my yb na wont rev freely over 6,000 rpm.
specs of the engine;
1. cosworth 9 bolt crank, cosworth turbo rods, acl duraglide main and con bearings
2. accralite forged pistons 12:1. cut to comp ratio of 10.5 : 1 for local fuel
3. twin weber 48's. ported to some extent. 42 venturi, 145 main, f16 tubes 210 correctors
4. newman cams 300/440, solid lifters

the engine pulls well from 3,500 rpm to 6,000 but then tails out quickly after 6,200. i expected the engine to rev up to 7,500 easily, but it just wont go.

any ideas why this is happening?