Motorcycle Repair: fuel, fuel vapors, full tank

Question
1986 vt1100 act like it is trying to run out of fuel at 45 on up;i have fuel flo;change carb;change spark control/change coil/wires/and plugs/ some time it will run smooth and it start doing it again;been runing like this over a year. Thanks Bobby

Answer
I've had three questions on this same symptom in the last four weeks.  The engine isn't getting enough fuel for some reason.  I'm not familiar with bike past 1982 but my guess is that fuel tank vent isn't working right.  There's a vent inside the fuel tank that runs down to canister filter that traps the fuel vapors. If you can run with the fuel cap open, it will eliminate that as the cause pretty quickly.  Just don't do it with a full tank of gas or you have a crotch full of fuel and that's very unpleasant.  It could also catch fire and that wouldn't be pleasant either.  Try it and let me know what you find.  The fuel pump could also not be delivering enough fuel to the carbs for some reason. Intermittent problems are tough to find, but I'm betting it's a fuel delivery problem.  When it acts up next time, pull the choke on.  If it gets momentarily better, it's a fuel problem.  

Regards,
Rich