I have a 1985 Kaw Eliminator, it seems as though the pressure from the bike cause the overflow tank to run over from the radiator cap. what can cause this problem.

So the overflow tank would be pressurized with a radiator cap. This would no doubt be higher than top of radiator and engine .Keep filling it without engine running and heater on hot, then start up, when engine gets hot and thermostat opens, water should leave overflow and you need to top up as required.

if you say there is steam coming from the "coolant only" overflow reservoir, then the first thing I'd inspect is the integrity of this overflow coolant tank. if you believe the cap is in working condition, take a careful look at the tank itself. so as to rule out any cracks or deformities which may compromise this parts ability to build and retain the necessary pressure for the cooling system to operate in the manner for which it was designed. hope this helps!

Sounds like you may have trapped air in the water system if so needs bled....or the water is going into your overflow tank or back into the radiator...That should happen only when the engine heat is a little higher then normal causing the radiator cap to open and allowing the pressure to push the expanded water into the overflow tank..if you are not experiencing over heating problem then...My bet is your radiator cap is getting weak..have it checked and changed...if necessary.good luck...

pressure causes the coolant to go into the overflow, as the engine cools it siphons back to the radiator. if the tank keeps running low you must have a leak some where

are you confused?
does the operators manual show 2 tanks.???marked AF, antifreeze?
is one, washer fluid.?
it look it up in the FSM but no engine stated.
http://www.amazon.com/2002-2007-Radiator-Overflow-Reservoir-Mopar/dp/B00E8PPE4A

i see one tank ,called BOTTLE here,
my guess is you lost the operators guide and are lost.
http://www.factorychryslerparts.com/showAssembly.aspx?ukey_assembly=612242&ukey_make=1064&ukey_model=15531&modelYear=2004&ukey_driveLine=7453&ukey_trimLevel=18498&searchString=tank

have a compression test done as that will most likely be a blown head gasket or cracked head allowing combustion pressure into the cooling system. A quick check is to remove the radiator cap and fill right to the top . Run the engine and if you see small bubbles in the water or it tends to belch out then it will be the gasket of cracked.

If the thermostat is not opening, the engine will keep getting hotter as you drive. You could be low on coolant and have air pockets in the system which would cause the temp gauge to not be accurate. Is the radiator fan coming on when the tank is boiling ?

This sounds normal. The coolant and any trapped air bubbles in the system will expand as the engine warms up, and cause this to happen.
You ought to be very very careful about opening the radiator cap on a hot, or even warm, engine, as you can get a face-full of scalding hot coolant.
The normal place to add coolant if needed is to the radiator overflow tank, normally a plastic tank off to one side, connected to the radiator neck by a smaller hose. This will have 2 levels marked on it, Hot and Cold.
The radiator cap is not routinely removed, and on some cars is even locked on by an additional clip.

Your overflow tank works on suction to return coolant to the radiator. Check the line and nipple ends for any cracking (cracking on the plastic radiator nippl is very commom). The radiator nipple is not under high prrssure like the fluid under the cap (inside the coolant system). It's on the otherside of the cap and will never "stream or gush" coolant (uless you are over heating badly). As the coolant in the radiator cools it creates a vacuum and draws coolant back from the overflow throgh a small hose. You might not see any leaks as coolant heats up and travels to the overflow. It might be a very small leak and you are usaully driving around and drying up before you see it. Even with a very small crack it will only draw in air and not liquid (as liquid is way heavier than air). Also, I've seen where people have taken out (or fallen out) the tube attached to the overflow cap that goes down deep into the overflow tank. That tube should be long enough to almost reach the bottom of the tank. It's hard to know if something is missing if you do not know it was suppose to be there in th first place but it needs to be there.

Until you get it figured out use regular water. Replace thermostat and radiator/coolant tank caps. Thermostat regulates pressure in the motor and allows radiator to cool. Caps regulate atmosperic pressure. Either or both being bad will cause you system to siphon and fail.