Got my new 350 Mercruiser put in my Sea Sport, it replaced a 305 Volvo, these are both Chevy blocks. Every time for the first hour heading out it cruises along nicely then will miss or drop off about 400 rpms then pick back up like nothing every happened. Feels like hitting a log or something, just instantly drops off for a quick second, then resumes right along. Will do this about a dozen times in an hour and won't do it at all a day later coming back.
THINGS THAT WERE ATTEMPTED THAT MADE NO DIFFERENCE;
-Timing has been re-checked. (made no difference)
-Gas tank emptied completely (yet never did this ever before, why now with a new engine???) (made no difference)
-Water/fuel filters changed (made no difference)
Next test is; Changed the coil and put in new batteries.
Why would it do this in the first hour of a run but not after fishing all day???