Motorcycle Repair: CBX1000 RPM Problem, nuetral, air cleaner

Question
I just purchased a 1979 CBX1000 with 15k original miles on it.  The bike is as clean as can be but will not rev over 5k RPM under load.  It has no problem reving higher in nuetral.  The air cleaner is brand new as well as the plugs and wires.  Any ideas would be greatly appreciated.

Thanks.

JR

Answer
If the air supply into the engine is good, and the air filter box as well as the air collector is clean (no animal nests inside) then the next suspect would be a blocked exhaust, especially when it revs OK without load.  What exhaust system is on the bike?  Let me know,  Jan