I would not go any further until first I checked to see if the butterfly's on the carbs are going full open. The second thing I'd check is the tach. There are a dozen different tachs and setting on them and who knows if yours was set right. They have a selector switch and fine tune calibration pot on the back. Check it. The are often off by a little ( calibration pot) or a lot ( selector switch). It's so easy and common to get them wrong that I don't trust them especially on a not so common Chrysler. You may have a plugged up high speed jet. Get it at fast idle and block the carbs one at a time with a rag. This creates a high vacuum and will pull that brown jelly that forms with today's gas thru it. Shortcut to rebuild but it works wonders on all small carbed engines. Did you trim it way up on the top end? Get it bouncing or cavitating and then just down enough until it stops. If it does not have power trim move that metal rod on the engine up or so it does not go down as far when lowered. Fwiw. My current boat did 26 mph at 4600 rpm when I bought it and it ran fine. My tach was selected wrong, the wrong prop was on it, the engine was mounted an inch too high on the transom and it did not have power trim. Tach was low by about 1200, I went from 15 to 17 on the prop, I added a good used power trim but it cavitated befor I could really air it out with the trim so I lowered the engine. All common stuff to experianced boaters and more important on small boats like ours. Yesterday it ran an all time high of 37.6 @ 5800 RPM. In other words keep tweaking and you will get it. 1983 16' Valco Bay Runner with Johnson 60.
