2000 RSV millie running issue

Hi guys.

Im new to the forum and also new to riding. Ive just started riding my 2000 rsv mille 1000cc
Ive had moto x bikes most my early teens but this is the 1st big road bike ive ever had.
The bike was my dads but due to his health he can no longer ride, he gave me the bike and ive only recently been able to insure it. The bike has been stood in hus garage for roughly 2 and a half years.
Anyway i finally got it on the road (what a feeling that was) she starts perfectly, ticks over nicely but after about 5mins riding as i apply the throttle, especially when setting off and changing up into 2nd the power just vanishes and i can hardly get the bike moving without destroying my clutch.
As im new to riding on road ive little to no idea where to even start so i thought id ask the pros.
Any help will be greatly appriciated, im new here so im not fully sure how to use the forum.
Thanks alot
Lewis

Blocked fuel filter,pipes inside the tank split??

Get it to aprilia performance in tamworth get griff and the boys to sort it out.....

Before i put bike on the road i was having issues with the pump injecting too much fuel so my dad altered it as it was constantly backfiring. Im wondering if he has somehow made it pump less fuel in that it needs. I drained all the old fuel and refilled with clean so i cant imagine its blocked again. I will have a look into those guys at tamworth though, thank you bud