a process and apparatus for improving fuel economy and exhaust emissions of internal combustion engines operating at high altitude conditions . the process is comprised of adjustments to spark timing carburetor idling and crankcase ventilation flow to lean out the fuel - air mixture an amount based on the relative elevation in altitude . the process includes the use or modification of a crankcase ventilation modulating valve having novel flow characteristics and produces much greater efficiency than would be expected under the high altitude conditions . the manifold vacuum between the engine carburetor and piston modulates the valve to provide a greater air flow when increased power is demanded from the engine . in addition to coordinating the spark timing with the modulating valve air flow , the process includes a concomitant special adjustment of the engine &# 39 ; s speed and fuel air mixture at idle .