I have 2 engines currently installed on 2 separate bikes. On 1 bike that I call "Ol Betsy" ,lol, I had the engine for about 9 months or so, and it running Absolutely GREAT! The bike itself has 27" wheels. Now on my new bike, I have a Black Limited Edition Slant head 80cc from Zoombicycles and I had it for about 3 weeks now. So far it went through roughly 3-4 tanks of gas which is about 2 gallons roughly. When I am running the 2 bikes side by side the older engine performs really really good, and the new black slant head just runs ok but is slower while there are 44t on each wheel. I did take into account that the 27" on "Ol Betsy" make a little difference but I really believe the slant head and expansion chamber on the new black engine should make a difference as well. Is there any reason that you guys can come up with? I thought that my engine isn't full broken in so I wont see a performance increase as yet. I am in the process of adjusting my carb as well.