A very pretty 2009 Suzuki V Strom 650 came into the shop last week. She looked showroom fresh, with less than 2k miles on her. Needless to say, she had been parked for years...and sadly, with a half full tank of gas...which, predictably, turned into a tank full of rust.
The owner had already cleaned the tank and replaced some or all of the fuel pump assembly, which lives inside the tank itself, but despite these ministrations, she would bog down under load. She idled like a champ, but just wouldn't pull when the rider cranked on the throttle. Very frustrating for the owner.
There are a number of esoteric possible causes for that behavior, but the most obvious thing to check is fuel pressure, which is a proxy for looking at fuel flow. This is a fuel injected bike, so fuel pressure is supposed to be pretty high, around 60 PSI. Fuel injectors just won't work right if the pressure gets too low. And if the fuel pump assembly cannot keep up with the fuel demand from the engine (as the engine control computer tries to get it to make more power), then the fuel pressure in the fuel supply line is going to drop.
Because spraying 60 PSI gas around your shop is a bad idea, selecting the right fuel pressure test rig is important. Fortunately, bike manufacturers have largely selected standardized fuel line fittings when they design their bikes. In this case one of the mid-size quick disconnect fittings was just the ticket and that allowed disconnecting the fuel line from the tank, plugging in the tester T-connector, reconnecting the fuel line to the Tee and then hooking up the pressure gauge to the open end of the Tee. The results can be seen at the links below...
Start and Blip Throttle and Advance to Higher RPM
What you see here is a significant drop in fuel pressure as engine RPM is increased...despite the fact that the bike is just sitting in the shop in neutral, not even spinning the back tire. So even with zero load on the engine, the fuel pressure drops...a lot. You might even notice in the second video that you can hear the RPM begin to drop (despite increasing throttle) when fuel pressure gets near 20 PSI. It is significant that the pressure drop is smooth. Pressure rolls off steadily as RPM increases. It does not, for example, spike low intermittently and then pop back, which might suggest an intermittent electrical connection to the (electric) fuel pump. Nope, this sure looks like plain old low flow. The fuel pump assembly just cannot keep up.
Now why say "fuel pump assembly" and not "fuel pump"? Because inside the tank there is not only a fuel screen / filter, there is also a pressure regulator, as well as the pump itself. Poor flow could be caused by problems with any one of them, not to mention the possibility that the fuel pump assembly components themselves may have been misassembled and are leaking fuel from the pressurized lines inside the tank into the low pressure area inside the tank itself. That would not produce a fuel leak that is visible outside the tank, but it could cause a pressure drop in the fuel line when fuel demand increases.
So, how did this all turn out? We don't know yet. Now that he has a solid answer on the fuel pressure question, he and his bike are back home awaiting the arrival of a new Suzuki OEM Fuel Pump assembly, which he will install after taking another good look inside that tank...to see if another cleanup is in order before installing the new pump. Stay tuned!