G450 APU
 

Dear All.
I am having a unique problem while starting G450 APU, Honeywell 160. It does not pose any problem during initial start but if it is switched on and again tried to start it will not (no ignition but rpm do increase till 22 percent)
As APU cools down it will receive ignite easily.

Not familiar with this particular APU, but this is a pretty common baseline design. (basically a GTCP 36-150)

Primary fuel nozzles need cleaning. Uneven fuel distribution pattern out of the nozzle is snuffing out the ignitor and preventing light-off. First start of the day and after sitting a while the fuel evaporates and ignitor does it's job. But once covered in fuel it prevents spark.

Otherwise unplug the fuel shut-off solenoid and give it start a couple times. Listen for consistent 'cracks' from the ignitors. If it is only working part of the time may be time for new ignitors or ignition exciter.