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Discussion Starter · #21 ·
cornerslider is on it: The design and layout of the intake ports and the way the injectors are aimed leads to a lot of "wall-wetting" in the intake ports. Shut the engine down, then that all evaporates and fills the intake runners with fuel vapor. A restart right away normally isn't a problem, because it hasn't had time to all evaporate. A restart after a few minutes (heat-soaking the intake system) leads to the intake charge being too rich to fire for a few seconds of cranking (meanwhile the injector is dutifully adding more fuel as you crank). If it's been sitting long enough to be cold, it's okay, due to a combination of the vapors dissipating and mixing with air in the airbox, plus the engine being more okay with a rich mixture upon an actual cold start, plus possibly the idle-air-control-valve opening slightly in preparation for kicking up the cold idle speed.

Cracking open the throttle a little upon warm start encourages more air to get into the cylinder, to dissipate the excess fuel.

The bit about whacking the throttle wide open when shutting down the engine in anticipation of a subsequent warm start, is to encourage the last few rotations of the engine to happen without fuel being injected, to hopefully get some of that fuel hanging around in the intake ports, through the engine and out.

The wall-wetting is a pain to tune around, too. A certain setting of a PowerCommander, for example, at a certain RPM and throttle position leads to different air/fuel ratio depending on what the engine was doing leading up to that condition. It's not bad at full throttle, where most of the fuel that the injector squirts in goes into the cylinder right away, but it's a nuisance at idle and near-shut-throttle. The stock ECU has wall-wetting compensation (they all do, nowadays - emissions) but it seems like there is only so much it can do.
Had a chance to test opening the throttle while shutting down and it worked. It fired right back up after 15 minutes of resting.
 

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Had a chance to test opening the throttle while shutting down and it worked. It fired right back up after 15 minutes of resting.
If that worked, I'd just go with that.... I honestly don't know if the R3 has the technology to sense higher elevations? Either way, I'd go with what "works" :cool:-
 

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It does have a manifold pressure sensor, but I'm not sure how the underlying logic works. It seems that a lot of the smaller bikes only have a single manifold pressure sensor downstream of the throttle, and they somehow infer barometric pressure from what that sensor says under certain conditions (e.g. before you start the engine, that sensor ought to be recording barometric pressure, because there can't be any pressure difference across the throttle).

Without drive-by-wire, there's also only so much the system can do. It could very well know there's a warm-start happening, but not be able to do anything about it, because it can't command the throttles to open (hence, you might have to do it yourself).

I've had the airbox cover off my ZX10R many times, and it pre-positions the drive-by-wire throttles before starting the moment you turn the key on, and then does what it needs to do afterward to achieve the correct idling speed once it starts. Without drive-by-wire, can't do that!
 
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