That screw is NOT a easy place to adjust max fuel delivery if built to factory specs. Deere uses it on some Stanadyne pumps to limit engine torque rise at certain times. Thus the reason why it's called a "torque screw." The torque screw limits how far the metering valve can open -which basically works like the butterly valve in a carburetor except it restricts low pressure fuel flow instead of air flow. The torque screw does not limit max fuel setting permanently (except by a small amount). It only slows down the flow. So, when a load hits the engine fast, it takes longer for fuel delivery to increase with the torque screw. Max delivery will only slightly increase if the torque screw is backed out IF the pump is set up to Stanadyne specs (determined by pump ID plate #).
You can take two Stanadyne pumps used on the same model tractor. One with the torque screw and one without. Both have the same max fuel setting. If you back out the torque-screw on the pump that has it, max fuel delivery does not change very much. Max torque timing changes and so does visible smoke at certain loads.
I don't know why you have one on a 4020 but you didn't give the pump model #. Regardless, it has little to do with your max fuel setting IF built to Stanadyne specs. Max setting is done with the setting of the rollers and Stanadyne requires they be shimmed to limit turn up beyond .005". I'm sure there are people who put pumps together and not follow Stanadyne specs though.
Here's one example of a Deere tractor that came several pumps. Some with and some without torque screws.
2030 ag tractor:
JDB435AL2442 - with torque screw - at max fuel delivery pump flows 10-12 ccs per cylinder at max RPM. Rollers set at 1.9635" to 1.965"
JDB435MB2805 - with NO torque screw - at max fuel delivery pump flows 10-12 ccs per cylinder at max RPM. Rollers set at 1.962"
If you completely back out the torque screw in the first pump - you gain a max roller width of .0015" to .003" which isn't very much. Each increase of roller width of .001" gives a 5% increase in max delivery.
Maybe somebody customized your pump, removed the roller limit shims, turned fuel delivery way up and then tried to control it via an added torque screw? I guess this might cut down on visable smoke a bit under certain condtions if your tractor is turned up beyond factory specs.
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