Top Speed Problem.please Help

good day masters , i have a problem on my MX
i use 14-36 sprocket combi and i got a top speed of 145 , but i can only reach it on the long straight road
on 14-38 i only got 130kph
on 14-40 only 125
btw my set

stock head
28mm carb
60mm bore kit
API R cdi Thailand

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