Not much to go on but it's obviously going lean. My first suspicion is in the fuel tank area especially since this is a new install that has never worked correctly. Since you have never had a long run I suspect all of your observations were done with a near full tank. My thoughts lean toward you having the engine feed line hooked up to the vent tube. The vent tube may be "in" fuel when level but "out" of fuel when tipped back.