Hi, I had a problem with my A4 the other day I can't explain. It had been running fine all season. It is raw water cooled. During a cruise back to port, the sound of the engine suddenly changed, was running very roughly, and slowly an odor could be detected inside the cabin. I checked the carbon monoxide detector and it was reading 80 to 100 (first time ever it read anything higher than zero). I shut the engine off. Tried starting it again later but it would only idle with the choke pulled out - when closing the choke it would stall. After a few tries and pouring some Mystery oil in the tank, there was a pop and then the engine started running normally again. I'm a new A4 owner, so have a lot to learn - can anyone explain this? sticky valve???