Help required?1981 250 lc carbs

[gazilla]

After lots of help trying to get my basket case rebuilt its still not revving past 5000 under load but will in neutral.
1. Could this be carbs (anyone got a working pair to prove or disprove this)
2.Could it be electrical (have tried coils and stator)
3.I have no other ideas?

thanks

Gaz