DELETE

What ignition does it have? Most stock Mercruisers do not have rev limiters. Most likely thats all the intake it has.

my 1997 has a revlimiter set at 4900 rpms

DELETE

mine is a stock carbed 7.4

I may pull off the ign and try an used HEI dist and wires and see what happens.

Besides the fact that you cant get any more rpms out of this motor, something is seriously wrong here. How is it possible that you can spin 3 different pitch props and still get the same exact top speed? Unless the slip changes drastically between these props, it is mathematically impossible.

It's possible, and mathematicly possble: anything can happen when dealing with a boat. Same speed with diffrent props happens all the time. I was takling with Johnson propeller in freeport yesterday and he said it happens all the time where guys a fustrated when they buy $500 - $1000 props and they get no change in speed or rpm.

Something very strange is going on. No way a 19p and 25p could have same run numbers, that's too big a size difference even with prop mods.
Are you sure the motor only turns 4600 ? Verify that the tach is able to travel beyond that so it is not a tach issue.
How could you get 61 with a 19p ? You'd really be twisting that motor.

Sorry props were 23" mirage, 25" mirage and 27" cupped and labbed all the same results. 4 blade 23-25" misc...