The carb is dirty. The reason it will top out sometime and other times it won't is because the slide of the CV carb is lifted by the negitive pressure of the intake charge moving thru the venturi. This neg. pressure is routed thru an internal passageway in the carb body that is partialy obscured.
My recommendation is that you take the carb to a shop where it can be cleaned in an agitator. An experienced tech would know how to check the carb body to make sure all the internal passagewats are free and clear.
As always, check inside the fuel tank to make sure it's cleanand shinny.
Elite 150's are the coolest scooter's ever made, espically the '85 and '86 D models.
Also, while the carb is off, replace the prone to cracking. original manifold with Honda's updated design.
Always tighten the header to cyl. bolts first and then the muffler securing bolts.
The Scooter Doctor
Motomech "One modification a day"
'92 BMW R100GS Bumblebee
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"The BMW Airheads, riding yesterday's technology the day after tomorrow".