Could be a not-attached electrical plug.
Start w/basics:
compression
spark
fuel
Place a spark plug into your COPs and be sure to ground it out & then turn the engine over. Does it spark the plug? No spark, check your sparkplug ground, try again. Repeat for all 4 cyl's.
If you have a compression tester, that's best to get a reading, but for a seat of the pants measure, if you could reach the sparkplug hole surface, you could use your thumb over the hole, and turn the engine over. It will turn over easier if you remove all the plugs, and do the test one at a time of course. Because the sparkplug is recessed you need something like a compression tester hose, to attach the gauge to and now you will get a discrete reading. Don't forget to hold the throttle open fully when you're doing the diagnostic for compression. Any significant variance in compression readings? If so, you need to find-out why. Repeat for all 4 cyl's.
Fuel:
open the bowl drain and see what comes-out. If you leave the drain screw open and you turn on the ignition, the fuel pump should fill the drained float bowl, and you will get a pee-stream out of the drain. Repeat for every cyl.
That should help to determine what's happening.
