What does the engine sound like when it is running at operating temps? Does it sound like it is stuttering? If so, that stuttering/stumbling sound means you are too lean, richen the mixture.

Are you sure that engine is brand new? If you engine does not stutter or stumble at operating temps, but simply slows down in revs and diesm that sounds like symptoms of an engine that has seen many gallons though it and has a worn piston & sleeve.

On a new engine, your poroblems should be the otehr way around: harder to keep running when cold with a tight pinch but runs better when at operating temps.

If tuning does not fix your problem, dismount the engine, disassemble it, and inspect the parts. Look especially at the sleeve and piston; check to see that they are not worn.