The original fuel line was 8 mm (almost exactly 5/16"). Vapor lines were 6 mm (1/4"). If you have a standard engine, the 1/4" would be alright. If you have a modified engine with less than 135-140 hp, then the original 5/16" is alright too. However, after 140 hp, if you are using a single Weber carburetor, the fuel bowl will empty out at full throttle due to a lack of fuel volume (not pressure) and will potentially damage the engine. At this point, fuel lines must be increased to 3/8". If you have twin Weber DCOE's, no problem usually...they have much larger fuel bowls, plus there are two of them!