New Motor Questions, 1950 Johnson SD-20

[gjonz]

Glad she is running! I like these motors. Reliable, yet more challenging to run than latter OMC motors.

Enjoy! She is a nice piece!

Greg

[david-bartlett]

Greg,

There is a definite learning curve associated with these motors. Starting one is a lesson in humility if you do not follow the instructions to the letter.

After that, they are quite a handful for sure, but a nice running engine. I like it!

[garry-in-michigan]

Since the bottom of the tank is below the carburetor, it is pressurized off the back of the low speed center bearing rotary valve. They can be set up to run on the 6 gallon double line pressure tank. Carburetor/rotary valve set up similar to the Evinrude Lightfour and Zephyr. Same bore and stroke as the FD model. I often wondered how they would fit on the FD driveshaft/exhaust housing.
. . . . . . 😉

[opposedtwin]

Shouldn’t water be pumping out of the big hole of the back of the leg? Or is that only if it’s moving rapidly through the water?

[garry-in-michigan]

It uses a pressure vacuum system. Water is forced into the opening on the leading edge of the exhaust outlet by the propeller wash. The water is drawn out by openings at the trailing edge of the propeller blades. There is no "tell tail". Any leaks let air in, causing it to overheat.

[sedover]

New to the forums but have been involved with the AOMCI off and on since 1985.

I have two SD20’s that I purchased around 1973-75. One was a runner and still is and the other was somewhat cobbled but not butchered, so it provides me an excellent spares engine. I was working as a Johnson/Evinrude mechanic at the time (still am) and purchased these immediately. Never saw any others come through the dealer which had been in biz since the 40’s. I was also able to buy the few NOS parts that we had on hand.

This week I pulled the SD out for recommissioning after nearly 40 yrs of not being on a boat. It had been run in a test tank maybe 15 yrs ago. At that time it was run, fogged, the fuel system drained, and put away.

The sea trial was on a 14′ Sea Nymph and it ran great. I brought along an infrared temp reader to monitor the cooling system performance which I had no real experience with. As a matter of fact it had never been run on a boat at planning speeds while in my possession. Strong runner and the separate mag and carb controls allows one to really dial it in.

From what I could decipher the front intake only feeds the exhaust cavity coming up from the bottom and then out the discharge hole.

The intake at the exhaust snout eventually ending at the aft side lower gear housing assisted by the prop cools the powerhead. Powerhead temps ran in the 120’s to 140’s with the "dimple" (lower bump of exhaust manifold0 hit 200-220.

Steve

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