This engine is the replacement for the 45HR hydro and the eventual discontinuation of the 45RS to be faded out shortly, although the main item spares will be available for some time. Designed to be more powerfull with altered timings, stroke and bore. Based on the configuration of the 21 VAC motor which is the upgrade on the 21 RSV valvola. Engine has a CNC machined large bore carb and comes with the seperate third channel mixture valve.