I had the very same problem with the same engine. With a 5x1800mAh NiMh AA pack above the fuel tank I have the CG at the rear end of the recommended range. After only 6 flights (it's winter here) it seems to me that the recommended CG is very much on the safe side. Next time i will move it a bit more rear, about to 6 3/8". Instead of adding useless weight try to add an alu nose cone. (anyway just a few backfire at start-up and the plastic will be damaged) After building I think that the engine had to be mounted more forward compared to the manual. Probably they counted on a much heavier engine. I don't like to have the battery in the nose because I'm used to check the battery and the cabling very often (each cell one by one, I came from the r/c car racing scene ). So I want to put the battarey pack to where it belongs. I think after that I have to re-position the engine about 1/4" forward on the mount. The cowling will go with the engine also forward and two pieces of hard wood will be glued to the firewall to hold the screws.