Gum formed during fuel storage can deposit on induction system surfaces, intake valves, stems and guides. To test for gum content, a 50 mL sample is evaporated in an aluminum block bath for a specified period under controlled conditions of temperature and flow of air. Electrically heated baths for determining existent gum in aircraft turbine fuels by steam-jet evaporation and in motor and aviation gasolines by air-jet evaporation. Unit consists of a high temperature evaporation bath with 100 mL test beakers and, for aircraft turbine fuels, a steam generator and steam superheater. Air/steam pressure regulator with gauge and a flowmeter adjusts air flow per ASTM specifications. Stainless steel jets deliver air or steam flow to the test wells through removable brass conical adapters. Microprocessor PID control provides quick temperature stabilization without overshoot. Fully insulated, aluminum block design assures safe, efficient high temperature operation. Dual LED displays provide actual and setpoint temperature values in °C/°F format. The bath is protected by an overtemperature control circuit.