Glad to see this raised some discussion. Interesting points. I have a
few more thoughts (don't I always!). I recalled today Pat Mullen's
(Ross' dad) excellent write-up on SU tuning
http://home.istar.ca/~datsun/att06.htm
He points out that one must be careful to check the float needles for
leaking prior to setting the mixture. High fuel pressure and a leaker at
idle can cause you to set the mixture to lean on one carb. Doesn't look
like the problem here.
Detonation appears most likely. This could be caused by too much advance
(BTW I set the advance the same way as Victor), but again why #3. *If*
the #3 rod journal does get less oil than the others, this could cause
the #3 piston to run hotter, since oil is sprayed from the big rod end
on the cylinder walls and the bottom of the piston. This provides lube
and cooling.
Now here is a question for Victor, since the damage appears very close
to the #3 spark plug, does this plug perhaps pertrude into the
combustion chamber more than the others (the ground electrode can heat
up and cause detonation). Champion makes serveral different thickness
spark plug washers to deal with this problem.
Todd Osborn
68 DATSFUN
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