Atco HY 14" Plug #

Hello,

I am having problems getting a Atco HY running. At first the engine started and ran well but would not idle very well. The throttle cable was quite sticky so I replaced it. I thought that the screw in the side carb piston held the cable, but, found that it holds the needle. I have replaced everything,but, not sure if I have the needle in the correct position. The needle moves slightly is tat correct? I am getting a good spark. Maybe the plug needs replacement? What number is it? The engine sometime backfires and runs the wrong way for a couple of revolutions. I have not touched the timing.  Any ideas?