When the tank is nearly empty, there is a large amount of air in the tank. Air is easily compressible, so the fuel pump doesn't have an overly difficult time pumping gas out and creating a slight vacuum in the tank as a result. The less fuel in the tank (i.e. the more air) the easier it is to pump fuel out, decompressing the remaining air (relative to outside atmospheric pressure).
However, you go to the gas station and fill up. Now there is very little air left in the tank. The pump tries to pump gas out of the tank, but there is almost no air volume in the tank to expand, and fuel sure doesn't expand under pressure. The end result is that the volume of fuel the pump is able to pump out is severely restricted. The engine starts running lean, so it bogs down.
You switch the choke on, which enrichens the mixture, so this helps, but eventually there just isn't enough fuel to run the engine properly.
If you let the bike sit for a while, eventually enough air leaks through the gas cap, and the bike will run again - until more gas is pumped out, and the problem occurs again.
This is easy to diagnose: Next time this happens, loosen the gas cap. If you get an inrush of air, and the bike suddenly comes to life, then you know that this is what is going on.
Sometimes this is a combination of several issues: clogged gas cap vent, clogged fuel filter and/or weakening/failing fuel pump. One on its own might not be enough to produce symptoms, but two or more at the same time can cause the bike to bog down or quit altogether. You might clean or replace the gas cap, only to find that a few months down the road, the fuel pump dies completely.
If removing your gas cap does bring your bike back to life, take it off and let it soak in a pan of Seafoam overnight. A lot of times this will fix the problem. You can also try boiling it in a pot of water for a while. Either way, you might want to do a fuel pump volume test to ensure it is operating correctly:
Fuel Pump Operation Test
Remove the right fairing inner cover.
Turn the ignition switch OFF.
Disconnect the engine control module connector.
Short the BLK/WHT and BLK/BLU wire connector terminals with a jumper wire.
- Fuel Pump Test 1
WARNING: Keep gasoline away from flames or sparks. Wipe up spilled gasoline at once.
Turn the ignition switch ON, let fuel flow into the beaker for 5 seconds, then turn the ignition switch off.
Multiply the amount in the beaker by 12 to determine the fuel pump flow capacity per minute.
Fuel Pump Minimum Flow: 640 cc (21.6 oz)/minute
- Fuel Pump Test 2
