Oil! I have to run mine at 28:1. I use klotz, for no other reason than just cause the hobby shop had it. I started at 50:1 and had overheating problems in the cowl. At 32:1 it would run ok but would have occasional problems. Ran good without the cowl but I didnt like that. 28:1 solved it and has run great for 2 seasons now. My ryobi is a 12 year old wore out retired weedeater motor. But still runs great, 6700rpm with TF 18x8.
Edwin