Abstract: A tamper resistant mixture adjustment screw arrangement for a carburetor. The arrangement includes a carburetor body having at least one air/fuel adjustment screw threaded therein. The adjustment screw has a threaded shank and a head portion. The head portion is defined by a smooth top surface and an undulant, uneven side surface capable of being engaged and mated by an adjusting tool having a complementary undulant, uneven side surface capable of being engaged and mated by an adjusting tool having a complementary undulant, uneven surface for initially adjusting the air/fuel mixture in the carburetor. A blocking curb extends from the carburetor body to a level which at least substantially corresponds to a projecting extent of each adjustment screw and being closely spaced to the screw head to prevent the screw from being turned by commonly available tools, but to permit the screw to be adjusted by the adjusting tool.