What happened
After completing refueling, the pilot of N912ASS took off to proceed with a banner tow operation. During the climbout phase, the engine lost power. The pilot attempted to restore normal engine operation, but these efforts were unsuccessful.
In response to the loss of power, the pilot initiated an emergency descent into a soybean field for a forced landing. During the touchdown phase of the emergency landing, the aircraft nosed over.
The investigation
A post-accident examination of the engine showed that it operated normally within the range between idle and maximum RPM. However, an examination of the carburetor revealed that the float valve would stick in the closed position, which effectively cut off the fuel supply.
Investigators also reviewed the operator's refueling policy. It was determined that a blend of 100 low lead aviation fuel and automotive gasoline was used in aircraft authorized for such use via a Supplemental Type Certificate (STC). N9128D had been granted an STC for the use of automotive fuel.