Jim Walker's control line patent (I have a copy) shows the O&R spark timer rigged to a 3rd line, and a spring to pull it back to idle. That sort of works, but the O&R timer, if loosened enough to permit 3rd line operation, tends to get even looser, and even fall off! I think it would take some re-design of the stock timer to make that work. I've seen Walker fly a Fireball using that system. It worked for him.