1990's Yamaha 2 Stroke 50HP engine ID and lower unit assembly

[seasponge]

Hi there,

I have an old Yamaha 2 stroke 50HP motor. The nameplate has 50DETO 6H5 L on it. I am forever finding it difficult to get information on it when looking up parts and manuals. It is a model sold in Australia, but I believe the same model may have been sold in the US under a different model number.

I am trying to confirm the year and US model number and am looking for an exploded diagram of the lower unit assembly.

I have some questions on adjusting the shift rod, but would like to see an assembly diagram before i attempt it.

thanks.

[boscoe99]

The numbers that follow the letter L are needed. They were on both the main identity plate (the one with the 6H5 L) and on the round alternate label that was affixed to the block.

More than likely it will be the equivalent of a US model 50TLRU.

1996 50TLRU Yamaha Outboard Parts

[seasponge]

thanks for the reply Boscoe. The full name plate is 50DETO 6H5 L 412466

[seasponge]

More than likely it will be the equivalent of a US model 50TLRU.

1996 50TLRU Yamaha Outboard Parts

Thanks. I had an issue after removing the lower unit, and I found this thread http://www.yamahaoutboardparts.com/f...p-th22459.html which help me put it back together.

I had removed the lower unit to flush out the water cooling system. After I put it back together and got it in the water, we had problems getting it into reverse gear. Took it out of the water. After a search of the internet for solutions, found the linked thread above and followed the steps below:

we then put the throttle in reverse (it did not engage), disconnected the shift rod (not sure of the correct term), pushed the lower part of the shift rod down whilst turning the prop until it engaged into reverse gear, then reconnected the shift rod with the barrel nut and locking nut. Checked it with the muffs on, then put it in the water. The problem seemed to be solved, we can now engage reverse, it sits nicely in neutral, and engages forwards ok.

The problem now is that whilst in forward, the motor will now make quite a heavy clunk/knock/shudder. It doesn't happens whilst engaging, it happens after that as we throttle up. As we start to increase speed, we might get one clunk, then its ok, and as we get towards full throttle, the knocks happen continuously till we back it off.

I'm not sure whether its related to what we did when taking the lower unit off and fixing the shift rod, or whether its just coincidence.

It has been suggested to me that the shift rod needs adjusting. Following the steps in the thread, where can we make fine adjustments, if that is what is needed.

[boscoe99]

The 25HP model uses a completely different shift mechanism than does the 50HP model. No comparison between the two can be made.

The 25HP model is shifted when the shift rod is raised or lowered. The 50HP model is shifted when the shift rod is rotated clockwise or counterclockwise.

With the 25HP model the shift rods are coupled using a barrel and jam nut.

With the 50HP model the shift shaft in the lower unit interfaces with the shift rod in the mid section. Both have splined ends. One is female and the other is male. The male simply slides into the female section when the lower unit is installed.

I am confused by your mention of the use of a barrel nut on your 50HP model.

[seasponge]

I called a Yamaha dealer with the name plate details. They told me that it is actually registered as a 1984 model. DETO was the australian model number. If that is the case, the only model I could find on the Yamaha parts website is the 50ETLN. So from what I can determine, that is the model I've got. This is the lower unit assembly drawing I can find for it

The shift rods are definately coupled with a barrel nut and lock nut, and the mechanism appears vertical not rotational.

[boscoe99]

I stand corrected. I misread some data I have and thought your serial number was produced during the mid 1990's. It now appears that your model is from the circa 1984 ~ 1985 time frame, as best as I can tell. See data below. And yes, it does use a barrel nut to attach the two shift rods.

1985 50ETSK Yamaha Outboard LOWER DRIVE 1 Diagram and Parts

The best way that I know to make the connection is to first put the lower unit into reverse gear by pushing down on the shift rod as far as it will go. Make sure that it does not slide upward. Then, shift the motor into reverse gear. Connect the two rods together, first the barrel nut and then the jam nut and they should be properly connected together.

[seasponge]

thanks Boscoe, what you've described is exactly what we had done.

There is no problem engaging, the noise/clunk occurs after forward is engaged and we are already moving, and then again nearing full throttle. When the noise/clunk happens, it doesn't seem like its slipping out of gear, there is no excess revving or loss of speed, just a large clunk then keeps going.

What other fine adjustments can be made to the shift rod and how? From the assembly diagram, i can't really determine how the movement of the shift rod up and down engages forward, reverse or neutral.

By the way, are you able to explain what the assembly items 59-63 are? It appears to be a hose connecting to nowhere. On my motor, the nipple item 61 has snapped off inside the hose, but I can see anywhere for it to connect to, nor know what it is.

[boscoe99]

There are no other shift shaft adjustments that I can think of. And, I have not a clue as the noise that you are hearing.

Here is a link to the drawing that shows the control mechanism and other half of the shift rod system.

1985 50ETSK Yamaha Outboard CONTROL ENGINE Diagram and Parts

The other parts that you mention are for the pitot tube connection to a hose that will run from the motor up to the helm for a speedometer. A small hole in the leading edge of the lower unit is the pitot tube. Water impacts that hole and first creates water pressure which is then converted to air pressure. Air pressure up the hose to the gauge activates the speedometer.