Re: mercruiser I/O top end problem
What you say above is the way it should behave if I am reading it correctly. To be hyper-correct, planing is when the boat exceeds the speed of it's bow wave and is indicated when there is no water on the transom. The water can't keep up, so when you look at the transom all water is "wooshing" away behind instead of coming back and hitting it . . .
How do you know your little boat goes 25? I trust that the speed is correct, but I am curious.
To verify RPM you probably need to find someone with a portable shop tach. This can be one that works with the plug wires or what's called a Phototach which are getting really cheap. A Phototach reads a piece of reflective tape that you can put on basically anything to see what the RPM of that particular thing is. In this case you'd put it on the front crank pulley.
Another thing you should try is to look at the back of your tach and see if it has a switch for 4, 6 and 8 cyl. They "count" differently depending on the number of Cyls. I can't keep this straight in my head, but one way or the other makes them read faster or slower than actual i.e 8 cyl vs. 4 etc. This may not be a solution in your case, but it is worth a try.
Finally, all props "slip". Do some research on this, but we are not saying that the hub is slipping, but you need to imagine a prop going through a big hunk of clay. If you could turn it through the clay, a 19" prop would go 19 inches forward in one revolution . . . Well water is not as thick as clay, so when it goes once around it doesn't push this big old hull a full 19" through the water. It probably goes around 15" or 16" which is 22% and 16% "slip". 22% is a lot, some great setups get below 10% slip. Your numbers when plugged into a prop calculator indicate huge slip numbers in the order of 50%.
Now they can also "slip" mechanically (bad), the hub of your propeller is mounted to your propeller shaft, the propeller itself is mounted to the hub with a rubber or similar link. This link could be spinning so that the prop shaft may go one revolution and your propeller only .75 . . . That's 25% slip mechanically, not "hydraulically" (water), so if you add 25% mechanical slip (could be in the gearcase too) and 25% slip hydraulically (clay vs. water example) you could be nearing the numbers indicated by 20 MPH and 5500 RPM. BTW, I don't think this is the case.
Next, you have gear ratio. Most 140s whether they are Mercs or Volvos or OMCs are around 2:1 ratio. This means that the engine has to turn completely around twice to get the propshaft to turn once (2 to 1). It is possible that there is a different ratio in your drive unit. I don't believe so for two reasons:
1) Normally that would go the other way indicating that the boat should be even faster (1.5:1) and,
2) I don't believe that a 140 would swing a 19" prop if it was a lower numeric ratio (faster) than the 2:1 . . .
Oh, to verify gear ratio you need to turn the engine over by hand (remove spark plugs) with the drive in gear until the prop turns precisely one revolution. The number of revolutions of the engine is you gear ratio. Twice around = 2:1 ratio. Not quite twice say 1.8:1 etc.
These issues as you have found are perplexing. Your numbers say you are going over 40, the real world knows your not, so the numbers are wrong. What I find odd, is that it does sound like it is planing and all of the other stuff, poorly running engine, poor maintenance to date, 19 inch propeller would indicate that it shouldn't be that easy.
Just for fun and it is possible that this is confusing us, does it have a hydrofoil on the drive? Whaletail looking thing? This could account for the low top speed and relatively simple to plane . . .
