Usually when you can’t get full rpms or speed it’s a belt or clutch issue. Confirm you have correct belt # 0627-028. Belt deflection approximately 1.25”. Does it reach higher speed and rpms when track off the ground? Do you know the history on clutches whether they’ve been serviced at all? How many miles on sled? Have valves been serviced?
start with the basics-compression check, both sides equal?-siphon some gas from bottom of tank and put in clear bottle overnight-check for dirt and jelly like goo in botom of bottle-thats what ethonol does and will mess up your carbs or injectors so if so get rid of all the fuel and start fresh then see if choking it at top speed helps clear it out-if so its probably not getting enogh fuel and stuff needs cleaned-check for air leaks on rubber flanges where carbs/injectors clamp to engine-run it wide open on stand to see if clutches r opening and closing all the way-look for the obvios 1st before spendin a bunch of time and money