1979 25 Evinrude no top end

[Hawk Bay]

I have a 1979 25hp 2 stroke with relatively low hours despite its age. It ran well until recently. It starts okay and first two trips it ran fine. But I now get no top end. Full throttle delievers one third power at best. I installed new plugs and gas tanks, hoses and gas are new, but clearly not the answer. Tune up last year did not resolve issue. Any suggestions?

[PhrozeN_FisH]

First thing I do with a running problem is check compression. Have you done that? Next would be to check for spark on both cylinders... If you have good compression and spark the next check is fuel. Possibly could be fuel pump, carb, filter... Let me know what you find out.