A suggested long shot! We had to drill out the breather hole in the fuel cap on our N7 to allow more air flow into the fuel tank as the breather hole in the cap often got blocked by dust and material. We then always covered the fuel cap with a couple of thicknesses of rag, held on with an O ring to keep the breather hole from blocking up. The same problem applied to our l2's in the 1970's. The problem will only show up under high load and high fuel consumption situations. Partial blockages in the fuel lines, usually at a tap or inside the tank also caused us problems. The R series have a separate fuel tank breather. Cheers.