To start off a 24 ft pontoon is a tall order for that engine. With that being said it can be done but you will never get good performance. So were to start, first I would start checking the cheap stuff. Compression test, if your compression is low nothing else will matter. Inspect plugs and make sure all cylinders are firing. With the engine off make sure carbs are going all the way open when you move to WOT. Are all cylinders getting fuel? Make sure engine is mounted correctly not so deep to cause drag and not so high to cavitate. I do not think the coil is the issue but if it is damaged replace it's just for good maintanace. I would also hook up a spare tach to make sure the old was was accurate and use a GPS for speed. With this engie/boat combo I doubt you ever exceed 18mph. Was the last test with the 13p prop. If the engine checks out ok you may find yourself going to a smaller prop with a lower pitch to get to your 5500 rpm range. After saying all that, if the tach is correct and it was the 13p prop there is a mechanical issue. Usually either compression or carb related.
