Strange compression readings on a 948

[Fingolfin]

Last night I decided to give Mog's engine a compression test. It's a 948cc, but since some former owner was smart enough to remove the engine number plaque, I don't know if it's a high or low compression engine (it probably is a donor engine from an MG Midget). Here are the numbers:
Cyl. 1 (frontmost): 110 psi
Cyl. 2: 118 psi
Cyl. 3: 115 psi
Cyl. 4: 105 psi
As all of these numbers are within 12 psi of one another, I'm not worried about a head gasket or piston ring leak. But I've read that a low comp. 948 should give about 130 psi, and a high comp. should give 150 psi, so these numbers seem abnormally low. Any ideas?

(For the record, I ran the engine until it was quite hot, jammed the throttle open with a brick, and removed all the plugs and the coil lead. The engine has not been disassembled in the time I've had it.)

[bmcecosse]

Pretty poor readings - as I think you already guessed. The gauge may be reading low of course. Usual thing then is to load a bit of oil into each cylinder - and do the readings again. If they jump up considerably - the rings are worn.

[Fingolfin]

Thanks, Roy. I did as you suggested, with the oil -- the first three cylinders gave me the same readings as earlier, then the fourth gave me a reading of only 30 psi! So I tried the third cylinder, which had only just tested "well" a few moments before, and got the extraordinarily low reading. Therefore, I think the compression gauge is bad. I'll go borrow another tomorrow and try again.

[bmcecosse]

Either that - or the starter motor/battery had died.......

[Fingolfin]

Oh my no -- the starter put out lots of oomph, and the battery's been on the charger.

[Fingolfin]

I obtained a second gauge and took readings again, having burnt the oil off.
Cyl. 1: 125 psi
Cyl. 2: 130 psi
Cyl. 3: 123 psi
Cyl. 4: 120 psi

These are a little more in line with one another, and not indicative of leaks. But they remain low, if the engine is high comp. I can't imagine any Midgets were equipped with low comp. engines, and this engine is almost definitely from a Midget...

With oil in the cylinders, the readings were uniformly between 130 and 135 psi in each cylinder except the fourth, which once more gave a dramatically low (40 psi) reading when oil was added. How could it be that compression is "high" without oil, but very low with oil? Possibly the gauge is getting gunged up with the oil...

[Fingolfin]

For what it's worth, I tried the test again on cylinders 1 and 4, but this is after the engine had dissipated the majority of its heat -- it was perhaps around 80 degF/30 degC.
Cyl. 1: 143 psi
Cyl. 4: 145 psi

So clearly cylinder 4 is okay, at least when the engine is cooler...

[JOWETTJAVELIN]

Is it running fine? If so all is well. Rebuild it only when oil consumption becomes excessive, say 100 miles to the pint.

[Fingolfin]

Okay. It's running all right -- a tiny, almost unnoticeable intermittent misfire, which may be down to poor mixture (I need to get a new jet and needle). No oil consumption I can see, though I've only driven it about two miles since first firing the engine.

[JOWETTJAVELIN]

Well cold starts account for 80% of all engine wear - so drive it more for longer!

[bmcecosse]

It's not a dramatic increase when you add the oil, and they are all similar - so I would say it's in ok condition, just doomed to low compression. You could get hold of another head and rip 80 thou off it - that would improve matters!

[katy]

As it hasn't been run much for the last year, (maybe more), I'd suggest, for the time being, just leave it alone. Wait 'till you get it on the road, driven it for a few weeks, have a few hundred miles on it, giving it a chance to settle in and then recheck the compression. Maybe reset the valves while you're at it.