Revving too high under acceleration.

[ksubigbuck]

Motor is a 1977 115. I took the boat out for it's first cruise today, after months of repairs to the boat and motor. It started right up and I idled out onto the main lake. I then slowly applied throttle and the boat was almost on plane at 10 mph when the motor suddenly revved really high and the boat came off of plane. I tried to get on plane several times and it did this each time. It also looked like it was trying to shoot a roster tail or somehting. It felt like the prop was slipping, so I decided I would anchor and fish for an hour then go back to the ramp. After fishing I thought I would try one more time. This time instead of slowly throttling up I just hammered it. This time it didn't slip, and I was going 40 mph in no time. I cruised for a minute then turned around. I slowly throttled up and increased to 40 mph again without any slipping.

Anybody have any idea what could be causing this? I checked out the prop and it is very tight, I don't think it was slipping. Could it be popping out of gear? I didn't hear any gears clicking or grinding. Maybe it was just heavy cavitation because the motor was trimmed to high or something?

Thanks,
Hunter

 

[jbjennings]

Re: Revving too high under acceleration.

It does sound to me by your rooster comment that the motor was possibly trimmed wrong. Is your cavitation plate on your lower unit parallel to the bottom of the hull? If it is jumping out of gear (which I doubt it is because you said it stayed in gear once you got it on plane) it will feel like you hit a stump. That isn't how you described it.
How does the boat feel when you gas it slowly? Is it pushing the bow up more than normal? If so, it may be trimmed to high.
Got pics of the motor fully trimmed down?
JBJ

 

[ksubigbuck]

Re: Revving too high under acceleration.

The cav plate is parallel to the bottom of the hull when it is trimmed almost all the way in. It didn't feel like I hit a stump, it was a smooth transition. The bow did come up a couple of feet, but nothing out of the ordinary for most boats.

I can take a pic of the motor/transom, but I would have to find my camera...no idea where it is.

Thanks for the help,
Hunter

 

[Mas]

Re: Revving too high under acceleration.

Did you change the distribution of weight after fishing that might cause the stern to rise? Maybe that "big one" that you caught??

Also, you may want to place an adjacent mark on your prop & hub to see if the marks move after running. If the marks move, your have a spun prop..which can be fixed.

MAS

 

[ksubigbuck]

Re: Revving too high under acceleration.

Thanks Mas. No, the weight distribution did not change (I didn't even catch any fish).

I'll mark the prop and hub and see if they move. But, like I said earlier the hub seems perfectly tight.

Thanks for the help,
Hunter

 

[tashasdaddy]

Re: Revving too high under acceleration.

hubs will slip then not slip, but they finally give out.

 

[F_R]

Re: Revving too high under acceleration.

The slipping described is exactly like ventilation. High mounted, trimmed out, and you'll get it. Ever see a race boat take off? Some of them are almost impossible to get going.

 

[ezeke]

Re: Revving too high under acceleration.

How far out from the transom is your thrust rod set? What propeller are you using?

 

[dieselcat]

Re: Revving too high under acceleration.

Sounds like to me you have a spun hub on your prop.If you back your boat into the water and leave it on the trailer,start it and put it in gear and try to go farward and it still revs up like you say it's doing,then i would put my money on a spun hub!hope this helps

 

[theviolentvicar]

Re: Revving too high under acceleration.

I would have thought this was a classic ventilation scenario. If the behaviour you describe is repeatable, then its seems to me to be unlikely to be a spun hub - the more power you apply to the prop, the more likely it is to spin, and you are getting the exact opposite. Hard acceleration changes the trim of the boat coming out of the hole as compared to gradual acceleration - it seems likely to me that during hard acceleration you are altering the boat's trim at some transitional phase (between in-the-hole and full plane) so that the ventilation just isn't happening.

 

[ksubigbuck]

Re: Revving too high under acceleration.

I think it might be because the motor is mounted kinda high. The cav plate is about 1/4" to 1/2" above the bottom of the hull. When I had the motor trimmed all the way in it had no problem getting on plane. In fact, it took off like a bat out of hell! I'm going out again tomorrow, I'll try to get the prop to slip on the trailer, and try my backup prop if I need to.

Thanks for all the replies,
Hunter

 

[ksubigbuck]

Re: Revving too high under acceleration.

How far out from the transom is your thrust rod set? What propeller are you using?

I don't think I have a thrust rod, it is PTT. I'll have to check on the prop.

 

[tashasdaddy]

Re: Revving too high under acceleration.

don't do it on the trailer good way to break a winch post, and put the bow into the back of the tow vehicle. also horrible for the boat ramp.

 

[The_Kid]

Re: Revving too high under acceleration.

I don't think it's a spun hub. Once they spin they keep doing it, they don't fix themselves.

If you have power trim it may have just been trimmed out to much, or it may have been as simple as just having a stick lodge against the lower unit when you were idling out. I've had this happen quite a few times when leaving a fishing hole that was full of brush. Most of the time I just shift to neutral then reverse for a couple of seconds to dislodge it, and everything is fine after that.

 

[ksubigbuck]

Re: Revving too high under acceleration.

I'm heading back out right now. I'll let you know how it goes.

Thanks,
Hunter

 

[jay_merrill]

Re: Revving too high under acceleration.

I've seen clutch dogs that are failing do what you describe - sometimes they'll lock in and sometimes they won't. I've also seen them lock in when power is applied rapidly, although I have seen them fail to lock in under that scenario just as often.

Go it another go, paying attention to trim and ventilation issues and let us know what happened.