Mostly, as was the case with mine also,....this is caused by crud in your fuel tank (rust etc) finding its way to your carbs and blocking up all and everything including your float valve and seat - hence the float valve is not doing it's job. I just finished cleaning out my fuel tank using 1st electrolysis (covered on this forum) followed by a 10min soak and swish around with a litre of diluted rust-kill acid and a flush out with a garden hose. Last step - I threw in a litre of ATF and tossed that around for 5 mins or so, tipped out the contents and wiped out the excess with a chux multicloth wrapped into a ball on the end of a flexible hose that I could shove down the filler cap and use as a mop. Left the residue as a lining on the tank and fueled er up again. No more carb overflow. Rust seems to be gone. You can sort out the carb(s) by removing the float pin to drop out the float..the float valve falls out - so watch it and catch it... then undo the valve seat nut and clean out the filter attached. Also, remove both jets and inspect for blockages and clean. I was able to do this with carb rack in bike - but to #1 carb only - as others do not show symptoms. But can do #4 easily also. 2 & 3 would be a pain and would be best anyway - to remove the rack and clean it properly.
Mine is a CB750 - assume your 650 to have similar needs. Good luck.
Mark
