Hi Guys, a curious one. My 85 ZN 1300 starts perfectly when cold, however on occasions it will not start when hot, unless I give it full throttle. Just cant figure it out. Air filter is fine. So sometimes with a tiny bit of throttle it starts when hot but if it doesn't then I need to go full throttle immediately or else it will just keep turning over without firing. Any ideas? The bike is a Naked version and I wonder is there anything I could have done as I took all the computer stuff off, replaced all instruments etc etc. I feel it is something to do with a sensor or the brain? Any help welcome. Kenny
Sounds like you're experiencing vapor lock. All that latent heat from the engine heating up the fuel in the fuel lines leading to the injectors. If you have a slightly leaking injector or 2, the fuel rail pressure drops and will allow the fuel in the lines to boil. The injectors have a lambda time ( on time) for various throttle positions and rpm. Gas vapor is light compared to liquid fuel so the injectors are not squirting enough fuel to keep the bike running until the vapor is cleared.
Usually, on fuel injection systems, when you first turn on the key, the fuel pump runs to charge the fuel line up to operating pressure, then it shuts off until the injection computer sees RPM indication, telling the computer that fuel pressure needs to be maintained. This is not enough time to clear the vapor from the lines via the fuel pressure regulator valve on the return line to the tank.
2 tests you could do-
Put a pressure gauge on a test the fuel rail pressure ( in this case supply line pressure). Run the bike then shut it off and watch the pressure gauge. It should hold pressure at operating pressure indicating that the fuel injectors are not leaking down. If they are, then the proper fix is to have the injectors serviced and or replaced.
The other simple test to prove the theory is to have the bike hot and stopped to initiate the no start condition, then in an attempt to start it, turn the key on and listen for the fuel pump to run and stop, then turn the key off for a couple of seconds, then turn the key on and again listen for the fuel pump to run and stop. repeat this for 4-5 times, then try to start the bike. If it starts better than it did before, then you know that vapor lock is a problem. What happens here is that by running the fuel pump for an extended length of time will push a lot of the fuel vapor back to the fuel tank via the fuel pressure control regulator. Because of the fuel supply lines like the ZN has ie. the short pigtails from the main supply line to the injectors, it's not possible to clear all of the vapor, but this test should help get the bike running properly quicker.
NOTE- This is general fuel injection testing based on a standard fuel injection systems. I don't consider myself a motorcycle fuel injection specialist but prior experience on auto fuel injection systems and snowmobile injection systems has given me a sound background in fuel injection.
Hope this helps.