Lew,
In a message dated 96-10-01 12:24:26 EDT, ldove@macomb.com (Lew Dove) wrote:
<< ..... 3000+ mile trip later this month ..... I still lose/consume oil, so
I checked the engine compression,
cylinder # compression (psi)before oil compression after oil
1 124 133
2 132 135
3 127 131
4 133 137
the plugs in cylinders 2 and 3 are a nice clean tan; the plugs in the
cylinders 1 and 4 are sooty and black. My conclusions are:
All the pressures are low, but perhaps I have a bad gauge. The rings are
not sealing properly and are probably worn. Nigel is a '65 MGB with a US
spec. engine.
What do you think? I haven't driven it long enough following the oil
lines and cooler replacement to know about oil loss/consumption now.
Would you trust it for a 3500 mile trip? >>
One quart of oil in 500 miles is nothing to get upset about. One quart in
300 miles is still cheaper than an overhaul. A few drips is no big thing as
long as you don't mind the oil spots on the floor. When you get large oil
puddles in the garage it's time to fir the gaskets and seals.
Smoking and black plugs can be caused by rich mixture, but then it would be
#1 & #2 plugs, or #3 & #4 plugs, or all plugs. Any other combination is not
fuel mixture. If the rings are bad enough to cause compression loss, it will
spray oil droplets out the tail pipe and go about 200 miles or less to the
quart, sometimes 50 miles to the quart.
Most common cause of black plugs and smoke (especially on start-up) is loose
valve guides, allowing a little oil to flow down the stems into the
cylinders. This doesn't have any effect on compression until it gets so
loose that the valve heads bounce around and won't seat, and that's pretty
loose. As long as it's running OK and not smoking too bad and not oiling up
the car behind you on the road, just keep adding oil and drive on.
My MGA compression numbers at last check:
1 130
2 125
3 113
4 137
Not particularly unusual as the block is not real flat and the head gasket
usually leaks a bit between #2 and #3. Also my 1500 is low compression
model, never gets more than 145 in the best of condition, maybe could stand a
head shaving.
If you had high compression flat-top pistons you would expect 175 in good
condition, but for dished pistons anything from 125 to 150 is reasonable, so
yours looks pretty good (as long as your not racing for a living). Ideally
you would like to have them all within 10 psi so it idles smooth, but if you
don't mind a little rouhness, anything within 20 psi is OK, will just sound a
bit strange at moderate speed and full throttle.
Anything over 90 psi on all cylinders should start and run OK, below 90 it
could be hard to start when cold and also low on power. Since I race my MGA
regularly, when any one cylinder gets below 100 psi I usually yank the head,
but do a blow down test to see where it's leaking first. If I was a bit
more serious about the racing, I would have the block decked, shave the head,
put in flat tops and look for 165-175 at all times. Would also have to run
hi-test fuel.
Trust it to go 3500 miles? Of course. It's an MG. Drive it 'till it drops.
Barney
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