Hi I am looking for some new ideas to help find the reason for a 250 crossflow that breaks down at around 4000 RPM. It has had a full rebuild. specs are , 10.2:1 comp , aussie speed 4 barrel intake , Holley carb fed by electric pump at 6psi , Camtech Cam with over 500 thou lift ( cant remember exact specs without looking it up} Blue Racer valve springs. Crane roller rockers. Exstensive head work, all ARP bolts ect ect . Ignition is MSD 6al and MSD coil , at present has TFI dizzy fitted locked at about 25 deg. Put this in to check if it made any difference, Had a Bosch electronic in it that was re-curved by Performance ignition. swaping them made no difference. swapped leads and coil also. Any ideas appreciated. would it be possible for valve springs to cause this at such low RPM

The one time I had an XE do that to me, it was lack of fuel pressure.

My old XF did a similar thing once also, was a loose fitting centre dizzy lead (the coil lead)

Could also be lack of fuel to the secondaries ?