I've had tire to Dyno chain-guard interference with a Schwalbe 2.35" tire on a Huffy 40mm rim; the steel guard was the more simple style, not the classic one on the deluxe or base models. Not sure if the overall chain length, sprocket sizes, and relative placement of the axle/hub within the drop-outs was a contributing factor. Was about to notch it, but then switched to 2.1" black.
I've tried adjustment once, but then the guard front would hit the cranks.
I have a used bike with a previously tweaked chain-guard (black); it seems that they will bend easy enough, and may even flex some in hot and cold weather.
I have seen only one other Dyno with a plastic chain guard, (that one was online), and it was also cracked.