Car finally started but died on the way back from test drive

[Ray Sta.Juana]

I was able to get the car started after changing the spark plugs. The old
ones had some dry carbon buildup and as soon as I changed them, the car
started after a few cranks.

Went driving for about 5miles and a block from my place the car just died.
I'm gonna pull the plugs to see how they are but I'm lost as to what is
causing them to foul up if they are the problem. Any ideas? TIA

Ray S
98 Sentra SE w/mods
04 Kawasaki Ninja 250R