Help! Jetting, drive system or other cause?

I have a gilera dna50, water cooled 2003 bike. When bought it had clearly been modded, the exhaust showed signs that the end pipe had been removed and refitted suggesting the restrictor was removed. Revved to 7500rpm before engaging clutch and very tight powerband between 7500 and 9000 so clearly had tight springs etc. however, it died on hills and occasionally struggled to accelerate at all so i played with the springs and cleaned the carb.
It has a white naraku contraspring (1000) and i replaced super stiff springs with looser black naraku springs (1500). Rollers are 5.5gm, idle jet is stock 32 and main is 62.5. Airfilter is stock and clean, slide needle on 2nd from bottom clip.
HERES THE PROBLEM - IT DRIVES LIKE S#\~
The bike engages better now at 5000rpm, idles a bit low but manageable. Accelerates to 1/4 throttle and tops out at 6000rpm and 18 mph. If i hold at 1/4 throttle it runs smoothly at these high revs to low speed.
Open the throttle more and it dies. Between 1/4 and 3/4 throttle the bike revs, dies, revs, dies in a brumm....brumm...brumm.... Brumm way. If i then push to WOT it revs a bit more cleanly again at 6500 - 7000rpm and sits at 18-20 mph and a little rough. NOW... if i hold on WOT at the top 7000 rpm and 20mph and hit a small decline the bike gets a life and flies up to 9000rpm and 45-50 mph! WTF. I can fly at this speed and performance as long as it stays flat or declines and i hold the throttle on. Close the throttle and its sick again.
I cant tell if these symptoms are carb related, drive related, something faulty like choke, pistons etc or something electrical.
I believe the top end had a rebuild at some point recently. The choke screws are stripped so i havent checked the choke yet. I have no electrical knowledge so havent tested the cdi , coil etc.
The main issue seems to be the 1/4 to 3/4 range where it runs like a one legged dog with a thorn in its paw.

 

Sounds like a mixture of poorly setup carb and wrong weight rollers. Try upping the roller weights slightly and adjust the carb so it idles better and move a notch on the needle. And keep playing around until it's sorted

 

stripped screws on a carb are a problem, could be letting air in there. There's not a lot that can be done with that so probably best to change the carb, then start working from there. From the rev/die/rev thing you say, i'd guess at wrong jet size.

 

On the stripped screws on enricher part, that was my bad, the screws are stripped on the head where the screwdriver sits, not the threads. When you suggest jets, the stock is 56 and i have 62.5, so only 10%' do you think i need to go richer to feed the mid throttle range?