Try this one:
SimConnect_AddToDataDefinition
(m_hSimConnect, DEFINITION_SIMOBJECT_SET_ENGINE1_THROTTLE, "GENERAL ENG THROTTLE LEVER POSITION:1", "Percent", SIMCONNECT_DATATYPE_FLOAT32);
and then after
float fPercent = 0.9f;
SimConnect_SetDataOnSimObject(m_hSimConnect, DEFINITION_SIMOBJECT_SET_ENGINE1_THROTTLE, nESPID, NULL, 0, sizeof(float), &fPercent);