Mercury 25 HP 2 stroke slipping out of gear??

My 2005 25 HP merc outboard has begun randomly revving and losing speed as if it is slipping out of gear. This happens frequently on turns but also on straight runs at relatively high RPM's. It's a temporary slip; when I ease out of the turn it catches and powers up. If I keep the RPM's at about 3000 it doesn't seem to happen. I did not experience this before this boating season. ANy insights on what's going on would be appreciated.

Re: Mercury 25 HP 2 stroke slipping out of gear??

Sounds like a slipping prop hub or you are trimmed too high.

Re: Mercury 25 HP 2 stroke slipping out of gear??

schematic said:

Sounds like a slipping prop hub or you are trimmed too high.

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Knowing nothing else, I will take the former.....slipping prop hub. Mark it and run it and measure it. If the lines are no longer lined up, get yourself a new hub or prop.

Mark

Re: Mercury 25 HP 2 stroke slipping out of gear??

Texasmark said:

Knowing nothing else, I will take the former.....slipping prop hub. Mark it and run it and measure it. If the lines are no longer lined up, get yourself a new hub or prop.

Mark

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Thanks for your ideas. I'll be checking the prop this weekend.

Re: Mercury 25 HP 2 stroke slipping out of gear??

Well, I put on a new prop but had the same problem. Revs up and loses thrust at top speed. I guess I will try to adjust the trim. I'm not optimistic because this problem started this season and I didn't see it last year. I didn't run the boat a lot last year so maybe I just missed it.

Re: Mercury 25 HP 2 stroke slipping out of gear??

Ok then it looks like prop ventilation problems. Since this is a new problem and one assumes that you have had the boat for awhile and had no problems, what is different this year? If not trim or engine mounting too high on the transom, a lot of ventilation problems can be traced to something immediately in front of the prop that causes bubbles to form and the bubbles are what causes the problem that you seem to have.

What have you changed with respect to the bottom of the boat? Keel added, hook in hull, contaminated hull via barnacles or algae, deformation caused by running over an underwater obstruction?

Mark

Re: Mercury 25 HP 2 stroke slipping out of gear??

Mark,

Thanks for your suggestions. I've run out of boating season here so further investigation will have to wait till spring. One clarification though. I bought the boat used last year so last year was my first with it. I didnt use it a lot the first year but, as I said, I don't recall the slipping until this year but I didnt do a lot of wide open running so I cant say absolutely. The motor is mounted on a Bass Tracker in a factory built combination so I'm guessing that the transom mount is pretty standard. I guess I'll have to do some trial and error checks in the spring. My main concern was that it might be something internal to the motor, but that seems unlikely so I'll try adjusting the trim and see what happens. Thanks for your advice.

Re: Mercury 25 HP 2 stroke slipping out of gear??

To help us help you you have to learn the proper terminology. Sorry, but we only respond to what we are told.

Ventilation is when the prop sucks in air in combination with the water it is supposed to be pushing. Since air compresses and water doesn't, any air causes the load on the engine to drop which allows it to unload meaning that rpms increase directionally proportional to the load that has been relieved.

Trim, props, bottom of the boat conditions, hull shape , weight, weight distribution, water conditions, engine prop vs water level height, on and on and on, all contribute to the unknown "performance equation". It is really a complex operation, for higher performance operations especially, and has to be walked through step by step. As I said below, the best thing you can do is find a knowledgeable friend that is water savvy and have him get involved in your rig and help you to maximize the performance and get you the "fun machine" that you spent your money to obtain. You can talk to us all day, but unless we are there and witness the reality of your situation all we can do is guess. Sometimes we are lucky, other times just shooting in the dark.

HTH,
Mark