Now I am guessing.....
Fuel is not the issue if the dyno operator is not totally lost and out of his mind.
Less likely but not imposisible, ignition issue. Operator should be able to tell you.
Now with the information we have, my gues is single springs.....the drastic drop off, i.e. valve float.
That is not a natural power drop off, or cam issue or exhaust restriction.
It is a serious cut off.
+1000 to 1500 more rpm is hidden in there and plenty of more hp.
Verify the fuel and ignition first, then in any case, get a good double spring setup if you intend to go anywhere.
Or....as an afterthought, a good single beehive retrofit. Now I am not sure if that is that option is so easy.
I agree with all above and I would also check for any boost leaks.