power drops as RPMs go up

[ststeveo]

I have an '88 225hp evinrude. When I get to 2700-3000 RPM the power drops and the RPMs take off so the Prop is no longer taking the load. I had the water pump replaced last fall. Is something not put back on right. The mechanic took the lower unit and put it back then I put the prop back on. The nut is tight and the pin is on but when I get to putting it past plane the power just drops and the engine whines.

 

[kenmyfam]

Re: power drops as RPMs go up

Sounds like a spun hub on the prop.
Do you have another prop to try to see if things improve ??
If not mark the hub and prop in line with a permanent marker or paint marker. Go for a ride till it does it again then check to see if the marks still line up. If not then the prop needs to be either re-hubbed or replaced. Both would be better then you will have a spare for emergencies.
Good luck and let us all know.

 

[ezeke]

Re: power drops as RPMs go up

Picture may help; click the thumbnail, please.

 

[tashasdaddy]

Re: power drops as RPMs go up

agree^^^^

 

[ststeveo]

Re: power drops as RPMs go up

i pulled the prop to replace. It is a michigan 14 1/4 x 17. Only michigan carries this. Can i go with another size or do i need to keep this size? It seems the cavitation motor anode broke loose and hit it (threads broke inside lower unit) causing the spun hub. prop still looks ok and i will see if i can get it rebored. Can i keep running without the cavitation anode? Will it cause any performance problems? I don't think i can get a bolt to thread in and hold one.

 

[ezeke]

Re: power drops as RPMs go up

You can run without that anode, but you need to have at least the square one at the top of the gearcase and the two on the bottoms of the transom brackets in good shape.

If you have a seal kit and the right tools, it's not hard to change that anode; You might want to see what a service department would charge - they don't even have to take the gearcase off the motor.

If the boat ran well on the old prop, you can get a matching MW prop here at iboats. What's the part number on the hub?

 

[ststeveo]

Re: power drops as RPMs go up

ok- gt the prop rehubed and replaced it. After a dead battery and a vacation i took it out today and same problem. Did the prop respin or could there be a transmission issue here causing this?

 

[kenmyfam]

Re: power drops as RPMs go up

How much motor is in the water ????

 

[ststeveo]

Re: power drops as RPMs go up

The trim is stuck up slightly but plenty of the engine is in the water. The boat begins to plane then as the rpms go over 3000 is slips and the power drops while the rpms are up to 5000.

 

[kenmyfam]

Re: power drops as RPMs go up

How is the anti cavitation plate in relation to the bottom of the hull ??? even ? above ? or below ?? and by how much ??

 

[ststeveo]

Re: power drops as RPMs go up

5 inches from the top of the cavitation plate to the hull bottom, seven inches below surface. The engine is trimmed at an angle so the highest point is referenced.

 

[kenmyfam]

Re: power drops as RPMs go up

Is the cav plate higher or lower than the hull by 5 inches ???
How about a photo ???

 

[d.boat]

Re: power drops as RPMs go up

5 inches from the top of the cavitation plate to the hull bottom, seven inches below surface. The engine is trimmed at an angle so the highest point is referenced.

If your cavitation (anti-ventilation) plate is 5 inches ABOVE the hull bottom, I believe you're getting prop ventilation when the the boat is on plane. Without knowing more.

To me, this tells all:

The trim is stuck up slightly but plenty of the engine is in the water. The boat begins to plane then as the rpms go over 3000 is slips and the power drops while the rpms are up to 5000.

Yes, it's in the water when sitting still, but when the boat is on plane, the water only comes up to the bottom of the hull! The rest of the boat / transom is out of the water, therefore the engine is not in plenty of water! Probably only about 1/2 of the prop is in the water, therefore not enough bite.

You haven't told us when the trim got "stuck up slightly" . My guess is that the engine worked fine when you were able to trim it lower? Further guess is that the change in perforemance has nothing to do with the work done on the waterpump, but that might have been coincidental with the trim malfunction? Can you let us know about that?

 

[ststeveo]

Re: power drops as RPMs go up

2 years ago the power trim motor died. I ordered a new one and spent last summer trying to get the trim motor off and change it. After consulting with mechanics its turned out that the whole bracket would need to come off and most suggested leave the prop in the water. Thats what I did this year but the shallow water pins are up and even though the manual release for the trim piston lets it down it is not down enough. Is there a manual relief screw or valve to get the pins all the way down?

 

[ststeveo]

Re: power drops as RPMs go up

I have taken it out and tested it and as soon as it planes off it is racing and losing power. Max I can get is 8 kts before it happens- I will have to find someone who can get the motor all the way down.

 

[tashasdaddy]

Re: power drops as RPMs go up

there is a tilt release screw in a hole on the port side of the transom mount. unscrew let motor down, close screw. may take an inpact wrench to open the screw.

 

[ezeke]

Re: power drops as RPMs go up

The manual release screw is recessed in your starboard transom bracket.

In most situations the anti-vetilation plate should be parallel to the water when the boat is on plane. The a/v plate should be even with or only slightly higher than the bottom of the boat.

 

[ststeveo]

Re: power drops as RPMs go up

The tilt piston screw is on the starboard side. I need the trim pistons release screw, the two pins that start to engage the engine for the first few inches until the tilt piston takes over.