CB550F sooty spark plugs - why?

So here's the problem my cb550f1 seems to have

New spark plugs completely soot up with dry black soot. They get soo covered in soot, that after about 500km of driving they no longer spark enough to start the bike. Sounds like a classic rich mixture, simple enough right?

The problem is that I've looked the bike over and worked on the bike together with a real expert - the guy makes a living repairing and selling SOHC CB's, he knows what he's doing. We just can't seem to find the problem.

- Bike is equipped with electronic ignition
- Carb's have been synchronized
- Carb's have been completely reset to stock with new jets and o-rings (38 slow, 98 main, 22mm float height)
- Stock new honda airfilter
- New rubber boots on both sides of the carbs
- All four plugs soot up
- Idles perfectly, runs fine first 1/3 of the throttle range, sometimes (not always) gurgles and stutters a bit in the 2/3 throttle range, runs strongly at the top throttle range
- Once in a while a carb float 'hangs' and overflows

Any ideas on what the culprit could be? Or on method/steps on how to identify the problem? Help would be greatly appreciated!

Problem could possibly be the "new" jets. It's not uncommon for aftermarket replacement jets to have a fairly wide margin of error over the OEM specs. If you can, I'd try to find a way to measure those jets and make sure they are up to spec. Based on the photos it looks like you're running pretty rich, and that would be consistent with having larger jets than necessary

Run a compression test on each cylinder and report back. It's also worth checking your ignition timing.

If it's OK at WOT, suggests that the issue is is on the needle. Start by checking needle and needle jet for wear.