Try our new info resource - "Aladdin's Cave" (Main menu)Just added a separate link to Ash's Dropbox thread (shortcut)
If you need to keep running it like that initially, then change the plugs to #8 as it'll give you more safety margin. This won't fix anything but with the plug tips getting very hot (white colour on #7) then it risks making pre-ignition which will lead to melted pistons if you're not careful. Do you know the needle position and what size the idle jets are ?
Did you remove the inlet manifolds? always replace the o-rings in them if you did as they grow hard and won't reseal after removal. It's the heat cycle which does for them. I'd imagine that the engine is sucking air in from somewhere causing the weak mixture.
I may be able to help; where about in A'dam are you?Can you make it to my garage near the Weteringcircuit?
I've been talking to Mark Paris (Hondaman) a lot lately. Did you know BTW he's writing a book on the 500/550, the same type as he did for the 750. Anyway, he recommends removing the advance/retard springs and cutting a half turn off the end of them both, reshape to match original curl and refit. The stronger spring evidently changes the engine from its characteristic "switch ON, Switch OFF" torque sensation at 5000+ RPM to one of more linear response with more torque on hand all the time.