re: Why low compression on one cylinder?

[Jay Negrin <jmnegrin@domain.elided>]

Nick asks (edited):

<<My 1971 Giulia 1300 Super has all the good bits from a 1975 2000
Berlina (engine, gearbox, axle etc). It has developed a rough idle, flat
spot at
about 2000 and the ability to foul plugs at will.

It's been a long time so I cannot remember the exact figures but one
cylinder has much lower compression than the others. The thing I don't
understand is that the low compression seems to only exist at low revs.
Above 2000, it is fine.

I drove it last night for the first time in many months last night. It
revs
fine and pulls well, especially above 2000 rpm.>>

Tuppence from California -

Have you done a wet and dry compression check?  Put a couple of squirts
of motor oil in the spark plug hole before doing a compression check.
If it brings up the compression in that cyl, then likely the oil has
(temporarily) sealed the rings, which may be worn.  Worn, or broken
rings = oil in the cyl.  That leads to misfire, if it fouls the spark
plug.

What does the spark plug look like?  Is it fouled with oil, as one would
expect from a oil-soaked cylinder?  Or, has is burned too lean, and is
it overly white, and burned looking?  Are ALL of the spark plugs fouled,
or just the one that has low compression?

When did you replace the spark plug wires?  Are they old and crossfiring
amongst themselves.  Look at the engine idling, at night, with no other
lights on.  If the plug wires are cross firing, from being old and in
need of replacement, then it will look like a cute little forest fire of
small blue lights, all over the plug wires.  DON'T TOUCH THEM.  High
voltage, remember?

If plug wires are OK, and wet/dry compression check shows little or no
difference, AND you still have low compression in one cylinder, then you
may have a burnt valve.  Likely exhaust, as it is the hotter of the
two.  The motor would, I expect, seem weak at lower rpm, and then seem
to pick up a bit once under way.  Lack of compression from a leak in the
cyl (the burnt valve) would make it hard for the spark plug, assuming
all else is OK, to do its thing.

Like I said, just a couple of pennies worth from LaLa Land.  Worth
exactly what you paid for it.  Here's a wish it is something that
doesn't need unbuttoning of the motor to correct.

jay Negrin
ARO So Ca
76 Alfetta GT

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