2006 Ninja 250. 19,000 miles I was riding down the interstate, doing a steady 70 mph, no problems at all for the first 40 miles. Then I notice that the engine very slowly seems to be losing power. It's weird, but soon I've got the throttle pinned and I'm down to 3rd gear just struggling to maintain 55 mph trying to get to the next exit. I notice at this point the engine is getting really hot, too--much more than it should be for highway riding. I pull off the interstate and into the first gas station and pull the side fairings. First thing I notice is that the coolant reservoir under the seat is bubbling vigorously. It's also a funny brown color. I'm almost positive it was green or blue last time I checked it. Once the engine cools down a bit, I pop off the radiator cap. Engine is filled with coolant, although it's brown in there as well. Very strange. I put it all back together and start it up. Everything back to normal. A few minutes later, it starts getting hot again, and starts losing power. I pull off the next exit, and it stalls out on the ramp. I push it to the nearest parking lot. It will not start again. The starter relay clicks on, but the starter doesn't move. The battery is fine, lights and horn work, but the rear wheel will absolutely not budge in gear, even dropping the clutch down a steep hill as in bumpstarting. I keep trying, but only succeed in wearing down the battery. I wait about two hours while I eat something at a gas station, then try again. Now the engine will turn over, but the battery seems to be rather dead, no doubt from all the previous efforts. Bumpstarting will get it to turn over, but it doesn't stay catch, so finally a get a jumpstart from a passerby, and get it running--but it's a heck of a lot louder than before. I try to head home like this, but within a mile the sound gets worse and worse, to the point where it sounds like I'm shooting a machine gun into gravel. I pull to the side of the road and give up and a friend picks me up. A week later I got a chance to look at the bike. I pull off the clutch cover to take a look inside, and make sure I didn't do something silly like leave my watch in the engine when I'd last changed the clutch plates. Looks pretty good inside, though the oil is slightly murky. Started up again, the bike is still loud as hell when idling. It sounds like I took one of the mufflers off. I checked the exhaust for holes or leaks, but found none. The exhaust fumes from either cylinder (it's a twin) seemed to both be about the same level, and about the same temperature, though I didn't run it long at all. The noise is a very sharp, percussive sound, a bit like a small firecracker. It doesn't sound metallic, though. It happens in beat with the engine note. It seems to come from inside the engine. I had to leave before I could take off the tank and look at the top end. So, what's wrong? I'm guessing it's not something rattling around inside the engine. I'm guessing it's not piston slap or engine knock. I couldn't find any leaks on the headers, and it sounds like it's coming more from inside. Maybe a bad exhaust valve? Would that explain the heat, the noise, and the loss of power? Any other suggestions? When I pull off the top of the engine, for what am I looking? Last time I had a problem like this, it was simple to diagnose: the engine didn't run, the cam chain was snapped, and there were shards of valve rockers all throughout the top and bottom end. This time, I haven't yet found anything out of the ordinary (though I still haven't looked at the top side).