I have a 1986 ranger 390v with new freshly built 1994 200 efi 2.5 liter merc. I was running a 1986 merc 200 2.4 liter and 6" jack plate with 24" trophy. Boat would run low 60s with 2 people. Now i have the 2.5 efi and 9" jack plate i can only get 55 with the same prop and less rpms. whats the deal??? Running 4.25 below pad and if i raise motor i loose water pressure. Boat feels like a slug. I had to put 9" plate because i installed hydraulic steering and if trimmed up all the way it would hit the transome. now it clears fine but lost alot of performance. It wont turn a 25 tempest. HELPPPP?
Before you added the 9" plate did the motor run okay or did you add the plate before you tried it out? The 9" plate may have set the motor behind the swell of water behind the boat which now lets you get air into the system or the water pump could have been installed wrong. What's your water pressure at idle and at WOT?
Water pressure should drop at idle after a few hrs. run time to 1-3 lbs. That motor can run WOT with 10-12lbs of WP. How much does it drop when you raise it up? Mercury makes high speed scoops for that lower unit if it's losing WP at speed. Part number is 17280A2 I think. They also make a limiter so the motor will stop before it hits the transom.
You might have a motor issue. The 2.5 200 put out 202 HP from the factory, the 2.4 200 put out 212 HP, so you should only be about 1 mph slower with the same setup. With an extra 3" of setback, you should as fast or 1 mph faster. I suspect your motor is too low. For every inch setback you go, you should also go up half an inch, as a general rule. So if you went from 6"to 9" setback, you should be running about 1.5" higher. That should add a couple MPH, You may need to go up several inches on your plate.
That boat was never designed for a 9" which is actually a 10" plate and more isn't always better in set back. There is a swell of water that forms behind a boat. As it passes through the water it pushes water down and too the side and once it passes that point it the water rushes back in and forms a swell or wave higher than the level of the surrounding water but only for short time and depending on the boat it could be 6" to 18" behind the hull. The other thing I wouldn't trust that old wooden transom with a 10" plate unless you've had it rebuilt.
Got it fixed! That hull requires a measurement of 24.5" from the hook on the mount to the bottom of the hull. I had to drill new holes in the jack plate and lower motor. Got a new boat. ! Great hole shot, sits high on the pad , Rpms at 5800 and running 67.5 on the gps!!