Motorcycle Repair: engine dies when tank is half empty, tank valve, honda shadow

Question
I have a 1986 Honda Shadow VT500.  I have no problems until I reach about the halfway mark in the fuel tank (around 80-90 miles).  After that, the bike will start normally, but it will die after five minutes or so.  However, if the bike is already hot (i.e. I am already riding it) I can ride it all the way past the 100 mile mark with no problems.  It is only if I START the bike cold after already crossing 80 miles or so.  Once I fill up the tank, everything works normally again.  Also, I don't seem to have this problem in hot weather, so I think the engine temp may have something to do with this.  I don't think I had this problem before, but in the last few months it has been pretty consistent, and it's driving me crazy.

Thanks very much,
Jon

Answer
Jon,

sounds a bit like a vapor lock in the tank
or fuel lines. There are a few other possibilities
though.
If it works okay with a full tank I suspect
a fuel problem like a weak fuel pump, partially
plugged fuel filter or lines or a faulty fuel tank valve or
petcock. The valve has a filter up inside the tank.
Maybe the lower part of it is plugged.
-Check for any fuel lines running too close to the
engine and getting too hot.
-make sure fuel tank vent is clear.