79 Kawasaki kz750 sputters?

My bike has been running off lately, but last night it was terrible. It would idle fine, but if I gave it gas it would barely rev past about 3k, and if I tried to go from a stop, it would bog down and die. If I actually got it going, it would rev up okay until about 5k, where it would sputter and want to die. It ran fine when it was hot outside, (85F) but it was about 50F last night, so I'm guessing it's running rich? I can only change the idle screw jets on the carb, so how could I fix the top end power?
 
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