Here you go. Two pages from the '48-'51 Chevy Truck Shop Manual that describe function and show the spring (second link/page).

Heat Riser Function - '48-'51 Truck Shop Manual p. 6-4
Heat Riser Spring Image - '48-'51 Truck Shop Manual p. 6-5


1947 2nd-Series GMC FC152 3/4-ton
Follow the story in the DITY

1953 Chevy Bel Air Sport Coupe Powerglide