I just bought a 2004 vulcan classic 800 how do i clean out the fuel line and gas tank

More than likely you have a bad fuel petcock. No fuel should go from tank to carb without the engine running because the fuel is pulled by vacuum. Remove line from petcock an have it set to on or reserve position and if fuel comes out it is bad, and let's the fuel continue to flow to the carb and seep into the engine oil also. Mine was bad and had my oil thin as water with all the gas in it. Wonder it didn't wash a bearing out of the engine. Check and drain oil immediately if the petcock is bad.

The fuel on/ reserve adjustment (fuel petcock) is on the bottom left side of the tank opposite the key switch and near the choke. The fuel line extends from that and will drain the tank if you remove it. You will likely want to remove the tank if you plan to drain a lot of fuel. That will make it a little less messy

Didn't know they made a Vulcan 400.

BUT, if you meant a Vulcan 500, maybe this'll help:

http://www.bikebandit.com/houseofmotorcycles/1992-kawasaki-vulcan-500-en500a-fuel-evaporative-system/o/m150088sch55170

Yes, its the needle and seat (in the bottom of the carbs) that controls the flow of gas into the carburetor, when they get ANY dirt (of any kind) between the needle and the seat..this does not allow the flow to shut off completely. This fills the float bowl until it over flows out the needle jet/pilot jet and it runs into the cylinders and back into the air box/air filter.
Your fuel system needs cleaning (NOT WITH ANY ADDITIVE) - actual disassembly and cleaning..INCLUDING the carburetors and add a fuel filter to the fuel line between the tank and the carbs..

You will need to remove and clean the carb. When you run one out of fuel it allows all the trash in the tank to go into the carb. The fuel leaking from the overflow is trash in the float needle and needs to be cleaned.

Disconnect the gas line from the petcock (should be in ON or RES position). Remove the seat. Remove the bottom bolt of the dash and lift it up, then disconnect the two wiring harnesses and the speedo cable. Remove the bolt under the dash on the frame mount and the bolt at the rear of the tank. Lift the tank, and you can disconnect the one or two (CA models) vent lines. Place the tank so that the weight is not resting on the petcock.

There can be especially if you have a California model, I would suggest hooking them both up and then run the lines to there proper place, not sure which model you have but one is likely the drain for the gas cap area, if you don't hook that up you'll have gas dripping on your engine when its hot, probably not a good idea !

it can...please check clutch lining also.
sliding clutch can cause hi rev but minimal speed

Make sue\re to add fresh gasoline to the fuel tank and make sure it is entering thr carb.

I'm pretty sure it does. I own a 1995 Vulcan and I bought a 2003 tank and looked at the mounting brackets and fuel connections all looked to be the same - but don't put this in stone. I haven't been able to put it on yet because it's still out being painted. There should be no difference in the fuel tank mounts from 1996-1005, but as always, check with a professional if this information is critical.