Jerry I am using Smittys on my 235 with 2 holley webber carbs and I have found that when my carbs are a little out of sinc the mufflers get to popping on one side or the outher. Try to adjust the carbs and see what happens before you spend a bunch of money on mufflers. I am using the same set up as you on my engine with the exception of the carbs.
Hope this helps.
Tom
