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I have been working with one of my motorbikes this year that started to show a similar problem for no particular reason. Bogging just off idle and a massive surging on/off throttle effect on a constant throttle at 3 - 3.5 k rpm in long corners - not at what you need with a 1300 cc 145 hp four cylinder bike with massive torque! The FI system is a lot simpler than a car- throttle position sender operated from cable operated throttle bodies, manifold pressure sensor, coolant and inlet air temperature senders, fuel pump in the tank supplying fuel rail to four injectors in four throttle bodies, plus an oxygen sensor before the very small cat, and the ecu of course.
Before getting too involved I pumped the fuel tank dry and refilled with fresh UL98 - which unfortunately has 5% ethanol in it.
Not a mechanical problem - only 12k miles on it, no noises, compression fine, doesn't burn any oil, standard exhaust and air filter.
A very similar engine architecture to a car engine, all alloy Yamaha design, 16 valve, chain cam etc.
I checked everything, earths, all the senders, live outputs, resistances. Fuel rail pressure, Throttle body sync done recently but checked again, replaced plugs, coils and coil leads and connectors checked, coil primary and HT plug voltages and currents checked with an Ignitionmate. Nothing deviated from the makers figures in the manual hot or cold. Pulled the oxygen sensor, to read the colour, nice brown/tan colour but with a red blotch on the end. Looked up what the red blotch can mean - Possible metallic contamination. No fault codes recorded, this system is pre- OBD and displays any fault codes in dash diagnostic mode. Test rode it with the O2 sensor unplugged, got rid of the surging but still bogs down off idle. Now, do I drop £200 on a new O2 sensor and risk that's the problem? So for a similar cost I fitted a Dynojet fuel controller (not an advert Avant!) which is loaded with their map to improve driveability on this model year bike, also gets rid of the ratty over-weak idle mixture and gives a limited amount of manual mixture adjustment in the low middle and high rpm ranges.It just plugs in on the injector loom and runs with the O2 sensor disconnected. Result? Near perfect, had to adjust it to give it a bit more fuel at the idle range, now it's like a new bike.
No problem over here with emissions on a bike - there is no mot for bikes in France.
Why? The only conclusion I can come to is some sort of fuel contamination but what from is a mystery. Ethanol? Don't know, we can't get ethanol free petrol over here in France so I can't run a check on that.
Edited by focussed on 24/11/2019 at 22:35
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