What could make a fuel-injected, computer-controlled-ignition engine behave as if it had burnt contact breaker points, dirty carb jets or a closed choke?
I own a 2001 F4i with over 100,000 miles, my daily rider. I have taken pretty good care of it, including no wheelies, gentle riding and regular maintenance including fresh full-synthetic oil every 3000 miles.
It used to run fine until it began blowing off steam through the radiator overflow hose upon turning the engine off. I had the radiator cap replaced since the spring had grown weak so it wouldn't build up enough pressure, and a couple hoses were replaced as well. After that, it began sputtering, misfiring and lost most of its power, specially above 6,000 RPM and when hot. I had a surplus fuel pump so it was replaced and that helped a lot, but it was not enough. We spent a full day checking every wire for continuity and found many gremlins in the sensor sub-harness which were taken care of; the injectors were ultrasonically cleaned and tested OK. Spark plugs are Iridium units, replaced about 3,000 miles ago. Coils were replaced about 10,000 miles ago. All vacuum hoses connected to the throttle body were replaced.
However, the engine still behaves almost the same. It runs somewhat better when cold, and runs smoothly enough below 6,000 RPM but above that it just won't run properly, and the throttle works only halfway: More than half throttle will make the engine groan and lose most of its power. Top speed is about 100 mph on level roads. It also runs better at lower altitudes. At 10,000 ft above sea level it runs really bad. It detonates and backfires, specially when engine braking.
Any ideas? Thanks for helping!

 

Lots of things could cause your bike to perform badly. First check would be the battery to make sure it is getting a good charge and that the cells have enough electrolyte in them, that's if your battery isn't a gel one! Check across the battery terminals with a multimeter and the engine running and see if the voltage rises to 13.5/14.5 volts around 3000/5000 revs. A faulty rectifier will not allow a full charge to the battery if you don't see a rise in voltage so should be replaced. If the battery is ok, check for air leaks in the intake and exhaust systems. The coolant temperature sensor can cause the ECU to ask for more fuel than is required if the sensor is faulty. Usually this appears as white smoke from the silencer. A faulty ignition unit and/or ignition timing could be the cause too. Hope I've given you some ideas to work with to try solving your problem? Check camshaft/valve clearances to make sure they are not too small, which will affect the time valves open and close and hence give poor running.

 

it almost sounds like a blown head gasket.

also check the throttle sensor. they can get flat spots that make the computer not know how far the throttle is open.

how is the fuel mileage? low mileage can indicate that it is running rich. that would point to the pressure regulator, coolant temp sensor, oxygen sensor (if your model has one, california models do) or map sensor.

any smoke from the exhaust? blue is oil. check compression. 100k is a lot on a motor.
white is coolant. check the head gasket.
black or grey is fuel. check those things that make it run rich.

how worn are the plugs? pull them and look closely at them. the will often tell the story of what is going on. they may just be worn out.

hope this helps. good luck.