Based on your details, my first guess would be an air leak---maybe not correct, but the "Surging" you described is indicative of a lean mixture & it did respond to more fuel---for what its worth, try this---with engine running steady RPM's somewhere above idle, spray "Automotive starting fluid", on intake manifold connections,high speed needle & back plate to engine connection (not in carb air intake), if there is an air leak, the starting fluid will be drawn in & the RPM's will change---higher nitro % will only make a bad situation worse as it advances ignition timing even more---YS's do like high nitro, even up to 30% when all else is right!---keep us posted