Today I have built the gas mixer. We had an old one, but it was quite small, so to be chore that it wont choke the flow I built a new one. The construction consists of a 2 pipe that ends in a 3 pipe. The producer gas comes in through the 2 pipe and the air comes in through the side on the 3 pipe. A throttle valve regulates the air. The valve is controlled by RC-servo.
I made the gas mixer in two parts so it should be easy to inspect and clean. The connection hosing is made of soft rubber to prevent vibrations to go in to the car body.