Nx50 lacking power (not gy6)

[A-T-Scooter]

Hey y'all I bought a 1981 Honda nx50 express sr 2-stroke scooter originally it had a oil injection system but I've bypassed it and am mixing my fuel it runs excellent on the trophy stand but driving is a different story it bogs like crazy on take off then sling shots you and goes like hell till you get to 20mph then it's done that's all it has I managed to push it a little further and got it up to 25 and it was like hitting a speed limiter in a chevy cavalier kept nose diving then picking back up after it slowed down... from the factory these things would do 40-45 but with age and lack of a proper replacement belt your looking at 30-35 why am I only getting 20-25?

[A-T-Scooter]

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