T-Racer T 7 Plus Surface Cleaner is designed to be compatible with Kärcher K 4 to K 7 series pressure washers. However, I do not have specific information regarding its compatibility with the HD 650 model. It is possible that the fittings are not directly compatible, but this would depend on the exact specifications of the HD 650 model. I recommend that you check the technical specifications of the product or contact a Kärcher dealer for more precise information.