<VV> Bad Idle (66 140)

[N. Joseph Potts]

My very stock 140 has developed a terrific miss at idle (consistently on one
side, it seems) that seems to disappear at higher engine speeds (although
that MIGHT just be a masking effect of the higher rpms). Pulls smooth and
strong as always. Here are symptoms that I imagine MIGHT be related.
     On that side, I attain the best idle with the mixture screw cranked so
far out that I don't think the needle is regulating at all - the spring
under the screw head is completely extended and the screw turns freely. This
condition existed BEFORE the incident described next.
     A couple of weeks ago, a few minutes after I started the engine up from
a warm state, I heard a terrific banging on the side in question that
sounded to me like a thrown valve seat (I've heard two of these, on this
engine, in the past year, and replaced both heads). The head was recently
(thoroughly, expertly) reworked by one of our vendors of such services, so I
would be very surprised if in fact a valve seat has worked loose. This
banging sound was accompanied by a solid (all rpm ranges) miss, again
totally in keeping with thrown-valve-seat symptoms.
     Then, as I limped home, it cleared up. I know thrown valve seats can do
this, although neither of the (intake) valves that I had thrown previously
ever fell back into place as may have happened now. I haven't had the
problem since, but I HAVE had this bad idle, pretty much from that date.
     I performed a compression test - everything is normal, but I note a
loss of about 30psi on one cylinder (only). Percentage of lowest to highest
remains 78%, as before.
     I removed the rocker cover on that side to check for the telltale low
valve stem - none such. I removed the push rods for the low cylinder to see
if either was bent - they weren't.
     Does this form any sort of coherent picture to anyone? If so, I'd love
to hear what. Educated guesses are very welcome.
     Thanks.

Joe Potts
Miami, Florida USA
1966 Corsa coupe 140hp 4-speed with A/C

PS  Carburetors all rebuilt within past two years - stock except for
ball-and-cage float valves. All ignition components stock and sound (Dale
rebuilt distributor) except using new-ish Clark's silicone spark-plug wires.