ORIGINAL: GaryBarry
I have had this checked by MANY experts and they have all come to the conclusion that it is not the tank and lines, so please don't go down that route,
There really isn't anything else that will cause it to go lean if the fuel draw at the spraybar is good.
I have about a dozen .40fp and clones, magnum, tower, etc. I use them in funfly planes that gets tossed around plenty, never had a problem with the engines going lean.
Keep your tank positioned very close to the engine, is all I can offer.