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Formerly reliable MG

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Bob Claffie

Bob Claffie

Jedi Knight
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What is a reasonable compression reading for a '72. Apparently I have a burnt exhaust valve, as one cylinder is down to 45#'s. The other three are all around 95/100. All stock and reringed about 12K ago. Thanks, Bob
 
Bob,
I'm guessing that 95 to 100 would be reasonable on the other cylinders also.

It's fairly common to get a small piece of dislodged carbon stuck between a valve & seat when compression testing. Worth a try, to give the end of the valve stem a few "taps" with a hammer & drift which will dislodge or break up the carbon. Tap hard enough to make sure that the valve reseats hard. Also make sure that the lash adjustment hasn't "temporarily" changed for the same reason.

I have had this work several times over the years.
D
 
My '72 read around 140 -145 right after rebuild with the factory 8.9 pistons. With the 8.0 stock, I would say around 120 but certainly 100 is good enough for combustion and be fine if there is no oil consumption.
I just looked in my Bently manual and the 18V engines have an extra letter in the specs which I wonder about. It has a cranking pressure of 170 lbf. /sq. in. I don't know what the f is for but in the 18GD and 18 GG enginespecs it has regular lb/sq. in. specs of 160 lb/sq. in. for the high compression engine and 130 lb/sq in for the low compression engine.
Bob
 
Been many years now and things have changed, but a good "purging" at 5000 rpms plus was always "instant tuneup" in my day. Not sure how you drive the car (I ran a '70 high compression and a '69 MGC), but short drives just seemed to never warm up the engine, blow out the carbon and clean out the compression chambers. Doesn't sound very scientific, but a good night time run across an interstate pushing the yellow line on an early B (about 103 mph) used to make them purr like kittens in the morning. OK, that's illegal in most states.
 
One could assume lb/f to mean foot pounds.

The Workshop Manual Supplement states that the 18V engines have 160 lb/sq.in. and are 8.0:1 comp ratio.

Frazier
 
Just noticed that my Workshop Manual is copyrighted 1973, so things had probably changed by 1978. Sorry for butting in.

Frazier
 
Thanks all, I've been having a little "phht,phht" in the exhaust for a few weeks now with progressively poorer running so I guess it's not just a stuck valve but I will try that hammer trick before turning any wrenches. When I rebuilt this engine it was honed but not bored. At the time it only had about 50K on it. I did the rings and bearings as a precaution, not to fix anything in particular. Car has never been especially quick as my son's '71 will run circles around it. (I built his motor too).I can live with the lack of power but need to get the smoothness back. Thanks, Bob
 
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