I know this will sound weird but I know a person who was having problems with a larger gasser. It would start and run up to about half throttle. When the throttle was opened farther the engine would backfire and run irraticlly. It was driving everyone nuts. Come to find out that the magnet had fallen out and had been reinstalled backward with the wrong side of the magnet facing outward toward the sensor. They turned the magnet around and it ran great.... I was not there but was told about this while we were having a "Do you believe this" session. Bill