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?

 

I expect it is the Webber’s, ideally swap to throttle bodies and get someone who knows their stuff to map it. If not get someone who knows Webber’s to set them up and check timing etc.

 

Webers should be ok at that rpm, especially 48s!!

 

What valve spring have you fitted It could well be valve float

 

Totally, but only if you can get them set up right. The ones I had on a pinto were terrible, in my experience Webber’s are flat spots intermingled with running rich.

 

Check you cam timing,iv found that if inlet cam is out on my Yb to advanced will choke the engine,worth a look in,Carbs or other would run lean or rich would thought