New system build help

Oct 6, 2018

I am building a new system. I got all of the parts, assembled, and when I go to power on the computer it spins the fan and starts powering, but just restarts about 9 seconds later (nothing ever shows up on the monitor). I initially had my dvd Roms plugged in, but then unplugged those. At this point it is bare bones without anything plugged in but the necessities. (Ram, processor, and cpu fan)

I have an I7-8700K processor
Gigabyte z370 Aorus gaming WiFi rev 1.0 motherboard (using the onboard video via hdmi)
DDR4 2400 2x4GB ram.
Cosair 430 watt power supply. (I realize this is smaller than ideal, but was trying to use this from my past system which was working fine when disassembled)

As stated, I attempt to power it on and do not get anything showing on the monitor. It self reboots after about 9 seconds. Unfortunately I do not have any other systems to swap components with so I’m trying to decide on my next step. I’ve built around 10 different systems in the past and never had an issue with putting one together. Not sure whether to start shipping back the motherboard and or CPU or if there is possibly something I’m missing.

Thanks for the help