Tried bleeding the coolant system on my cbr400 NC23 earlier.

It's really weird. I removed the pressure cap, filled it with coolant and kicked her in. the coolant starts bubbling, excellent that means the air is being pushed out.

So all is fine, until that is it reaches just before the point where the fan would normally kick in. It just started spewing out coolant like anything, big gulps of it.

The coolant would sink down out of sight, then blip the throttle and all of a sudden a massive gulp of coolant is thrown out...


Ahhh, typing this now im pretty sure i know where i have gone wrong, i shouldn't blip the throttle, right? As that is obviously going to spin the water pump faster and it'll push the coolant out.
It was however doing it randomly at idle.......

I turned off the engine and filled up the coolant and it's taken a good litre.....

I'm yet to get it to the point of the fan kicking in.


The one thing i did notice however is that the hose at the top of the rad was much hotter than at the bottom...
I appreciate that the rad is there to cool the coolant but seeing as the bike isn't moving and the fan isn't on then surely both the top and bottom of the rad should be of around the same temperature.
So the rad could be partially blocked?????

I could be wrong but does the bottom hose only get hot when the thermostat opens allowing coolant from the engine into the radiator to cool down. Meaning the themostat hasnt opened so it hasnt got hot enough yet (normal operating/running temperature)

cbr400rrn wrote:I could be wrong but does the bottom hose only get hot when the thermostat opens allowing coolant from the engine into the radiator to cool down. Meaning the themostat hasnt opened so it hasnt got hot enough yet (normal operating/running temperature)

Yeah, you're right. Stupid me. Obviously the coolant isn't going to be allowed to flow through the rad as you want to heat up the coolant quicker to get it to operating temp quicker.


The thing that was screwing me up was everytime i went to bleed the system it just spewed out coolant.
After posting that message i thought "yeah obviously it will, it's pushing air out"

So i got a tin, let it spew into the tin, then put it back in the system. It took me about 10minutes to figure out that everytime it spews it out and i turn off the engine to re-fill the coolant it's obviously just trapping more air.

So i decided to bleed it via the expansion bottle.
Fill up the system, shove the pressure cap on and allow it to fill up the expansion bottle.

This works because as it pushes coolant into the bottle it's also pushing in air. So once the bottle is filling up i'd shut off the engine and the pressure in the coolant system would pull the coolant back through, the air has already risen to the top of the water (it was bubbling) and provided i kept the expansion bottle topped up as coolant was being pulled back into the system then no more air would be drawn in.

Worked a treat :D:D