2001 st Workhorse no power

[BGONE]

I'm stumped.... I've searched the forum and tried things I've found but no success. When its under any type of load even just flat ground it will hardly pull it's self with slight back fire through carb. But point it down hill or put the rear end on Jack stands it runs great. I've pulled the carb and cleaned, cleaned out pulse tube to fuel pump, replaced fuel filter and all fuel lines. Before I go any farther I want to ask for suggestions. Would spark plugs, or valve adjust cause any of this. This whole thing started over night it ran great when I parked it then a week later it was this. Any help would be greatly appreciated