Yamaha made these little two strokes from 1966 to 1971. Displacement was 100cc for US & Europe markets and 90cc for Japanese home market because ofb license classifications in Japan. First 100cc model was named YL1 "Twinjet". The power of those 100cc was 9.5 hp @ 8500 rpm. First 90cc model was the "AT 90" (see pictures). Ther is a YL1 Twin Jet version with electric starter (dynastar) named "YL1E".
The Twin Jet power unit has served to design the first 125cc sport model "YAS1" in 1968.
YL1 evolutions :
AT 90 evolutions :