I have a 1976 75 hp Johnson Stinger. Model 75elr76D. The motor runs, but does not idle. I took the carbs out and noticed that the lowest carb had the little brass tube inside the larger brass tube (venturi tube) was corroded and no longer attached. I can not find a reference to this tube in my repair manual. How does one replace this part? It seems pretty firmly attached, and I can not find anything on line about it!