1948 Johnson TD 20 rough idle

Mikej 41

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Jul 10, 2013
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My 5HP TD 20 will not idle down. If I go too far down it quits. What can I do to correct that?
Thanks,Mike
 
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JasonAych

Petty Officer 1st Class
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Aug 10, 2008
Messages
274
Re: 1948 Johnson TD 20 rough idle

My 5HP TD 20 will not idle down. If I go too far down it quits. What can I do to correct that?
Thanks,Mike

There is a what is called a venturi (mini carb) at the front of the engine on the crank case. There is an adjustment needle (L shaped rod). Idle air/fuel mixture is adjusted there. I feel for it under the front of the cowl. They are a little bit of a challenge to get right on. I shut the low speed jet off (turn it all the way in gently) and get the high speed adjusted favoring lean then open and adjust the low speed at idle. The reason I do this is both seem to affect each high and low speed. Its just the higher rpms are more sensitive to the high speed adjustment and low rpms/idle are more sensitive to the slow speed adjustment. There are other methods out there including what Johnson recommended.
 
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HighTrim

Supreme Mariner
Joined
Jun 21, 2007
Messages
10,486
Re: 1948 Johnson TD 20 rough idle

5/8 out from seated on the low is almost perfect on all of my T and H series of motors.

Also, have you been through the carb? Lines replaced? Spark jump a 1/4" gap on a tester with a bright blue ZAP!!?? Running on both cylinders? What is the compression?
 

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,195
Re: 1948 Johnson TD 20 rough idle

Begin at 5/8 turn. From there it is very touchy. Turn it in till it spits in your eye, then back it out till it quits spitting. And don't be looking at it, so you don't get gas in your eye.
 

Mikej 41

Recruit
Joined
Jul 10, 2013
Messages
4
Re: 1948 Johnson TD 20 rough idle

There is a what is called a venturi (mini carb) at the front of the engine on the crank case. There is an adjustment needle (L shaped rod). Idle air/fuel mixture is adjusted there. I feel for it under the front of the cowl. They are a little bit of a challenge to get right on. I shut the low speed jet off (turn it all the way in gently) and get the high speed adjusted favoring lean then open and adjust the low speed at idle. The reason I do this is both seem to affect each high and low speed. Its just the higher rpms are more sensitive to the high speed adjustment and low rpms/idle are more sensitive to the slow speed adjustment. There are other methods out there including what Johnson recommended.

Thanks Jason. My Dad bought this motor in 1957. It was at our summer place and got a lot more garage time than water time. I will try what you suggested. Thanks again.
Mike
 

Mikej 41

Recruit
Joined
Jul 10, 2013
Messages
4
Re: 1948 Johnson TD 20 rough idle

5/8 out from seated on the low is almost perfect on all of my T and H series of motors.

Also, have you been through the carb? Lines replaced? Spark jump a 1/4" gap on a tester with a bright blue ZAP!!?? Running on both cylinders? What is the compression?

I have cleaned the carb. Float is good. I have not checked the other items. I will do that. What should the compression be?
Thanks,
Mike
 

seahorse5

Rear Admiral
Joined
Jan 24, 2002
Messages
4,698
Re: 1948 Johnson TD 20 rough idle

If the motor does not have enough oil in it, and/or the brass bushings are worn, it will have a hard time idling slow. Carboned up or stuck rings can do the same thing. Use 1/2 pint per gallon oil-gas ratio and inspect the rings through the side cover to see what shape they are in. When in good condition and tuned up, those motors will tick over beautifully.
 

HighTrim

Supreme Mariner
Joined
Jun 21, 2007
Messages
10,486
Re: 1948 Johnson TD 20 rough idle

If the motor does not have enough oil in it, and/or the brass bushings are worn, it will have a hard time idling slow. Carboned up or stuck rings can do the same thing. Use 1/2 pint per gallon oil-gas ratio and inspect the rings through the side cover to see what shape they are in. When in good condition and tuned up, those motors will tick over beautifully.

Very good point here. These motors NEED the oil. Don't let Bob the Builder from down the street tell you that with todays modern oils 50:1 is ok, even if it is Amsoil
 
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