Karhunen scored 31.5 kills in World War II; 25.5 of his air victories were achieved while flying the Brewster F2A Buffalo, an American fighter.

[1] Soon after the outbreak of the Continuation War, on 25 June 1941, when he was 1st Lt, Karhunen led the 3rd Flight (of eight Brewster Buffalo) of Lentolaivue (LLv) 24, based at Vesivehmaa.

[2] On 9 July, Karhunen was one of the dozen Brewster pilots led by Major Gustaf Magnusson on a patrol over Lahdenpohja, that in a ten-minute air combat shot down eight out of 15 Soviet bombers and fighters.

[5] Eleven days later, on 12 August, Capt Karhunen was involved in the heaviest air combat over the Karelian Isthmus, while leading six Brewsters from 3/LLv 24 on a patrol.

Karhunen, who claimed two I-153, his fourth and fifth air victories, recalled: While carrying out a reconnaissance mission over Mikonselkä railway station.

[7]After the end of hostilities, Karhunen, the captain and commander of the 3rd flight of LeLv 24, recalled: The Brewster model 239 was good against the older Russian fighters, Polikarpov I-153 Chaika (Gull) and I-16.