I have never seen an engine get lean when theday gets hot. Normally if anything I have to lean it a bit. I bet he just isn't richening it enough in the first place. Tank position will not change how lean or rich the needle position is. It is only an issue when doing aerobatics. That is a tank that is too high will go lean when inverted and visa versa for a tank too low. But how lean or rich after adjusting the needle only depands on how you set the needle.