Two suggestions:
1) How good is the mounting? I find carb/fuel issues often caused by poor vibrations - these need to be checked. Older models often need higher vibrations to work as well as when they were first acquired.
2) When were the lines fully blown through? Hold it at max revs for at least ten minutes to clear any residue in the lines. Let it cool and repeat, and I expect you'll get improved response and a lot more power.