Tuning a 1939 Johnson 5hp #AT-39

[arttheaggie]

I am trying to get this motor tuned up to put on my 17' Sailboat.
I completely disassembled it, cleaned the carb, installed new spark plugs, regreased the lower unit, cleared all lines (fuel and water). I only has one needle valve on the carb (I can get it to start and run at higher speeds but can not get it to idle, it does this best at 3-4 turns open from all the way closed). There is also a needle valve behind the fuel tank that looks like it controls the lubrication of the crankshaft. It has to be between 2-3 turns from closed. I have tried various fuel mixtures from 24-1 up to 50-1 and cannot seem to get it to a low speed idle. I have had it running for over 15-20 minutes at a time with the head tempurature only reaching upto 180 degrees at the hottest measured with and infrared temp sensor. Any help with this would be greatly appreciated.

Art Self

 

[GeneH]

Re: Tuning a 1939 Johnson 5hp #AT-39

Art, The correct ratio on this engine is 16 to 1. Anything with less oil than this will fry the engine. The little wheel with the numbers on top of the carburetor is the high speed mixture adjustment. The slender needle coming down behind the right side of the fuel tank as viewed from the front is the low speed mixture adjustment. The carburetor on this engine is really made in two separated parts. Very different! You cannot adjust "the oil going to the crankshaft" on this engine other than by changing the ratio while you are mixing gas and oil. There is a website for older engines run by AOMCI. Try a search on that if you don't get what you need here. Good luck!

 

[samo_ott]

Re: Tuning a 1939 Johnson 5hp #AT-39

Yes, 16:1 Dont fry that old beauty! And do try www.aomci.org for better help.

 

[arttheaggie]

Re: Tuning a 1939 Johnson 5hp #AT-39

Thanks for the input. I just got back from testing it at the dock. After reading more I had went back to a 20-1 ratio. I ran it for over thirty minutes without it dying. Figured out that the needle behind the tank had something to do with the low speed idle. Still have not dialed it in on the low side yet but I am getting closer. I am off to check the other site out that you all mentioned. Thanks again.

Art

 

[GeneH]

Re: Tuning a 1939 Johnson 5hp #AT-39

Art, No- 20 to 1!!!!! Yes- 16 to 1!!!! That needle is the low speed mixture adjustment. These engines will idle down to putt putt putt when everything is right.

 

[Chinewalker]

Re: Tuning a 1939 Johnson 5hp #AT-39

Those are really neat motors. I've had a couple that were tuned well enough that you could start them by hand - literally! I could grab the rope sheave with my fingertips and give it a sharp twist to start it! Very nice runners...
- Scott