Posted by JD Seller on June 25, 2016 at 18:32:42 from (208.126.198.123):
I have worked on many JD 3000 and 4000 series gas tractors. I posted back in April about the problems and things I have tried on them to "fix" them so they run correctly. Never really having much luck.
Against my own better judgment I bought a JD 4020 gas. It has a wide front end, JD canopy, and a real nice Allied quick tach loader. Truthfully I planned to just take the wide front, and loader off an put them on a diesel JD 4020 I have. The trouble is this tractor runs like a Swiss watch at idle or a little above. After that it is belch and stumble while having zero power to even pull itself let alone use it for anything requiring HP. So we have been using the gas tractor just around the shop with the loader. Wednesday I need a loader at another location. I drove it over. What a trip!!! LOL Had to drive at 1200 RPM in 6th gear to get there.
So Friday I decided to tear into the JD gas 4020 and find out what is wrong. Well the spark plugs where black as coal. They had less than 10 hours on them. So I installed new spark plugs and that helped a little bit but not much. So I took the carburetor off and brought it home. I went to take it apart to clean and found out it was an almost "new" Robert's rebuilt carburetor. Every thing was new, clean and seemed to be OK.
So I grabbed my JD 4000 series service manual and did some "light" reading last night. I read the entire section on the Zenith carburetor. I learned a few things.
1) The idle adjustment screw is actually adjusting air flow not fuel flow. So in actually increases the fuel drawn as it restricts the air flow which increased the velocity which makes it pull more gas through the idle circuit.
2) The accelerator pump pistons are adjustable. The lower one has three notches on the rod. The closest notch to the top is FULL flow and low fuel economy. Meaning the most fuel shot in during acceleration. The lowest slot is for lower fuel flow during acceleration and Highest fuel economy.
I did not know either of these things. I always assumed that in on the idle adjust screw meant less fuel. Plus I did not know that you has adjustments on the accelerator pistons.
So I reinstalled the carburetor with a new air filter just to be sure that there was not a restriction on the air intake side. I started with the idle set at 1 1/2 turns out and the load jet at 2 1/4 turns like the manual says to do. The tractor started right up. I let it warm up and then adjusted the idle leaner as I still has too rich of a fuel mixture at idle which was black smoking a little bit. I ended up at 4 full turns out on the idles screw. I then adjusted the load screw. I ended up at just a little over 1 3/4 turns. The tractor runs pretty good. It will take load and accelerate like it should. There is little dark exhaust too.
I went an bush hogged ten acres of pretty big horse weeds down. The tractor pulled great. It will be interesting to see if it will last. If it runs better I might leave the loader on the gas tractor as I will be mainly using it around the shop and house. It will not get high usage.
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