Will not shift into higher gears

[bpctx74]

Suddenly started having trouble shifting into higher gears. At first I could get into the higher gears but couldn't shift down, now I can't get into 4th or 5th at all. Any suggestions where to start? Shift forks? Clutch assembly? 

[don73tx750]

I'd think if it was the clutch all shifting would be affected, not just 4&5. The first thing I would look at are the shift claw springs.
#2 here
www.partzilla.com/parts/search/Yamaha/Motorcycle/1974/TX750A/SHIFTER+2/parts.html
and #24 here
www.partzilla.com/parts/search/Yamaha/Motorcycle/1974/TX750A/SHIFTER+1/parts.html
If those springs are intact and have decent tension, it's probably time to split the cases and check the gears and shift forks.