99% of the time this as a bad cable, especially if the problem shows up after winter layup. If there is a corrosion issue inside the cable sheath, it gets worse while sitting over the winter. Come spring, the cable binds or is locked in one position. Oil or grease rarely help and if it does, it will not last long.
The most common areas where these cables fail is where they bend or where they may contact something hard like the motor when it tilts up. You can feel the exposed part of the cable for lumps or for spots that seem to be stiffer than others areas. That is a sure sign of internal corrosion. Once the outer plastic covering gets cut or nicked and the internal spiral cable is exposed, it is only a question of how long before the cable corrodes.
The remaining 1% of these cases are either throttle mechanism failure ( or even a tension screw going wacky) or an issue with the motor linkage itself. That is easy to check by removing the cable clevis pin and seeing if the motor linkage is binding.
If you have to replace the throttle cable, replace the shift cable at the same time.
