I'll have to look into it. I have been adding Marvel Mystery to every tank.I like Iridium too, but I would bet the bit of shuttering you are getting is from fuel spray not being even. Try fuel system cleaner (I really prefer LUCAS) and then use LUCAS fuel additive ALL the time. I have for years, and on my carb bikes, carbs stayed perfectly clean and were lubed to the touch. It really helps fuel injectors to stay clean and lubed. Ratio is 400 to 1 so (I purchase by the gallon and use in everything I own) and a gallon lasts a loooooong time!