▁world ▁almost ▁exclus ively ▁used ▁two - stroke ▁engines . ▁A MA ▁racing ▁classes ▁had ▁two ▁classes : ▁ 1 2 5   cc ▁and ▁ 2 5 0   cc ▁two - st ro kes , ▁with ▁no ▁provision ▁for ▁four ▁stro kes . ▁Most ▁rid ers ▁considered ▁four - stroke ▁engine ▁technology ▁to ▁be ▁largely ▁util itar ian ▁and ▁un comp et itive , ▁re leg ated ▁sole ly ▁for ▁trail ▁r iding . ▁ ▁In ▁ 1 9 9 6 , ▁the ▁A MA ▁changed ▁racing ▁rules ▁to ▁allow ▁ 4 5 0   cc ▁four ▁stro kes ▁to ▁comp ete ▁in ▁the ▁same ▁racing ▁class ▁as ▁ 2 5 0   cc ▁two - st ro kes . ▁Yam aha ▁engineer ▁Y oshi har u ▁Nak ay ama ▁first ▁came ▁up ▁with ▁the ▁idea ▁of ▁creating ▁the ▁first ▁compet itive ▁four - stroke ▁race ▁mot oc ross ▁bi ke . ▁The ▁Yam aha ▁Y Z 4 0 0 F ▁was ▁developed ▁to ▁fit ▁into ▁this ▁category . ▁It ▁solved ▁the ▁power ▁d ile m ma ▁by ▁borrow ing ▁super bi ke ▁technology ▁and ▁giving ▁the ▁Y Z ▁a ▁five - val ve ▁head , ▁liquid ▁cool ing ▁and ▁a ▁ 1 2 . 5 - 1 ▁compression ▁ratio . ▁ ▁In ▁ 1 9 9 7 , ▁Yam aha ▁rock ed ▁the ▁mot oc ross ▁world ▁with ▁the ▁introduction ▁of ▁the ▁Y Z 4 0 0 M ▁prototype , ▁a ▁concept ▁motor cycle ▁which ▁borrow ed ▁much ▁technology ▁from