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My 4020 gas smoked like an IH diesel when I first got it a few years ago. I believe the long intake manifold makes it hard to get a decent mixture to the end cylinders (lots of condensation until the manifold heats and JD does call for manifold heat up to 90 when not loaded hard) and to the center two cylinders at the same time. However, I found a few things to make it burn a leaner mixture and never touched the carburetor settings. 1. I made sure the choke opened all the way. The choke cable sheath wasn't anchored near the carburetor and would open only half way. 2. I cleaned junk out of the intake. 3. I cleaned the air cleaner (dry). 4. I pushed the dome of the Donaldson precleaner up instead of down where it had been pulled from over tightening over the decades. 5. I replaced the float needle and seat. 6. I went to a higher heat range AC plug, No Champion plugs. 7. I put in copper wires. 8. I timed it to 20 degrees before top dead center with .020 point gap while at 2000 rpm per the owner's manual. The first year I burned 400 gallons, the next year doing the same work, I burned 250 gallons. I've not changed the plugs in at least 150 hours. I have to use the choke when starting, often even when its warm. I don't have manifold heat on because some unkind soul cranked in a pipe plug into the heat shrowd of the intake manifold and split it. It doesn't like 10% ethanol, probably because of the cold manifold that I ought to fix some of these years. You could try to reduce the output of the accelerator pump in the carburetor. You could increase the distance between the float and the flange to lower the fuel level and so lean the mixture over the entire running range of the carburetor. Gerald J.
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