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Discussion Starter · #1 ·
I've got a Merc 25 hp 2 stroke. When I open it up to get on plane I have to do it slowly. If I open the throttle all the way then it seems to rev too high kinda like a car's clutch when it's starting to slip. But if I rev it up slowly I can get on plane, it just takes a little longer. Also how could I adjust it to take the shake/sputter out of it when I'm trying to troll slowly? Any suggestions? Thanks
 

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Not too sure about the first issue, but I do know that those shakes and shudders are there for good. After all it is a 2-stroke!

Back to the first problem, is it a tiller or console drive? Either way check your control cables under the cowling, if they have jarred loose the motor could be having troubles staying in gear under high torque.
 

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OK, even though It is a tiller, it will still have control cables that run from your handle to your throttle/shifter in your cowling. Just pop the lid and make sure everything is snug. You can also play with the throttle (motor off) and see if your cables are moving around (they should be held in place by some means.
 

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Pull your plugs and check them for spark by grounding them to the head while pulling. I dont think that if you have a dead cylinder on a 2 cyl engine that it would still get you on plane unless you have extra power.

Had a 15hp with a dead cylinder that would almost plane a 11ft alum.
 

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The first problem that you are having sounds like cavitation. The prop is sucking air when you try to accelerate too fast. Its the same problem that occurs if you jump from wave to wave with the boat and the prop exits the water (if only partially). In your case, its highly improbable that the prop is actually exiting the water, even partially. Whats probably happening is, as the transom is forced down as you accelerate, it forces air down as well, creating if you will an underwater bubble. This bubble is enough for the prop to lose 'grip' on the water, thereby increasing revs with no corresponding increase in boat speed. If you accelerate slower, the transom is not forced down to the same degree and the prop maintains its bite.

Correct the problem by looking in your o/b owner's manual. The problem will lie in two areas - either the load is not evenly distributed in the boat or the angle of the o/b to the transom is incorrect. In either case, it should be an easy fix. If I was a betting man, I'd bet on the transom angle but then again, if you have ever seen me play the slots, try the other one first.
 

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Discussion Starter · #10 ·
I like the way you think rivahman. It does seem to do exactly that. Seems like it's running out of water to grab. I have noticed too that if I'm loaded heavy it doesn't happen so much. If it's just me or me and a buddy it seems to happen more frequently. Where would I find those specs as I don't have the manual for my motor? Thanks again for the replies. p.s would it make sense this happens because the stern of my boat is very very flat?
 

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Hi Bertoli

Alot of manufacturers have owners manuals available online. What you want to make sure is that the angle of the o/b to the transom is as close to 90 degrees (vertical) as possible. If necessary, add spacers to correct the alignment. If this doesn't work to correct the cavitation, try to ensure that the load is somewhat evenly distributed in the boat. You will never get it totally right because unless you have your 300 lb mother in law in the bow all the time ( :naughty:), the stern portion of the boat will usually have more weight, with the weight of the engine and fuel tank (s) and the operator (if its tiller operated).

More weight in the bow or at least forward of the last third of the boat length will help ensure that the transom isn't pushed down too far on acceleration. Barring that, this problem usually manifests itself in hard acceleration at startup. Lay off the throttle a bit till the boat starts to plane then you should be able to apply full throttle.

Been there..done that. We all learn from experience.
 

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Hey Bertoli,
I had a problem similar to that last year. After damaging a prop and changing it out for a new one, I added a river guard. The first time out I hit it and it wouldn't pick it up and just cavatated. I had to ease into the throttle, then she picks up to plane no problem. Sure was confusing at first though because we could'nt figure out why a new identical prop would cause that, but didn't take into account the new river guard might be deflecting the water away from the prop.
Crazy D
 

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Have you had any contact with the bottom lately? or debris? All current props have pressed rubber hubs that provide shock absorption for impact and save the gears and shafts. Once they have slipped they will grap at idle but will slip like a bad clutch until you get some momentum and they will slip everytime you hammer on the throttle wether it's from a dead stop or cruisin to WOT. That or the tilt pin may be too high so the prop comes out of the water.

F.Y.I Rivahman unless the boat is a tunnel hull the boat can't force a large bubble of air into the water at take off. The boat will carve a nice trench below the surface of the water which causes the water to rush back making the boat wash behind it. The motor angle can cause the prop to come out of the water as the bow raises and the boat plows to get on step. As long as the anti-cavitation plate (flat plate above the prop) is level or below the bottom of the hull the prop won't pull air due to the anti cavitiation plate. If it ever so slightly above the bottom of the hull the "trench" the hull carves in the water will cause air to enter the prop causing cavitation. You are quite close in your thinking I just wanted to clear up the boat forcing air into the water thing. This effect doesn't occur until you're movin a 60 mph with a lot of lift or a tunnel
 
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