Just to clarify, and maybe help MikePA, is the JD420 you speak of the 50's 2cyl upright engine (Dubuque, IA) (that was not the 2cyl horizontal that MikePA first attached)? The description poom-poom-poom isn't the one that was descriptive of my JD420 utility ('58), but I can't write a better one down. The upright 2cyl was a higher rpm than the horizontal ones, and had ~4" holes instead of 5+" holes. The sound that MikePA first came up with for your description sounded like two engines, one steady 4cyl in the background and a hit-n-miss in the foreground. Or the background was the mechanical linkages on the hit-n-miss. (There is a GT JD420 but I am sure that is not the one you refer to in your post)
Case-IH Farmall 45A, Kubota M8540 Narrow, New Holland TN 65, Bobcat 331, Ford 1920, 1952 John Deere M, Allis Chalmers B, Bombardier Traxter XT, Massey Harris 81RC and a John Deere 3300 combine, Cub Cadet GT1554
Do you have any of the larger diesels like in the R?