I got some time on my Yamaha FZR600 today, not finished yet but it's running better than it was. It has an after market exhaust and K+N twin pods on it and it was running very , very lean. So lean that it would not rev over 3k rpm and would spit and backfire furiously - it was a proper fire hazard totally unrideable. I had put much bigger jets in - standard were 102, I went up to 127.5 as experience told me that was a good starting point. although it ran like that and would rev up to the red line when fully hot it was still a pig to start and had a massive mid range dead spot.
I concluded that the needles needed to be raised to enrichen the mixture between idle and full throttle - the needles were set third groove down from the top - there were 6 grooves on my needles. The needles have most effect at 14 - 3/4 throttle, although they have a small effect outside of that.
I went all the way and put them at their highest setting - my gut feel was that the K+N's were flowing way too much air and the exhaust wasn't helping matters.
turns out I have gone a wee bit too far - the bike revs really cleanly now but there is some black smoke at mid to high throttle suggesting that either I have gone 1 groove too high on the needles or the mains are a wee bit too bike or maybe a bit of both.
Tomorrow I shall drop the needles 1 groove and see where that takes me. This will make a good, cheap track bike for somebody.