Went over to help a friend with his motorcycle Friday. Some (long) time ago I use to do this for a living and he had an odd problem. Seems he had replaced the ECU with a "high performance" one, and shortly after that the engine began refusing to run longer than to get through 3rd gear. Sounded to me like a fuel starvation problem to me.
Start with the gas tank outlet which has evolved to a Q.D. fitting with a check valve. Remove fuel line and push check valve seat open. Fuel drips out one drop at a time. Not Good.
The Mechanical manual for the bike shows the valve as having a short filter attached to the inside, just like the Bad Old Days. Remove check valve and discover that the picture was of a short section of a long hose connected to something way up inside the tank. Remove hose from valve and find the valve works just fine. Blow air up the hose and find it doesn't.
Check the book and discover that inside the gas tank are enough doo dads to assemble a Kholer Magic Flush Eco-Dyne toilet. To get at this, one first removes the seat, then the trim running up the middle of the tank, then the retainer ring using Harley-Davidson special tool # XXXYZZZ.VV. Alternatively a ball pien hammer and a big flat nosed punch will do fine. Some things never change. Wiggle and jiggle the assembly and it will slowly come out. Visualize a mammal giving birth to a starfish.
Disconnect the other end of one of two hoses, move the assembly far enough out to work with, remove a large clip which frees a housing, and inside the housing is a fuel filter. I swear only trans-dimensional engineering would have made this thing harder to get to. Yep, the filter was good and properly clogged. It looks like a Fram unit, $5 at O'Reily Auto or $25 at Harley Davidson. Plus labor. Fuel filters are routine maintenance replacement items. Replacing one should not be up there with open heart surgery.
Reassembly is the reverse of the assembly. My friend just had spinal fusion surgery and won't be needing the bike until July at the earliest so I can wait until another warmish weekend to reassemble the thing.