A refresher on the cooling system/overflow bottle operation:

The coolant reaches a certain temperature during engine operation. If the cooling system is operating correctly, the coolant will remain below the boiling point during engine running. When the engine is stopped, the coolant flow through the radiator stops. When the coolant stops flowing through the radiator its temperature rises. The rise in temperature causes the coolant to expand. If the cooling system is completely full, the expansion causes the radiator cap seal to open and a small amount of coolant to escape into the coolant overflow bottle.

As the cooling system cools the expanded coolant retracts. This retraction of coolant in the radiator (the level goes down) with its associated drop in pressure causes coolant from the overflow bottle to return to the radiator through a one-way check valve in the radiator cap.

--------------------

So, if your cooling system is working correctly (no leaks, proper pressure, all hoses good, system full, etc.) and you find the combination of a full overflow bottle and a low cooling system level, you must have a radiator cap issue.

Ron

 

Mine was doing that, and also pushing coolant out the overflow tank overflow tube. I'd find a fine mist of coolant under the rear fender. I purchased a higher rated radiator cap and it hasn't been a problem since.

 

Thanks a lot, Ron and the rest of you. You guys have been very helpful. I will study all possibilities. I will flush and drain the radiator myself this time and put in new coolant and test the bike. I read somewhere here, mistakes can be done during the draining process, there is one bolt below the water pump to drain the radiator but that only drains one side and the lower hose of the other side needs to be removed to drain the whole radiator. Is this right? The total coolant that goes in is 1.4 quart, is this right? I'll report back with the test results.

good point on the radiator cap possible problem, i was thinking of that yesterday as well. i do get this air sound coming out of the radiator each time i open the cap. the worse case scenario for this problem is the head gasket not sitting proper and i have to re-install it again

 

I'll lay my bike down on both sides to get as much fluid as possible out of the water pump drain. When filling, lay to the left to help fill and purge air.

 

Had to "lay her down", you say?

Ron

 

Yes, had to lay her down. And all from a standstill with the motor off. I need to work on my skills!


Question for you Ron...anything wrong with this method or suggestions for a better approach? It seems to work for me. Thanks.

 

That's a pretty good idea! I've always done it the old fashioned way - fill, run, check, etc. Maybe I'll try your way next time!

Ron

 

There's still a little burping after running and thermostat opening, as expected.

 

Thanks for your feedback, guys, i did the radiator flushing over the weekend and added new coolant, i also did the laying her down method then run the bike until it heats up. i rode the bike for about 100 miles so far and still had to top up coolant about 0.1 quart. i will do more riding and check the level, if there is no more burping to be done i believe the coolant level should stabilize, if it doesnt, then its definitely a leak somewhere.

just a thought, i feel the design of the radiator on this bike could cause this problem for those who intend to change coolant. from the exploded view of the bike cooling system, we can see the left and right radiator is connected via a hose at the top of each radiator. so after flushing and filling in new coolant from the right radiator where the cap is placed, the coolant will first enter the right side of the radiator and then the overflow will cause it to flow on the left side, hence the logic of laying her down. the previous bikes i had were just one single radiator and when you fill it up, there is little burping to be done.

 

I didn't want to start an entire new thread but have any of the 331/351 big bore guys on here re geared for better highway cruising? I want to do "minimalist touring" in the next few months and I'm debating if I want to BB my junk before I go.