The KHK that got it's top end re-op has been back with the for a couple month now and seems to have some of the same problem that it had prompting it's repair work - a recurring issue with the rear plug carbon fouling. The heads had plug inserts fabricated to allow 14mm short reach plugs and it's been running BR6HS NGK's. Most of his riding is in town a moderate speeds.
After running the bike on the dyno for break-in the idle and high-speed mixture settings were set for best power. These settings seemed to be fine for hiway running, but would result in popping on the over run. A richened idle mixture cured that. He does not use much if any choke to start with the warm(ish) temps here. However after a few miles on new plugs the rear plug will quite often carbon foul. The front plug shows a nice color and no sign of fouling ever. The rear plug does not look like it's an oiling issue - dry, black carbon that shorts out the center electrode.
So - a manifold leak seems unlikely as it would (you'd think) affect both cylinders. All adjustments are correct - spark timing, valve clearances, etc.
Is this something that was an issue with KHK's or other K Models for some reason?
Tempted to install an iridium plug in the rear, being more resistant to fouling, but that's just a band-aid over some other problem I fear.
George