Time for an update, albeit not a very good one I believe. As a reminder for new readers, initial symptoms were that the bike suddenly started heating up more than it used, noticeably, which lead so some piddling on the pavements on warm sunny town rides. I tested the thermostat, even removed it at one point, all OK. Changed the waterpump, no luck. Cleansed the radiator, flushed the cooling system, to no avail (no muck found anywhere, nothing seems clogged)
Since then, and very recently i did the following :
- driven a whole 15000 miles, no other symptoms (but always have been weary about hot summer days, avoiding towns as best as possible. However, fan works fine, and stops the needle from going into the red and stabilises effectively the heat roughly half way up.
Recent maintenance, 500 miles ago, as follows :
- Valve clearance : was slightly off, now perfect
- Oil changed : no water in it
- Coolant changed (many times) : no signs of oil or mess in it
- Filters all changed
- new sparkies
Unfortunately, noticed the bike was piddling again the other day, after only 15miles, in the cold, no town driving. Not at all the usual behaviour. I checked yesterday, as the engine warms up, pressure builds in the expansion tank, the level is way beyond max now (wasn't before - it was spot on), and it piddles. With the cap off one can see big air bubbles generating in the expansion tank and popping. It's not the usual piddling, after topping off with too much coolant, it's not continuous. So now the bike can't be used in my opinion - air bubbles will form in the cooling system and I risk overheating it now.
Spoke to a very helpgul forum member about this, and we both seem to have a good idea of what this may be. But I'll ask an open question anyhow, maybe we are forgetting something : what could cause air bubbles to appear in the coolant expansion tank?
I'm in the process of getting a new radiator cap, cheap second hand (but should be fonctional), to continue ruling things out. But now the time has come to find a real fix I do believe!