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Several observations here -- When a tractor is known to have been running well, and then suddenly acts up, I seriously doubt that a mechanical adjustment has changed all by itself. And so before we tear everything apart, let's think a bit. The experience you describe of the tractor firing only if the choke is closed, and the tractor failing to sustain when you pull-start it are highly indicative of fuel starvation. Bad sparkplugs will act this way too, however, I would consider it highly unlikely both would hard-fail at the same time -- but it CAN happen! So here goes: A replacement set of plugs is a very easy step, so do it and see if she runs. Regardless, leave the new plugs in the tractor. Actually, I would rather you tested ignition by seeing a blue spark across the electrodes of a real sparkplug attached to the wire's end -- not just a jump from wire to frame. This way you see the plug in action! I am assuming here that you have not lost compression -- if you can tell without a doubt when each of the two cylinders is coming up to TDC of its firing stroke, you have compression. And if not -- that's basic stuff! Go there and ensure tappet settings are between 0.017 and 0.018 inch! Basic rule: No compression - no run! Now, having ruled out lack of compression and the plugs (and you said you had fire from the magneto at the right time), we turn to the carby. Question # 1 -- When you pull-started the tractor and it failed to remain running, was the LOAD needle at least 3/4-turn out? You see, if the IDLE path in your carby is not open, the tractor will start hard but will sustain by throttling up and down -- drawing its sustaining fuel through the LOAD side of your carby. At any rate, set the LOAD needle at 3/4-turn out and forget that for now. Question # 2 -- The level of the float concerns me here. I don't think I am concerned about a bad, leaking float, but rather -- the setting. While it isn't likely that float level will have changed on its own, testing the fuel level in the carby bowl is kind of necessary here. Two quick (not that scientific) tests you can run are (1) Back out either the Idle or Load needle and check the end for "wetting." There should be enough fuel in the bowl to wet the needle's end. This observation is more successful in the morning at highest humidity that slows evaporation of fuel from the needle taper! (2) This one, valid only for the "H" tractor, is a "volume" test. Close the drain cock and open the fuel valves to allow fuel flow into the carburetor bowl. Leave it open for a full minute. Then close the fuel valve(s), and measure the amount of fuel you can drain from the bowl. You should collect between 4 and 5 fluid ounces! Float setting criteria -- if the carburetor's bowl were removed, this level would be achieved if the float is dead level as it closes the needle valve; a point where the float is between 3/8 and 1/2 inch below the carburetor's casting as the needle is closed. For "H" tractors, this distance is 3/8 inch. On the tractor measurement -- A small device called a float level tester is helpful here; install a hose barb in place of the carburetor bowl drain cock, attach a 10 to 12 inch section of clear vinyl tube and tie (or hold) the open end up along side the carburetor. Turn on the fuel and watch the clear hose; give it time. The level of fuel showing should be approximately 3/4 inch below the top of the bowl. You can see a picture of this set-up by going to the JD-H Restoration Site, and scrolling down to For Sale item # 8. Question # 3 -- Going further in this analysis without some data is simply prose at this point. I recommend you make certain the variables above are ruled out and then post back with the result so we can go from there. I will leave you with this: If you can manage to get the tractor running and she sustains, perform the no-wrench tests (LOAD & IDLE) on the carby as outlined on page 9 of the CARBURETORS article over at the JD-H restoration site (below).(PatB)
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