Could be a case of weak spark being blown out once you load the motor up.
Get some new plugs. Check coil connections for corrosion.

Double check both cylinders are firing. The engine will still rev out in neutral on one cylinder (trust me i know) But once you try and ride it, is very lazy.

Like was said earlier, exh should be warm, and should pulse. If one pot is out, it will still push air our, but it will be cool, and wont pulse.
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