Originally Posted by rstoughy

Have you checked it's timed it up properly?
It's very easy to time it up a tooth out.
On a Cosworth time the bottom pulley to the lug NOT the V.

Stick a ratchet / socket on, turn it over by hand to the marks and check.

I have seen ones a tooth out clipping the valves / pistons when running.
Got to be worth a check before going any further.

Hope it's an easy fix.