Replacement carb does not have mixture screw
I have a Craftsman YTS3000 with a 540cc Briggs Platinum - 21HP. (engine 331877 1371 B1) It sat too long and wouldn't start. I ruled out things until it appeared the carb was the issue. Per the parts pdf, it is a NIKKI carb # 591736. Got one cheap off Amazon - installed and it fired right up but.... It runs fast. All the time! Throttle adjustment does nothing. moving it to the "grass cutting" icon will kill the engine.
This replacement carb does NOT have a mixture screw. It is sealed and the description on the item says it is already set, so no adjustment is possible. Is this preset carb the problem? Tried messing with idle screw but didnt help. There's really not much to the linkage/install so I'm confident I didn't do anything wrong. Maybe I should have had the OEM carb rebuilt? Mower is usable, but pretty annoying always being full throttle - not a fan of cold starting this way. Any advise?