Let us know what happens.
Time to do some basic troubleshooting. Compression? Spark to all cylinders? Timing advancing? Carburetor opening fully? Carb might have a gummed up power circuit. Is the choke open fully? Other things mentioned, Exhaust passages clear? What’s the gear ratio? It should be on the decal near the upper gear housing
Similar boat as you.. I just checked and I'm running a 14-1/2 x 19 right now aluminum prop and have a 14-1/2 x 21 SS prop also in the boat I've never used. We top out right now around 42 mph, give or take, when trimmed up.
J.ust a thought, but would a spun engine coupler do this? I had a spun prop once that would hold for lower speed but then spin at higher speed. I had to crawl home. Not sure if an engine coupler works on the same principle but if there is a rubber part in the coupler it could be spinning at higher rpms?
Unless this 5.7 is really a 5.0 . If the gear ratio is for a 5.7 (1.50-1) the motor will not perform, period. You can get away with a 1.84 ratio.swinging a 17P prop
And this was determined by the TAG on the block just above the starter?