Gs40 good right out of the box the 51 gave me a fit for over a year. One day with no changes it starting running like a raped ape and five years and a few gallons is the best engine I have. I do have some OS , Magnums, and some Aviastars. It idles perfect ,instant transition, and screams with a 11x6 on an old Tower trainer. When Tower gets them back in stock I will get a couple more 51's.