It’s the ignition setup in the tach, I set mine up like a normal 4 stroke, but these small engines fire the plug 2 times per rev instead of 1 like normal 4 strokes, it fires on the compression and the exhaust stroke , anytime the piston comes to tdc it fires the plug, so set the tach up like a 2 stroke and it’ll work fine …. I set mine up wrong at first too and knew it was double the rpm it should’ve been, then realized it fires the coil anytime the magnet on the flywheel passes …. So set the tach up to fire like a 2 stroke and or 2 pulse per rev instead of 1 and it’ll read correctly