Purchased a new EMPI Weber knock-off from SOR, installed over the weekend with a new Carter fuel pump onto a 69' F145. Idles just great, starts on the first try...BUT...when you give it gas under load it boggs down to the point of not being drivable. Truck lives at 4000ft plus main jet is a 140 and secondary main is a 150. New plugs, new fuel pump, played with the timing with no real changes. Keep thinking the altitude and jets may be the problem. Any guidance is appreciated.