I have had two Webra's . Always 18/8 prop.Timing 5degrees btc @ idel. Carburator WT 76A. Wore the first one out after ten years of flying. Have you checked the screen. My first had more power then G-38
My friend and I had planes alike same weight and I could run circles around him. You can buy carburator fom V.E. Peterson. He has all the ones for all engines that or made @ great price
B&B Muffeler