I have a 15 year old walker with a Kohler ech740-3031 with fuel injection. Backfires badly from full throttle to idle. I have seen discussion on shutting down at half throttle. I never had this problem before. Spark plugs a little sooty, anything I can do with Eli to make it run leaner. Spark plugs were 2 heat ranges too cold. I can try replacing first.