What size jets have you tried? What other changes have been made to your intake/exhaust system? Since you mentioned 24 and 26 mm carb....I presume it's a 150cc scoot.
If the unmarked jet is the original, it is probably in the 105 to 108 range. If you are changing it out with a 120 or larger, is no wonder you're having problems. Need more info from you, to be helpful.
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