I installed a tezet ignition on my 251 ETZ and had a similar problem re backward running. This seemed to be due to limited adjustment of the sensor to the contact lump on the magnet wheel. It was not possible to achieve the 2mm btdc because the adjuster ran up against the securing stanchion and tdc had already passed ahead of the sensor! I’ve tried to upload some pictures of my solution which is to fix the sensor bracket to the stanchion with its clamping bolt. I will need to drill another hole in the ignition base to hold it properly in place so that the sensor does not move out of line or even foul the magnet wheel. It is now as close to the recommended position as possible. Fingers crossed when I eventually try it out!