I have an 87 F-250 with a carbed 460 that won't stay running. I just pulled the valve covers to reseal them and when everything got put back together, it won't stay running. I pulled the fuel screen on the carb and it's clean. The bypass that is immediately before the carb is clear. I checked the fuel pressure and it indicates 1.5 psi when cranking and drops to zero once the rig is running. I think it has one pump in each tank and it won't run on either tank. Any ideas? I need this thing running ASAP. Thanks.