5 0 F ▁ 2 0 0 3 – 2 0 0 5 ▁ ▁For ▁ 2 0 0 3 , ▁the ▁Y Z F ▁received ▁the ▁biggest ▁update ▁in ▁its ▁history . ▁First , ▁the ▁engine ▁disp lacement ▁was ▁increased ▁with ▁ 4 4 9   cc ▁in ▁comp liance ▁to ▁the ▁new ▁A MA ▁Mot oc ross ▁maximum ▁disp lacement ▁rule , ▁the ▁engine ▁now ▁produced ▁over ▁. ▁Next , ▁the ▁frame ▁and ▁pl astic ▁were ▁updated ▁for ▁a ▁new , ▁sle ek er ▁look . ▁Additionally , ▁the ▁Y Z F ' s ▁weight ▁was ▁decre ased ▁from ▁its ▁original ▁ ▁to ▁. ▁ ▁The ▁bi ke ▁made ▁trem end ous ▁power ; ▁however , ▁many ▁thought ▁that ▁the ▁bi ke ▁had ▁too ▁much ▁power ▁for ▁a ▁mot oc ross ▁track . ▁The ▁ 2 0 0 5 ▁Y Z 4 5 0 F ▁was ▁the ▁first ▁Y Z 4 5 0 F ▁to ▁feature ▁K Y B ▁tw in ▁chamber ▁cart ridge ▁for ks . ▁ ▁Four th ▁generation ▁Y Z 4 5 0 F ▁ 2 0 0 6 – 2 0 0 9 ▁ ▁The ▁Y Z 4 5 0 F ▁received ▁a ▁major ▁update ▁in ▁ 2 0 0 6 , ▁with ▁over ▁ 3 0 0 ▁parts ▁changed ▁and ▁improved . ▁The ▁power ▁was ▁smooth ed ▁out , ▁and ▁the ▁bi ke ▁was ▁made ▁easier ▁to ▁ride , ▁putting ▁to ▁rest ▁the ▁compla int ▁that ▁the ▁Y Z ▁had ▁too ▁much ▁power . ▁The ▁transmission