Tractor Sputtering - 1939 M

Good Morning,

I recently had my carburetor rebuilt and I have it put back on the M. My understanding is I need to adjust this knob with a flathead screw driver to eliminate the black smoke and to help the tractor run smooth.

My question is should I turn it all the way in and then turn it so much out? I tried turning it some last night and I couldn't find the sweet spot.

Also, I noticed when I turn the tractor off it leaks gas out of what I believe is the "weep hole" that is normal correct?


Thanks for the help!

first the points and timing must be set to factory spec. you cant set the carb till the ignition side is all good. even includes a valve setting.
on the carb set the main screw on the bottom to 2 1/2 turns out from its seated position. the set the mixture screw 3/4 turn out. then set the idle screw to 400 rpm. that engine will run as smooth as can be, and start with a touch of the starter. I use these settings on the work bench and takes very little fine tuning. it is kinda normal to have some gas leaking but when tuned correctly its not noticeable. you will set this more on start up when using the choke. your black smoke means your running rich so yes you will have gas leaking. that main jet on the bottom can be turned out 1 turn or so for heavy pulling but for yard work don't want to be fouling plugs.

Dripping gas out the weep hole is normal following a failed start, or sometimes after a shut down, but it should stop after a few drops. If it continues, or gets worse as it sits with the fuel valve open, the carb is flooding.

Flooding is usually caused by trash in the needle valve. If the tank is contaminated, or flaking rust, it will get in the needle valve and hold it open. It only takes a tiny speck to cause it to leak.

Be sure the screen is in place in the sediment bowl. Adding inline filters can have mixed results. Gravity systems don't play well with paper filters, too restrictive.

Something to try, close the fuel valve, drain the carb or run it empty, then open the fuel valve. Sometimes the inrush of gas will flush out the needle valve.

How is the air cleaner? If there is a wire mesh inside the canister, it needs to be periodically cleaned. If packed with dirt it will be restrictive. Also look for mud dobber nests, mouse nests.

Once the flooding is corrected (if present) a way to get the main jet close to adjustment, with the engine up to temperature, everything else tuned and functioning well, idle the engine down, open the throttle quickly, listen for the response. Start leaning (turning the high speed adjusting screw in) 1/4 turn at a time. Repeat the test until the engine falters on acceleration, then back it out, repeating the test until it will take sudden throttle without hesitation. A puff of black smoke is normal. That will get you in the ball park, it may need fine adjusting depending on the load, temperature, humidity, etc.

The leakage out the weep hole is not normal, common, just not normal. The Load screw (bottom front
straight slot) is a supplement to the actual main jet. The screw at 3 to 4 turns out from its full
in location should be OK to get where you are going. There are differences between the main jets
of Distillate carbs, and gasoline only carbs, but not to worry. Do not bottom the load screw into
its seat like it was a deck screw. Gently feel for the seat. The tractor will either barely run,
or fail to run with it all the way in. When it is out too far it will make black smoke out the
exhaust. If yours makes black no matter what, and weeps as noted, it could have one or more of the
following things contributing: the float level could be set too high, the needle and seat
controlled by the float could be leaking. The seat can leak where it threads into the chamber top.
The weeping needs to be fixed before any further adjustments.
With it warmed up well enough (uncomfortable holding your hand on the block) set the throttle at
High Idle (wide open, not under load). Adjust the load screw inwards until the engine begins to
act starved. turn outward until you see the first evidence of dark smoke. Turn in until that just
goes away. Now push the hand throttle lever to slow idle, and let it stay there for 10 seconds.
Rapidly pull the lever to high idle (wide open). If the tractor stumbles and doesn't make clean
transition, open the load screw 1/2 turn at a time till it does.
This will get the mixture close. Next, pull a tillage implement in soil to load the tractor. If it
shows a very tiny amount of dark smoke it is near perfect. Incremental turning inward, 1/4 turn
from there might clear that small smoke, as long as it doesn't make it stumble on opening the
throttle. I have not discussed the Low Idle mixture, it is in the archives in many locations. Jim

Did your rebuilder get the float height adjustment correct? If not the problems you described will be present.

Do this, start it up, turn off the gas. As it runs does the engine smooth out, or act up more, or no change until it runs out of
fuel?

(quoted from post at 08:49:44 04/01/20) Did your rebuilder get the float height adjustment correct? If not the problems you described will be present.

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Thank you for the reply. I plan to mess with the screw a little today to see if I can get it narrowed down. If I can't how would I know if the float is at the correct height? Is that something I can do or would the rebuilder need to do that?

Thank you for the reply. I will be trying this in a couple hours. Hopefully that is all it takes and it'll be good to go!

Least invasive: Shut off the fuel at the tank. Drain the bowl (plug at the bottom). Remove the bowl
of the carb while it is on the tractor. Get your measuring tools (caliper or small steel ruler).
Gently lift the float with your finger so the needle seats, don't push up hard) make the
measurement. I do not know the spec. The tiny tab that pushes up on the needle is gently bent to
adjust the level. Needle nose pliers (2 pair) make it happen. Don't bend the "arms". Jim

1 5/16 from the flat of the top to the bottom of the float.

Thanks. Jim