the block for level as best as I could" doesn't sound good enough. You need to understand how to determine if it IS flat, within .001" or so.

"It has probably been 300 miles since last retorque."
Time and heat/cool cycles are far more important than mileage. Contrary to many, I have measured and found that even the Payen gaskets DO settle over time.
Might want to send me an email for my take on head gasket tightening.

FRM
FR Millmore

FRM,

How do you determine if the surface is within .001 of being flat? Re: the head gasket, I drive the car every 2-3 weeks and 10-25 miles at a time. What is "your take" on head gasket tightening?


Keith
KTM Moore

Keith-
"How do you determine if the surface is within .001 of being flat?"
Standard machine shop practice.
You need a precision straightedge, or something that will serve as such, and feeler gauges to measure gaps between that and the part in question. Serviceable straight edges can be had at Harbor Freight, or as ground tool steel stock from industrial suppliers, or other things like planer knives. I usually use a piece of ground tool steel, 3/16x2x18, and treat it gently, but I have the high priced spread as well, in many forms, from 2" to 36" long. Designated precision straight edges can be very pricey, but are not necessary for occasional use. Once you understand the principles, you can make one yourself, or a correspondingly accurate surface plate. It's in books. One equal or greater than the diagonal across the head or block is best; short ones can be used for transverse or local faults, like between two cylinders.
Something like .005 total along the diagonal, or .001 locally are typical limits. You can usually get by with double that, if you understand the nature of the deviation, and the characteristics of the parts.

"Might want to send me an email for my take on head gasket tightening."
Ask for "Bolt Babble"

FRM
FR Millmore

FRM,

Thanks for the input, I really appreciate it.

Keith
KTM Moore

This thread was discussed between 28/04/2012 and 01/05/2012

MG Midget and Sprite Technical index

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