Change exhaust, and too poor blend of fuel

[slappy]

Check the exhaust is fitted correctly and tight,all gaskets are in place if there are any, an air leak could cause this problem.
You could refit your original exhaust and see if problem is still there, check the carburettors are correctly set up and there are no blockages in the pilot jets or main jets.
Is the bike running ok , any misfiring or lack of power?