There are two M84's under this listing, both of them have been modified. I quote Bruce Palmer's Vol II, "The M84 and the M97 carburetors main nozzle are internally vented through a tube pointing in the air stream."   The only known use of the M84 was on the early military 1941 WLA and only in 1941.  (The only other use was on the 1941 M97 TA). This modification was not use on any other Harley Davidsons and not used any other year(s) than 1941.