2003 TXT Wont rev out completely

[Wallyguy]

As the title says, the engine wont rev out to top RPM when under load (driving)
When I put it in neutral the engine will go to full RPM, (belt goes to upper level on clutch). When I drive it, it's power seems normal to a point then it won't rev any higher (belt not all the way up on the clutch).

Recent plugs, fuel pump, took carb off cleaned all and checked float. Compression even at 125 both cylinders.
Also did a leak down and I can get it to hold air at one point in the cycle.

It does seem to blow back through the carb a bit just when the motor is about to stop ( is this normal?)

Any thoughts? Places to look?

Thanks to any responses.