On Sat, 15 Nov 1997 16:38:21 -0600 (CST) Der schwarze Buccaneer
<js-allen@students.uiuc.edu> writes:
>..... I decided to buy and use a compression gauge on my MGB. .....
Anyone know what these results mean:
Cylinder Compression With Oil In Chamber
1 110 lbf 120
2 116 125
3 >> 26 << >> 26 <<
4 108 120
It's common for the head gasket to blow between #2 and #3, in which case
#2 and #3 both have the same low pressure reading, so not in this case.
A hole in a piston usually means zero pressure, and bad rings have to be
really bad to get the pressure that low, so also probably not either of
those things.
It means that 1, 2, and 4 are a little low on pressure, mostly caused by
slightly weak rings, but not enough to keep your daily driver from
driving. The rings on #3 are probably about the same. #3 either has a
burnt exhaust valve or a carboned up intake valve, or a valve that is not
closing all the way.
First order of business is to pull the valve cover and check the valve
clearances. Maybe one of the valves on #3 doesn't have any clearance, so
may not be closing all the way. If the valves do have clearance and they
are moving properly when you crank the engine, then it's time for a blow
down test.
For a blow down test you need to put air pressure into the suspect
cylinder. Take an old spark plug, break off the ceramic top and punch
out the center, so you have just the threaded steel base with a hole
through it. To this fitting weld on a quick connect air coupling. Put
this fitting in the spark plug hole. Rotate the engine to top dead
center on #3 with both valves closed. Hook up an air line to the special
fitting, set the pressure to about 20 psi, and listen. Open the throttle
by hand. If it hisses through the air intake you have a bad intake
valve. If it hisses at the tail pipe you have a bad exhaust valve.
Remove the oil filler cap. If you can hear a hiss at the oil filler you
have bad rings.
Bad rings are bad news. An engine tear down would be in order to install
new rings.
A bad exhaust valve will generally require removing the cylinder head to
grind the valve and seat (and presumably all the rest of them at the same
time). Wonderful opportunity to do a no-lead head job, since you're
already paying half the bill at that time anyway.
A carboned up intake valve may have an easier out. It could be Grapes of
Wrath time. I managed to weasel out of one of these once without having
to pull the head. The procedure is similar to changing valve seals
without removing the head. Rotate the engine until the cylinder in
question has the piston starting upwards on the compression stroke (both
valves closed). Push about 20 feet of 5/16 rope into the cylinder
through the spark plug hole, and turn the crankshaft with a wrench until
the piston compresses the rope up against the valve heads to hold them in
place. Back off the valve adjuster screw all the way and move the rocker
arm out of the way. With a little creative levering and maybe some help
from a couple of extra hands, you can then push down the spring cap,
remove the valve locks, and remove the spring.
Now apply a few drops of oil to the valve guide, and attach an electric
drill to the valve stem, preferable a variably speed drill or one that
doesn't turn too fast. With the drill spinning the valve at a slow to
moderate speed, pull up on the drill to press the valve head against the
valve seat. About a minute of this can clean the carbon off the valve
head and seat, or at least smooth it out enough to get most of your
compression back. Reassembly is the reverse of disassembly.
Now of course you would want to fix the cause of the carboned up valve so
it doesn't happen again. Suspect an overly rich carburetor, or excess
oil running down the valve guide. Too much oil down the guides means
worn out guides, which gets you back to removing the head for a valve job
and new guides.
Good luck, and let us know how it turns out.
Barney Gaylord
1958 MGA with an attitude
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