2004 Arctic cat atv 500 will not go faster than 40 km/h on flat ground and slows down to 20 km/h going uphill.
If low performance was the original reason you took it to a repairer, it is clear they have failed to discharge their contract with you and you have grounds for complaint and to expect the repair completed for little or nothing extra.
There are a number of possible reasons why an engine that runs fine would be down on power and if the fuel system really needed that big clean out, it could still be fuel related. A carb and fuel system that was in a bad state often needs cleaning a couple of times, especially if water was a problem.
Other possibilities are a strangled intake, a strangled exhaust, low compressions especially due to bad intake valves or bad valve timing.
Another (unlikely) possibility is that of ignition timing which will likely be electronic with programmed advance characteristics. It is unlikely but it is possible the programme could be damaged leaving the ignition timing too retarded at speed to produce proper power.
It is also worth considering things like tyre pressures, binding brakes, wheel alignment...
on Oct 27, 2018