I believe that TF is likely correct. It's almost unheard of for these vehicles to flare on 1-2..........the common flare is 2-3. If you're not engaging first gear, you believe the flare is 1-2 when it is actually 2-3.

You've got two problems.

The linkage for the Bowden cable has possibly gone "overcenter" and doesn't move when the throttle is opened. If this is the situation, all the shifts will be early...........doesn't matter what the throttle position is..........you won't get any revs out of it. But, more significantly, it won't engage first gear..........you only get 2-3-4. Count the shifts.........see if you only have two of them on the way up.

The second problem is the 2-3 flare............major success is usually the outcome with a new K1 spring kit is installed.