Heavy, Your problem is the cam. With 10.5 to 1 comp ratio and the small 268 cam you have way to much cylinder pressure. Too use that kind of compression on pump gas you will need to run at least the 274 extreme or the 280 or 286 magnum cam. Alot of guys are running into this problem. Give comp cams a call and I am sure they will tell you the same thing. That cam is a great cam but it only likes maybe 9.5 to 9.75 comp on pump gas with aluminum heads. With steel heads up to 9.5 MAX!