losing manifold pressure at altitude

[johnny]

I have a 1976 Riley P337 that I keep losing manifold pressure on the rear engine as I climb. we have swaped controllers, installed a new wastegate and the turbo seems to be within tolerance. I can only get 28" at 8500 feet while the front engine will go to red line. We checked as much as possible for any inductions air leaks. Does anybody have any suggestion - does anyone know anyone that can check the turbo before I purchase a new one?