A Light Sport aircraft pilot reported engine failure during a maintenance test flight and safe landing in a field.

Narrative

I was conducting a post-MX (Maintenance) checkout flight on my experimental CH-601XLB Zenith Zodiac. The aircraft had performed well with no issues for about 20 minutes. I decided to return to the airfield; ZZZ. As I started my descent and turn to the field (about 2.5 NM away); the engine quit about 10 seconds after setting idle power for the decent from 1300 ft. to 700 ft. (the base leg traffic pattern altitude). Just prior to deciding to return to the field; I had been running the engine at a high cruise rpm setting of 2;850 RPM. This setting results in high EGTs (nominally 1300 degrees F). I suspect that I encountered vapor lock as the engine transitioned from a high-power setting generating hot exhaust and hot engine case to the idle power setting that demanded a much reduced fuel flow rate. The mechanical engine fuel pump is mounted to the engine case. I suspect that as the fuel flowed more slowly through the pump; heat transfer to the fuel increased and allowed the fuel to vaporize. Carb heat was on through this entire sequence as it is needed to lower the EGTs at high power settings in my configuration.I attempted to re-start the engine in the air two times unsuccessfully. Once from the left fuel tank; the tank I had been using. And once from the right fuel tank. I had already turned on the electric fuel pump in preparation for landing. During the second attempt; I cracked the throttle a little more to see if it would help the engine start. Once I safely landed in an open field on the engine started back up and ran normally on the ground. The engine and fuel had cooled for about 10 minutes at that point.