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Professional Rebuilding of Carburetors and Magnetos for Antique Tractors
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Adjusting the Carburetor

Posted by Mike Shelby on 2/15/2011
Many people often ask about the correct settings on a John Deere one-barrel and two-barrel carburetor.
The tips given here assume the carburetor has been professionally cleaned and reconditioned, the float setting is within specifications, and the gasoline is fresh. Also, the cylinders have good compression, and the ignition system is in good working order.
Given these conditions, carburetor adjustments once made properly, do not need changing.
First of all, for adjustment purposes, the engine must be warmed up to the point that it will run without the choke on. Obviously, the choke is needed to get going in cold weather.
NOTE: With a properly reconditioned carburetor, more than a couple of flywheel revolutions at full choke may flood the cylinders. Some people prefer 7/8 choke especially on hand crank tractors.
Even after normal operating temperature is reached, the carburetor settings may still need fine tuning. Using the choke to keep the engine running causes the carburetor to supply fuel to the air stream through the main nozzle, not through the idle circuit(s).
When the entire idle circuit is 100% clean and the IDLE needle adjusted properly, the engine will run at low and high idle RPM with the load needle closed and the choke off. Won't pull the hat off your head, but will run.
The initial setting of the IDLE needle on a JD single barrel is 1-1/4 to 1-1/2 turns out. On a two barrel, the initial settings are the same.
The initial setting of the LOAD needle on a JD single barrel is 1 turn out. While the optimum LOAD needle setting can be determined with the tractor on a dynamometer, you can do very well by driving the tractor and watching the exhaust. Especially driving uphill, black smoke means the load mixture is too rich (turn in). No smoke and stumbling (won't pull) means the mixture is too lean (turn out). Usually a couple of clicks either way is sufficient.
NOTE: Do NOT attempt to adjust the carburetor on a moving tractor.
A JD two-barrel does not use a load needle like a single barrel. The two barrel has a load compensation screw in the center of the bottom stem; the usual setting is 2-3/4 turns out. The load compensation adjustment is especially useful for cold weather operation.
These settings will get the engine started. After the engine is warmed up, many people find that running the engine at half throttle is a good place to adjust the idle needle(s). This seems to be a good compromise for a parade tractor. If the tractor is a working tractor, you may want to adjust the idle needle at top RPM. For a tractor used in slow races, adjust the idle needle at low RPM.
If the IDLE needle is adjusted too lean, this will cause governor hunting and uneven running. If the IDLE needle is adjusted too rich, continuous black smoke will be evident out the exhaust stack. If you can't get the adjustment to come out just right at idle, there may be too much air leakage around the throttle blade causing fuel to be drawn through the main nozzle.
Remember, these carburetors are half a century old, and most are not perfect anymore.
The idle needle(s) on a JD two barrel should be adjusted independently of each other for each cylinder by temporarily disconnecting the spark plug wire, and adjusting the idle needle for the firing cylinder.
With the carburetor adjusted properly, the engine should accelerate smoothly (usually with a puff of smoke out the stack), and run smoothly at high throttle and smoothly under load.

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 Adjusting the Carburetor
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