This happens only on startup. The engine starts and idles ok but runs poorly for first minute after I move the throttle to mid range. After this minute something kicks in hard and the engine runs fine the rest of the day. Although, last trip this happened once after repeted stop and starts at different fishing holes. I have rebuilt the carbs twice, replaced all the coils and everything else electrical seems to check out ok in the driveway. I have been working this problem for over two years don't know what to do next.