4-stroke gas engines. I've built 5 so far, and no problems with any of them! Last year I tried a Robbin/Subaru 33.5 cc and it worked so well I had to build more bikes for family & friends to ride with me ! Now I have too many and I have to sell some to have room for some closeout deals I found. 2 Kustom Kruiser Slick Daddy's [brand new], and a brand new Spoiler still in the box! Of course I'm building them as soon as I have the room!