My '04 ZX10R with 34K miles developed an intermittent starting problem a few months ago. It would always crank but wouldn't always start. After reading dozens of threads and watching YouTube videos I finally found a few people with the same issue. It turns out the problem was a gray wire on the back of the ignition switch was broke at the solder connection to the switch. No one has written a comprehensive tread on this so here is mine.
The symptoms of my no start problem were unique so if you are having the exact symptoms listed below then this is most likely the source of your problem.
1. When key is switched on the fuel pump does NOT come on to pressurize the fuel rail (humming noise).
2. When the key is switched on the exhaust butterfly valve actuator on the right side under passenger seat does NOT go through a its normal check to verify position of butterfly valve. It makes a low whining noise as it pulls and pushes the cables.
3. When cranking the motor the FI light on the dash flashes but motor will not start.
To repair the problem access to the back of the ignition switched is required. The easiest way to do this is to pull the upper triple clamp. The ignition switch is held in place with security break-away bolts. There is no need to remove the ignition switch from the upper triple clamp. Just remove the upper triple clamp and flip it over for access.
Remove lower fairing. Remove left mid body panel, the small access panel under the headlight (one small Phillips head screw) and left inner mid panel (black plastic piece the turn signal is attached to.
Disconnect the ignition switch harness from the main wiring harness (8 pin white block connector) and feed additional wire harness through wire holder to provide the necessary wire length to remove the triple clamp.
Remove the large nut in the center of the upper triple clamp. Loosen the upper triple clamp pinch bolts.
Lift the upper triple clamp up and flip it over so you can access the back of the ignition switch.
There is a single Torx security screw holding the black cover on the back of the ignition switch. You do NOT need a Torx security bit to remove the screw. Use a pair of small pliers on the outside of the screw to remove. The cover pops off and you can see the wires soldered to the back of the switch.
Under the larger white and red wires you will find a small gray wire, approximately 18 gauge. Where the gray wire is soldered to the ignition switch it will be broken but will still make occasional contact. This is what causes the intermittent problem. This is a low voltage signal wire. Due to vibration, the fact the wire is a bit too short and my ignition switch harness had an extra twist in it led to the wire breaking. If the black covering on your ignition switch wiring harness has an extra twist in it, rotate the upper triple clamp and ignition switch to remove the twist. This will give you the extra wire you need to repair the connection. The black covering on the ignition switch should be smooth and not look like a drill bit or a barber's pole.
One issue with reattaching the wire is to be careful you don't put too much heat into the ignition switch and damage it while making the repair. It is a tight space and you don't want to burn the insulation on the other wires. For these reasons I did not use my soldering gun. I bought an X-Tronic Model 3020 soldering iron with temperature control an digital readout. It was $50 at Amazon and had good reviews. It worked perfectly.
I used 60/40 (60% tin and 40% lead) rosin core solder. Make sure the tip is clean and then tin the tip. After stripping back the wire, be sure and tin the wire before you attempt to solder it to ignition switch. Plenty of videos on this on YouTube.
I set the temperature to 650 degrees F and waited for temperature to stabilize. 3-4 seconds of heat on the ignition switch and the wire soldered on cleanly.
I plugged in the ignition switch to the main harness to check it. It worked perfectly. When you reassemble make sure the ignition switch harness does not have a twist in it because this will put tension on the wire. Once the triple clamp is back in place be sure to pull the extra wiring through the holder under the air intake duct and check the routing so it does not interfere with the turning of the forks.
Torque values per service manual:
Steering stem head nut (large nut in center of upper triple clamp: 78 Nm or 56 ft-lb
Upper fork clamp bolts 20 Nm or 14.5 ft-lb