1990 F-600 429 Lima engine
My brother in law has a Ford dump truck with a 429 and he is having some problems that neither my dad or I have been able to figure out. Sometimes the engine will run very well and run to ~3500-4000 rpm under load and others it won't go above 1000 rpm in gear or in park without backfiring and cutting out. One day it was running so poorly I had to hook it up to my truck and tow him home. It was fully loaded and my measly gas 6.0 and clutch didn't like that very much. Anyway, we started with checking that the timing was set to the factory spec of 10 degrees BTDC and varied it as far as the distributor would go in either direction. We cleaned the carburetor, fuel lines and fuel tank and changed filters because the tank had a lot of rust. We started it and it ran fine with good power while driving it up and down the road but the next day my dad called me and said it was running poorly again. It would sit at idle for several minutes before the idle would dip down momentarily before returning to its normal idle rpm. After that we installed a temporary fuel pressure gauge and have been observing the fuel pressure and it remains a constant 6 psi regardless of how the engine runs. We even spliced in a MSD coil I had laying around without any change. Today my dad disconnected the entire mess of vacuum lines attached to the engine to see if that would change anything but it didn't.
At this point we know it is getting fuel and air and think it has to be ignition related but can't figure out what. Sometimes we can start it up and it runs just fine but others it starts backfiring and stumbling as soon as you step on the gas. Is there anything else we need to look at?