Aerox hole in acceleration
 

Hello,
I have acceleration hole on my aerox.
It takes off good (flat, uphill with two riders), but there is a problem when there are two riders, it takes off good, revs as it should, but at around 10-15kmh revs drop and it hesitates, it takes around 5 seconds for it to get to 20-25khm then revs raise again and it accelerates good. This is even more noticable when riding uphill with two riders, it hesitates for a long time at this point/speed, but then out of a blue it strats reving and going like crazy.

I have Gianelli exhaust, new stock vario, 5g weights, stock clutch.
Everything else is stock. Belt is new.
I tried with and without the spacer in front of vario bushing, there is some difference how it behaves after 60kmh, but bellow that speed it is the same.

If riding alone I can also feel that hole, but it goes past it in a fraction of second.
It also pulls from take off better when engine is cold, choke is working, spark plug is brown as it should be, there is no bogging, it revs the same, it`s just not that bouncy. Could this be clutch counter spring too soft from age?
Thank you for help

I solved it. Previous owner didn`t rejet when he installed Gianelli exhast, I rejeted it from 62 to 68, malossi red sponge air filter.
Installed malossi white contra spring and stage6 medium rpm springs.