Running Hayabusa in a NASA Super Touring race car. Engine ran perfect last year after rebuild. The engine is running very lean and will not pull past 7800 rpm. We made no changes to engine for this season, except to use Lithium battery to save weight. Did a few easy laps in first 20 min session, ran OK. Did another full second 20 min session and ran OK. In third session ran harder and ran good for 15 min and then started to run lean and will not rev past 7800 rpm. I did change battery back, so far checked fuel system, we have adjustable pressure reg with gage and can see we have good fuel pressure to rail so would think could rule out fuel supply. We did experiment with raising fuel pressure the next week. Started cold and did a few laps and was able to pull red line. Took into pits and after heat sinking for about 15 minuets went out to verify if fixed and again will not pull past 7500 rpm so we called it a day. This week I am going to do a leak down test to make sure nothing damaged from lean running. But at a loss as where to start checking systems to determine the problem. Was hoping someone on this site may have had similar issue. Could use some guidance where to start diagnostics. Should add this is a gen 1 2004 bike. Was professionally tuned on dyno previously, we are not running stock map. Not using a power commander.