1962 Evinrude Lark IV (Gear Slipping in half throttle)

[J.R.]

I have a 1962 Evinrude lark 4 (IV)
Manual Shift

Runs like a dream motor however at lower than half throttle it never misses a beat and pushes my 1977 17' Basshawk boat just fine. once I reach 1/2 throttle it starts to slip and higher the rpm the more it slips so I must run at less than half throttle to run perfectly. does not push the gear shift out of gear or jump out of gear and make noises like clutch dog slipping.

There is no Noise, or anything just quits pushing a sec then pushes and stops ect. of course when it slips the rpm increases.

I would really appreciate a good post with Possible Issues and solutions.

Example:

1. Spun prop ( The Rubber or Metal teeth in your prop get wore and slip on the Hub of the boat)

A: Mark the position of the prop on the hub and after it slips a bit remove from water and check the marks.
A:Take Propeller to prop shop for having new rubber put in it

*of course I read another post of same issues a gentleman was having and gathered that info.

the problem with spun prop is there is a rod or pin that goes through the hub and prop on this model engine the the only way to spin the prop i believe is to shear the pin please correct me if im wrong .

You Guys were exact on the coils ect and where i messed up magneto wires when changing ignition switches and i am happy and thats fixed now if gear quits slipping ill be ecstatic with joy.

 

[J.R.]

Re: 1962 Evinrude Lark IV (Gear Slipping in half throttle)

I want to make sure i Note that only under pressure does it slip runs all day in barrel w/o incidence regardless of rpm.

This Motor will even idle when throttle is at minimum and never sputter, only issue is the forward gear slipping at 1/2 throttle and never tried in reverse to confirm if its only forward or not.

 

[lindy46]

Re: 1962 Evinrude Lark IV (Gear Slipping in half throttle)

Does sound like a spun hub to me. The center, brass part (hub) actually comes loose from the prop and will actually spin without the prop turning. You can remove the prop and test it out of the water. Put a small Phillips screwdriver through the shear pin holes, hold the prop firmly and try turning the hub with the screwdriver. You will probably find that the hub turns in the prop body. I just had one do the same thing - got it re-hubbed for about $40 and all is well again.

 

[Chris1956]

Re: 1962 Evinrude Lark IV (Gear Slipping in half throttle)

JR, If the gears were slipping, you would hear a loud bang. Mark the inside of the prop and hun with a sharpie. Reinstall and run it. After it slips check the match marks. You will see if it is the hub.

It could also be ventilation.

 

[F_R]

Re: 1962 Evinrude Lark IV (Gear Slipping in half throttle)

Does sound like a spun hub to me. The center, brass part (hub) actually comes loose from the prop and will actually spin without the prop turning. You can remove the prop and test it out of the water. Put a small Phillips screwdriver through the shear pin holes, hold the prop firmly and try turning the hub with the screwdriver. You will probably find that the hub turns in the prop body. I just had one do the same thing - got it re-hubbed for about $40 and all is well again.

The theory here is correct, however the practicality is all wrong. Only in very extreme circumstances could you ever turn it with a phillips screwdriver. Remember it has to restrain the power of 40 horses without slipping. No way could a man with a screwdriver develop that much power.

As everybody has suggested, make a mark on the plastic prop nut cone and another, aligning, mark on the propeller. Run the motor till the suspected slippage occurs. Then stop and check the marks. If they are no longer aligned, it slipped. (The nut turns with the shaft, inside the propeller)

 

[J.R.]

Re: 1962 Evinrude Lark IV (Gear Slipping in half throttle)

Please Elaborate on The Ventilation, How do I check that and what exactly is it? I have seen a mention of Ventilation/ Cavitation in other forum threads, I do appreciate you guys so dearly assisting me.

I am a Beginner at lower unit issues and in-particularly on Motors that existed decades before my Birth.


So Use baby Talk and take little steps with me and I will succeed LOL

I do need you to elaborate though and explain what ventilation is, how to check it, and possible solutions.

 

[J.R.]

Re: 1962 Evinrude Lark IV (Gear Slipping in half throttle)

I found the following at http://www.boatfix.com/how/props.html

I browsed the net and Forums looking for info on ventilation/cavitation




Ventilation and Cavitation
Ventilation occurs when surface air or exhaust gases are drawn into the propeller blades. The load on the propeller is reduced by the mixing of air or exhaust into the water steam causing over revving.

Anti-ventilation Plate: Large plate cast into gearcase housing directly above propeller. Helps reduce surface air from being pulled into blades.

Cavitation: The aeration (bubbling) and boiling effect of water caused by creation of a low pressure area. Generally caused by a solid shape (propeller blade) passing through the water, in such a position and speed, that a low pressure area is formed due to the inability to move through the water in nonresistant manner. An example is, a propeller blade that has a rough edge would not cut efficiently through the water, thus creating a low pressure area. If the pressure drops below the vapor pressure, a cavitation bubble will form in that region. These bubbles will collapse when they reach the higher pressure region of the blade. This causes a rapid change in pressure and can result in physical erosion. You may notice burns (erosion) at some area on the face of the blade.

Common rules of thumb:

Ventilation can lead to excessive slippage.
Cavitation can lead to ventilation and/or slipping.

 

[J.R.]

Re: 1962 Evinrude Lark IV (Gear Slipping in half throttle)

Now i learned what cavitation is and ventilation is:

How would I check or repair this issue(s)

Also: Does The position of the propeller in comparison to The bottom of Boat have any bearing on the ventilation/cavitation issues?

should all the hub, all the blades be completely below the bottom of the boat ?

 

[tx1961whaler]

Re: 1962 Evinrude Lark IV (Gear Slipping in half throttle)

Check for the spun hub first.
You won't be ventilating unless your motor is mounted too high

 

[J.R.]

Re: 1962 Evinrude Lark IV (Gear Slipping in half throttle)

ok, I changed Propellers to a slightly smaller ( less Diameter) prop. I monitored the Motors position Closely, The depth of the Motor, and the Boil of Water formed from the propeller running both in idle and forward at higher rates of throttle.

With the smaller prop i get throttle a bit higher before the slipping per say happens.

However it does occur at just over half throttle, So I am watching the boil an dthe depth of the propeller, I believe one The Motor is indeed not deep enough in the water , The water boil while Moving forms a nice space between the motor and lake water like Parting the water next to the engine.

Based on prior forums If the motor is indeed a few inches shallow and I place a set of reg 2 piece fins just above the propeller would that prevent the prop from geeting pockets of ait down to it?

 

[J.R.]

Re: 1962 Evinrude Lark IV (Gear Slipping in half throttle)

I also want to point Out I have a 1962 Evinrude Lark IV, on a 1977 Basshawk brand Boat Manufactured in Olive Hill, Ky. I know there is no way short of a Jack Plate/ Mounting Plate to change the depth of the motor Mounted in the Boat So if I can prevent the cavitation or Ventilation with the given conditions I would Like to know. I also know that My lower unit has over 16 pounds of pressure in it w/o any seals leaking i had those tested before i even hit the water as well as lower unit oil change.


Help is always appreciated Thanks Guys you make my boating experience so much more enjoyable making self corrections and that an achievement for me.