LOW RPM at Full Throttle TYM 353 HST help

[astro1949]

]my 353 HST is now running at about 21-2200 RPM at Full throttle (was about 2800 ??)
the cable is tight - it bottoms out on the engine
i changed the Air and Fuel filters ===No Change

it operates fine -this happened about a week ago .

any ideas ?

thanks,
bob