Maybe you should stick with the standard jets and rectify the fault first. Changing to non-standard bits, you will never know what the original fault was and could get a bit lost. Needle position is likely as Rajnish says.
quote:
All OK to 4,500rpm, then really hesitant (like running on one pot), then at 7500rpm takes off like a scalded cat and pulls hard.
Perhaps you have an RD400 engine in there?