RonH said:
I still find it rather hard to believe a cold engine gets flooded in a fuel injected engine? On an old carb engine you can pour gas straight down the carb and it will normally start right up.. So the fuel injection of this motorcycle sprays this much gas to foul the plugs where it won't fire up even cold?
The range of A/F ratios that will support combustion is narrower than you might think, somewhere in the range of 8:1 to 20:1. Get it outside these ranges and it won't fire. When an engine is cold though, it won't fire on a normal 14:1 as the fuel doesn't vaporize properly. Instead, you need something around 10:1. So the FI system has to get the A/F ratio into a pretty narrow range.
The problem, as I understand it, is that the system has no way to know during cranking that there is excess fuel. The ECU looks up in a table how much fuel to inject and then trims the number based on what the inlet air temp sensor says. If they miss high or there is some other glitch that doesn't allow the engine to immediately fire, then the problem can rapidly spiral downhill as the injectors keep adding more fuel which just adds to the accumulated/unfired fuel. And the plugs start to wet foul which makes things even worse. The situation spirals out of control and the more you crank with closed throttle, the worse it gets. Big 600cc pistons, a clanky decompression system, perhaps a marginal starter and battery..... it all might add up.
I think the reason WOT works (usually) is that they designed into the system a flooded-start protocol that mimics what we used to do with carb engines. A flooded engine needs no fuel and lots of air to purge the system of excess fuel and get the A/F ratio down to an acceptable number. On carb'ed engines, WOT got you the air, but it also tended to continue to get fuel.... eventually air usually won out. On the S10, Yamaha appears to open the butterflys and turn off the fuel. At least this is how most FI engines flooded protocol works.
The problem may be much more subtle than we think. I could see a scenario where some engineer poked a single erroneous number into an air temp compensation table that only bites in a certain narrow set of circumstances, but when it does, it bites hard.
- Mark
