Or do you just have a two-lever control with a coaxial arrangement like this?
We had one of these that produced exactly the same symptoms (also increased speed when shifting into forward). This particular one comes apart with the one bolt on the inside (
after disconnecting the cables
- forgetful mind, when you remove the bolt the throttle cable tongue comes off, so no need to disconnect, and the transmission cable stays in place), you can then pull the throttle lever out, clean and lube, and should be good to another several years. There is a plastic bushing between the two units, that can get damaged, especially if old, so some care is required. No idea if spares are available.