What is wrong with this
 

I have a 1990 Baja 250 ES. 7.4 MPI, bravo 1 with a nose cone and a mirage plus 23 prop.

It does not really want to go faster than about 40. If I trim the drive up much the prop slips like crazy. When I am going 30 or 40 even a small turn makes the prop cavitate and I have to pull the throttle back so it will catch again.

I don't even have a clue where start looking for what is wrong. Any suggestions?

Thanks
Gene Bopp

BB,
Easiest solution is , you're trimmed too high.
With the boat on the trailer , and the cavitaion plate of the outdrive parallel to the bottom of the boat , check your trim indicator position. If you're only interested in doing say 40mph , there is no advantage to be trimmed much higher than this;) .

Yea, there was something done to the prop. I bent the tip of one blade and had it straighted at a prop shop. Now the prop looks perfect and the hub is perfect. The boat was a dud when I bought it. I figured that the old engine was just tired. It spun a bearing and I put a new motor in, it still doesnt want to go fast. I dont think it is an over trim problem. I only trimed it a little, a few seconds worth. The gage doesn't work acuratly. But when i do trim it back down it will catch in a few seconds.

Thanks

Gene

TOO OLD --- It sounds like a spun hub to me also. It might just now be going bad. Try someone elses prop and see what the boat does. If it runs good then take the prop back and have him check the hub. If the hub is not bad then take the prop to someone else and have them check the blades for straitness.

OK, Better listen to what the boats telling you

Well, if the problem was just me over trimming it would be going faster. Right? I think that it should go faster than 40. How fast do you think it should go? If 40 is all this thing should do let me know.

Gene

That boat should be in the mid 50's with a 21 pitch.

How many rpm's is the motor spinning? Without that info it's really hard to tell if it's a spun hub or not.

How are you measuring that speed?

I don't believe the tach, it is a cheepie with an adjustment screw on the back. I haven't had time to put a real tach on it, but it is not spinning fast. I just had the prop hub off. I took it off to look at it. It looks perfect. I don't really know what a torn up one would look like, but this one looks like a new one. I am using an aviation GPS set to MPH.

I will get to test out any ideas on Thursday. Any help would be appreciated. Yea, i know. Set the tach!

Thanks Gene

Did you just put the nose cone on? is it straight, It may be causing it to cavitate and with power steering you may not feel the steering torque.
just a thought.

Just a story to tell.......back in my old days(???) I bought a Merc Mirage prop to go on my Merc 200 ob..... ran like crap.... just terrible. Had two mechanics & shops look at it.... then someone said- The thru hub exhaust is too small. changed to a SMALLER hub prop and it ran great. The larger hub was stopping up the thru hub exhaust..... the smaller hub allowed gases to exit over & thru the hub. It worked good....
Is the hub the correct one for your drive? Plugged?? Otherwise, I too would look at the slipping hub.:rolleyes:

To verify if the hub is slippping take prop off. On the back of the prop take a center punch & punch a dot on the center part & the outer part next to the inter one. Now after you come back from the water check & see if both dots are still lined up. If not, you are spinning your hub. If they are lined up then go back to your prop guy. I have had more props that don't work correctly after a "Prop Shop" worked on them after I took them in for minor ding repairs.

Something is just wrong. Even if you are over propped a little you should be doing more than 40mph. Try to borrow a good 21 mirage plus and see what happens. If you don't pick up at least 10-12 mph then you probably have an engine problem or some serious hull drag. Is the hull clean? I think a 21 mirage plus is the prop you should have for your boat-motor combination. I have a 260 Baja which is actually a 24 footer with a built in swim platform. With the stock 7.4L and a 21 mirage plus it would do 54-55 mph. Your boat should be pretty close to that if everything is working as it should. Good luck.

Thanks guys, I will try all that. Believe it or not, by looking and the prop wash i was wondering if the exhaust could be exiting around the prop instead of though. But i didn't know if that was possible. I have a swim platform on the back so i really can't see much. I think i need to put on a different prop and see what happens. The nose cone was put on a by a dirt bag shop in homestead.... After he had the drive for 3 weeks, I agreed to take the lower unit with the nose cone just to get the drive back. So i really didn't know anything about them.

Thanks again

Gene