More than likely the main jet is too small .
If you think about it ; when you back off the throttle the bike runs better , you are in fact reducing the amount of air ,the fuel delivery stays the same so the air:fuel ratio is where it should be .
If you have put a less restrictive exhaust or air filter on the bike ,you will need to jet up .
Do a plug chop to check out the mixture .
Blessed are the cracked, for they let in the light. - Spike Milligan
"When you come to a fork in the road ,take it ! When you come to a spoon in the road ,take that also ."