Yamaha 40-HP Gear Shift

Optimizing the performance of Boston Whaler boats
bobross
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Joined: Fri Oct 30, 2020 6:32 pm

Yamaha 40-HP Gear Shift

Postby bobross » Fri Feb 05, 2021 3:38 pm

I bought a 1988 Yamaha 40-HP two-stroke-power-cycle for use on a Boston Whaler boat I am restoring. The engine seller ran it [for] me in his driveway with the lower unit in a barrel of water, and it seemed to work fine. I'm am almost sure he shifted it into forward and reverse during the test.

Later, at home, I noticed the engine steering arm [tiller] was rusty, and thought about replacing it. It's a complicated job involving removing the power head and the lower unit. I got as far as partially removing the lower unit, and decided that this was more work than I wanted to do. So, I sanded off the rust and painted it.

When I re-connected the lower unit, I may have screwed up something. I have only tested it on the stand, without firing up the engine. The symptom is that when shifted into forward, the prop spins freely by hand and there is a "clacking" rotational sound. When I turn the crankshaft by rotating the flywheel clockwise, the prop turns clockwise as it should. But, I can also turn the prop clockwise by hand easily, as easily as I can turn it when it is in neutral. With the addition of the clacking noise. When this is going on, the crankshaft is not moving and the engine is not turning over. In forward, the prop will not turn counterclockwise by hand.

Neutral seems normal. [Something, possibly the propeller] turns freely without any noise.

Reverse seems normal. The propeller will turn counterclockwise when the flywheel is rotated in a clockwise direction. It will turn by hand if you overcome the resistance of the compression of the engine, but does not turn freely, and doesn't make the clacking noise. When in reverse, will not rotate clockwise by hand.

Hard to tell if the sound is coming from the mid-section or the lower unit. Sounds like both.

Just to be sure, I took the lower unit off completely and then re-attached it, following the manual. The problem problem [that is, the observed behavior] did not change.

I don't know if this is likely that the problem is upper, where the driveshaft connects to the crankshaft, or lower, where the shift into forward gear may be not happening fully.

[Apparently is soliciting advice on "the problem" and its remedy.]

jimh
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Location: Michigan, Lower Peninsula
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Re: Yamaha 40-HP Gear Shift

Postby jimh » Fri Feb 05, 2021 8:23 pm

Get the Yamaha factory service manual. There should be a procedure for adjusting the gear case shift linkage. Typically the linkage to the gear case is a rod that extends vertically from the gear case upward to the engine mid-section. There typically is an adjustment for that shaft length.

Have you actually started the engine and checked the operation of the shift with the engine running, observing what motion occurs with the propeller with each gear shift position setting?

Usually in an outboard engine there is a ratchet mechanism that allows the propeller shaft to be able to turn faster than the engine drive shaft.

bobross
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Joined: Fri Oct 30, 2020 6:32 pm

Re: Yamaha 40-HP Gear Shift

Postby bobross » Sat Feb 06, 2021 8:26 pm

Jim, I thought about what you said about the prop being able to spin faster than the drive shaft due to the ratchet. The sound was like a ratchet. I thought, maybe if it is under load (in gear)while the engine is running, there will be no clickety noise, just like there is no noise when I turn the prop (in gear) using the flywheel when the engine is not running.

So, I took your advice, hooked up the battery, fuel and water hose and started the motor. Once running, shifted it into gear and it seemed fine with no surprising or concerning noise.

So I guess this was a normal noise under the circumstances. In all my boating, I had never removed and reattached a lower unit and wasn't sure what to expect. So, I was cautious in the face of previously unknown noise.

Thanks for the advice.

dtmackey
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Re: Yamaha 40-HP Gear Shift

Postby dtmackey » Sun Feb 07, 2021 1:08 pm

Yamaha does not utilize a shift system where shaft length is a concern like Evinrude does. While Evinrude uses a shaft to push or pull (up/down) to shift, Yamaha uses a shaft that is actuated by a rotational movement. When the lowert unit is off the motor, accidental movement of this shaft or shift control can cause a misalignment of the splines on the shift rod which in turn can cause a shifting mechanism misalignment .

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jimh
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Re: Yamaha 40-HP Gear Shift

Postby jimh » Sun Feb 07, 2021 3:31 pm

I forgot that when Yamaha begin to copy outboard engines made by Evinrude and Mercury, Yamaha tended to copy the power head designs from Evinrude and the gear case designs from Mercury.

In some situations the propeller shaft can be made to spin faster than the drive shaft would normally turn it. For example, if the boat is underway in forward at some significant speed, and if the engine throttle is immediate cut back or the engine is shut off, the flow of water past the propeller will continue to spin the propeller while the drive shaft is stopped or is turning very slowly. If this occurs you can hear an odd noise—the dog clutch ratcheting.

dtmackey
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Re: Yamaha 40-HP Gear Shift

Postby dtmackey » Mon Feb 08, 2021 8:50 pm

jimh wrote:I forgot that when Yamaha begin to copy outboard engines made by Evinrude and Mercury, Yamaha tended to copy the power head designs from Evinrude and the gear case designs from Mercury.



There was a good amount of copying at a surface level design, but after working on Yamaha and OMC motors for 3 decades, I've come to learn that Yamaha imrpoved the many shortcomings of both Merc and OMC designs they used as a starting point, to become a dominant player.

I'm very please with my 250 Etec, but in earlier years, Yamaha had better electronics, carbration was some of the best and oil injection systems that didn't fail like VRO, not to mention crankshafts on 60 degree vs 90 degree offsets.

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