Abstract: Disclosed is a pressurized fuel carburetor system for an internal combustion engine. The sole movable component in the air intake passage is an accelerator-actuated throttle valve device operatively associated with a single fuel metering valve and with an automatic manifold pressure sensor operable to enrich and lean the fuel-air mixture. A fuel pressure regulator cooperates with a fuel pump and with a fuel return restricter connected to and governed by changing intake manifold pressure conditions to regulate fuel pressure at the inlet side of the fuel metering valve. A normally closed fuel check valve prevents flow to the fuel metering valve except when the accelerator is depressed. Pressurized fuel is subjected to multiple vaporizing stages including a first stage as it expands in escaping through the metering valve, thence through a set of small ports into a second expansion chamber and thereafter through a multiplicity of jets into flowing air hot exhaust gases in the air intake passage.