93 xj600 will not rev over 3000 rpm

[Sporter]

What would cause the rpms to rev up to but not exceed 3000 rpms, the bike has 633 miles on it, almost brand new, I've gone through the carbs multiple times, the valves probably need to be adjusted but I replaced the oil, fuel and eventually spark plugs,(which fixed the issue running without the choke on) and before it would rev all the way up but now, 3000 and that's it smh cannot figure it out plz help