if you can balance out the airplane by moving,battery,rx,servos try that first.it is normal to add weight to balance out.like was said here there are so many diferent engines and mufflers the weight diference can be as much as 1/2 pound depending on what size engine.you can move the engine forward or back on the mount some.my super stick i moved it about 3/4 of an inch forward to keep from adding extra weight.it looks funny but fly's perfect.ready to fly's like a nexstar come balenced out i havent seen many arf's that you didnt have to add weight or shift around your flight package.it is pretty much normal to add some weight,but if you can avoid it i would.