I just went through the most complete cooling system check and fix in the world LOL.
If the system is airtight and the cap pressure is good it WON'T boil. If it's boiling it's because the water inside isn't under pressure.
Most likely causes are
1) The aftermarket cap.
2)Air in the system. When I bleed I fill with distilled water and coolant/water wetter until it overflows, then rock back and forth and squeeze all hoses and refill. Then start it and give it a few good revs to spin the water pump and push air out. Let it get hot enough that the water rises to start to overflow then cap it, shut it off and let it cool. When it's cool top it off and do the whole start and rev process again then go for a ride. Let it cool and top it off one last time.
3)A minor leak somewhere either a pinhole in a hose or a loose clamp.
'97 GSX-R 750
Full D&D Exhaust, Dynojet Jet Kit, K&N Filters
CRG Levers, Vortex V3 Grips, ZG Double Bubble,
Stainless Steel Spiegler Brake Lines,