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yeah, the overflow bottle is just that,, for overflow and re-filling the radiatior during the ride. It is not an indication of fluid level in the radiator.

For the OP, since you are over-heating while in motion, I'd suggest that either you didn't put enough radiator fluid in during your stop.
But,... saying that.. you said that when you threw water on the radiator, behind the front tire, that steam came up. So, your radiator was that hot? If your water pump was broken then I wouldn't expect the radiator to get blistering hot. I'd expect that from the engine if the water pump was not pushing water through it.

I can't imagine a point where everything is working perfectly but yet the engine gets over-5 bars hot. If there was a constriction in the engine, then the radiator wouldn't get hot, but engine would.

even with no flow of water, the radiator is designed to cool down the fluid that is inside it.

Question for OP, when you filled the overflow with extra fluid you bought, and you continued to ride it, and then had to keep stopping, did you re-check the bottle? Was the bottle still full from filling it up? Or was it now dry?
 

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I have forgotten the thread where I got advice on my heating system from y'all but it fits in here also.

I drained my rad.
Flushed it once with distilled water; some grit came out.
A second rad full of distilled water is now in the bike.
I'm getting three bars and the fan comes on in slow traffic - but no fourth bar yet.

Major Question: My understanding is that you are supposed to check the radiator fluid level by looking at the marks of the overflow bottle. I had syringed-out 90 % of the anti-freeze in this bottle BUT none of the water from the rad has made its way into this bottle. So the rad APPEARS empty but is not. So what/how is the connection between the bottle and the rad?

Why do we look at the overflow bottle to tell how full the rad is.

If i look at the bottle now it indicates that my rad is almost dry but I know it's full.

Anybody? :serious
So Dave_j and captainfish explained it well. The overflow tank (or reserve tank) or expansion tank as it used to be called is there for the purpose of collecting expanding water from the radiator and then returning it to the radiator when the radiator cools down. The radiator cap does a good job of this, it bleeds a little pressure and fluid from the top of the radiator when the radiator gets hot, and the heat causes the fluid to expand. So instead of simply wasting the fluid by letting it fall on the ground, the fluid simply collects in a non pressurized tank. The when the radiator cools down, a vacuum is formed in the radiator and the fluid is sucked back into the radiator. THE TANK WAS NEVER INTENDED TO SHOW HOW MUCH FLUID IS IN THE RADIATOR. There are some systems that use a pressurized overflow tank that hold the full pressure of the radiator system. These expansion tanks have the radiator cap mounted directly on the expansion tank.

So for the OP, when you need to add water or coolant to the cooling system, you need to put it in the top of the radiator. First let the system cool down, then remove the radiator cap from the top of the radiator, add the coolant, replace the radiator cap, fill the reservoir tank, and purge the system (burp) of any air by leaning the scooter slowly from left to right, and refilling as necessary at the top of the radiator by removing the cap, filling, starting and stopping the engine, and finally replacing the radiator cap.
Myself, I find the radiator cap difficult to reach. It would have been much better if they could have put a long detachable neck on the top of the radiator that went up to just under the front panel, and placed the radiator fill cap there (similar to the 400). It would have been easier to reach, and a door opening could have been placed in the front panel, making it easily accessible.
Cheers!
 

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Under normal circumstances you maintain the coolant level from the reservoir tank and never at the radiator cap. The cap has two valves in it. One opens above the stated pressure on the cap directing coolant or air into the reservoir (coolant expands when hot). The other opens when the coolant cools down and contracts drawing coolant in from the reservoir.

Factory Service Manual said:
ENGINE COOLANT LEVEL CHECK • Keep the motorcycle upright with the center stand. • Check the engine coolant level by observing the full and lower lines on the engine coolant reserve tank. A Full line B Lower line • If the level is below the lower line, add engine coolant to the full line from the engine coolant reserve tank filler. To remove the filler cap, remove the maintenance lid.
If you have a coolant leak or some other problem (or you let the reservoir get low and air sucked into the radiator) then you can end up with air in the cooling system. Once the leak or problem is fixed you will then take off the radiator cap and fill from there, bleed, put the cap back on and fill the reservoir to the line. There is a small hose off the neck where the radiator cap is that goes to the bottom of the coolant reservoir. If this hose is cut or pinched the system won't work as its supposed to just as if you have a coolant leak or your radiator cap isn't holding proper pressure.

Factory Service Manual said:
AIR BLEEDING THE ENGINE COOLANT CIRCUIT • Add engine coolant up to the radiator inlet. • Support the motorcycle upright with the center stand. • Slowly swing the motorcycle, right and left, to bleed the air trapped. • Add engine coolant up to the radiator inlet. • Start the engine and bleed air from the radiator inlet completely. • Add engine coolant up to the radiator inlet. • Repeat the above procedure until bleed no air from the radiator inlet. • Close the radiator cap 1 securely. • After warming up and cooling down the engine several times, add the engine coolant up to the full level of the reserve tank.
 

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I was wondering about the bleeding or burping procedure for the coolant as I have filled my radiator earlier today, but have not as of yet, run the bike. What I've found in my forum search is the procedure is defined by using the same words (bleeding or burping) over and over again, but if one doesn't know what that means, it's all just gibberish. I might be getting old here, but can someone explain to me exactly in step by step, how the system is purged of air? I get that the bike must be rocked side-to-side, but is that with the radiator cap off or on? Then from there I refill, right? Then what--rock the bike again? Now when it says (in the last post above quoting from the manual) to "Start the engine and bleed air from the radiator inlet completely" is that where I put the cap back on and now rock the bike with the ignition running and remove the cap before the bike gets too hot to where I can't safely remove it without a high pressure scalding?
Somebody help this stunad please!
PS, Ledude's Burgman Center mentions 'burping' but doesn't expound on the procedure either.
 
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