That Marvel-Schebler die cast carburetor is a bear to tune. There's a big diaphragm that tends to split but not be replaced. It was such a b to tune that I have a service bulletin from Deere that says change to the Zenith. Many were changed. Zenith was also shipped from the factory, I don't know how they determined which tractors got which carburetor.

Get thee an operator's manual and the SM-2039 shop manual and ye can make it purr, though the section on tuning the MS carb is several times larger than the section on tuning the Zenith.

You can buy a new Zenith from Abiline, Shoup, and others for about $500 I think yet. Prices have been going up on that option.

Reading the op manual you'll find that carb heat needs to be on up to about 90 degrees unless plowing then it can be turned off at 70 degrees.

You need to check the intake plumbing to remove all the mud daubers, wasp, and bird nests. And then make sure the sheath of the choke cable is anchored near the carburetor, else you won't be getting full choke for starting or getting choke off for smooth and economical running. With anything but a NEW Zenith carburetor, its worth the few bucks to replace float needle valve and seat because it tends to wear from vibration and over feed the engine with fuel.

The first year my 4020 burned 400 gallons, after I tuned up and cleaned up, it got down to 250 doing the same field work plowing and all. It as a consumer of gas, though I plowed, disked, and planted the same ground with a MF-135 on about 135 gallons a year but it took 4 times as many operating hours.

Be sure the new plug wires are copper, not carbon. That will make a difference too. And forget that Champion made spark plugs they don't last in a vintage engine anymore.

The old generator wasn't great at charging, its only 20 amps and that takes a while each start to charge the battery. When I added an ammeter all I got was it bouncing from peg to peg even after I cleaned up the commutator and replaced the very worn generator pulley. My 4020 has an alternator and it charges in about 20 minutes after a hard start and batteries last three or four years instead of one with the long skinny battery and the generator. While ampere hours of the long skinny battery are comparable to a short battery, the long connecting bars inside cut down the cranking amps and the cold cranking amps. I built a new step and battery shelf for my 4020. It uses a side terminal battery to keep mud on top from drawing the battery down. I've different tires than any factory option too. Better for spraying.

Gerald J.