Welcome to the forum!
What you are describing sounds like 'slow return to idle' which, believe it or not, is usually caused by being too lean on the pilot (idle) circuit.
As your quad ages, fuel varnish builds up in the pilot jets, causing the idle circuit to operate leaner and leaner. The proper way to fix this is to remove the pilot jets, clean them out with spray carb cleaner and copper wire strands and reinstall.
A temporary fix (and to verify this is the issue) is to turn the fuel screws out (CCW) 1/4 to 1/2 turn each. Does it return to idle now?
Of course, the other issue may be a stuck throttle cable - you can see if this is the case by removing the throttle cam cover (triangular on the left side of the driver's side carb) and see if the cam returns to zero immediately when you let go of the throttle.
Another possible fix for varnished pilot jets is to add Seafoam (2-3X what is recommended on can) to your fuel tank and ride, ride, ride... often the Seafoam will remove the varnish, as long as it hasn't gotten too bad.
Good luck and let us know what you find!
BTW, slides are vacuum operated, so at open throttle you will see some modulation due to the pulsating vacuum of the single cylinder.
'01 660 Raptor, Burgard +2+1 arms, YFZ Shocks, LSR anti-vibe, CTR pipe, jetted, K&N
'01 Polaris Scrambler 400, K&N
'02 Honda 400ex, Big Gun pipe, jetted, K&N, Lonestar +2 A-arms, Lonestar anti-vibe
'02 Honda 400ex, DiamondJ +3 A-arm widener, Lonestar anti-vibe
New Toy: '16 RZR XP4 1000 Turbo
Last edited by QuadManiac; 01-27-2017 at 12:09 AM.