I have a 2009 YZ450F with 38.3 hours on the current top end. Runs like a raped ape and I really like the bike. I recently have had an issue and I can't figure it out, so I'm coming here for some advice/help.
I'm losing coolant in the bike. 2 Trips ago to the track, I was checking my fluids prior to going and noticed I was low on coolant. I was about 2 cups low - which I know is a lot. I get to the track and notice a fluid leak in the bed of my truck. I check the engine over and find I'm leaking coolant out of the coolant port right next to the exhaust. Of note, the coolant looked "dirty" - not necessarily oily, but it looked darker than normal (blacker than normal). I run engine ice in the bike so a darker blue as opposed to the normal color. I take it apart and find a bad gasket. I replace the gasket and the leak stops. Topped off the coolant and go ride. I put 1 hour of ride time on the bike. I get home and notice I'm low again on coolant fluid. I top it off. I then run the bike up and down the street for 5 minutes (don't want to bother the neighbors too bad). I check the coolant - no coolant loss.
Then yesterday I go to the track again. Thinking I'm all good, but a little concerned I go for my warm up. I ride and put 0.2 hours on the bike - a warm up and a few moderately hard laps. I come in, let the bike a cool for 5-10 minutes and check the coolant. I'm low again. So I top it off once again. Note that I did see blue coolant leaking from the bottom aftermarket skid plate (there is a sponge between the engine and the skid plate and I suspect I'm leaking from the weep hole and filling up the sponge which then shows a leak from the bottom of the bike). I did not notice a leak from the weep hole. Again, the coolant looks darker than what it was when I put it in.
I kind of suspect 2 things - bad seals in the water pump or a slightly blown head gasket. So to test the head gasket theory, I make sure the coolant is filled to the top of the fins and start the bike. I run it for 45 seconds and check to see if there is pressure building in the radiator. No pressure build up that I can tell - cap came off easily without any sucking force. I then start the bike with the radiator cap off. Water seems to be circulating like normal - no bubbles. When I hit the throttle, the water levels does rise in the radiator like one would expect as it gets more flow with higher RPMs. But water doesn't shoot out of the radiator - it doesn't even overflow out of the radiator when I hit the throttle about 1/3-1/2 open on the blip. So I'm kind of leaning toward it not being a blown head gasket. Does this sound about right? My question is what would be causing the coolant to get dirty/blacker?
So now I have water in the radiator because I don't want to keep wasting Engine ice at the tune of $24 a bottle to find the loss. I've flushed the radiator now a few times and the water is clear with little black spots on it - can't tell if it is small rubber chunks or what. I changed the oil and there is no coolant getting into the oil.
I'm kind of leaning toward a leaking weep hole of coolant when the bike is running really hard. Would it leak coolant more when hot versus cold (I would think it would leak more cold, but I may be wrong). This would mean the seals in the water pump may be bad or I need to replace the shaft, bearings, seals, etc.
Am I on the right track? Thoughts on the tiny black chunks in the coolant fluid?
Thanks in advance for the advice/help.