Hi Phil, welcome to Outdoorking.
I think we need a bit more clarification of the surging motion you felt. Were the back wheels slipping, perhaps on mud or gravel, when it happened? If so, that is normal: intermittent wheel-slip will give a surging sensation to the driver.
If it is not wheel-slip, and it is in the wheel-drive system, there are two generic explanations remaining: the engine is surging, which is usually a sign of lean mixture, or the wheel-drive system itself is slipping. I'll begin with the more common, and less dire, explanation of slip in the wheel-drive system. The Greenfields are subject to belt slip in the wheel-drive system, and most times this is caused by insufficient belt tension. You can test for this fairly easily. Point the machine forwards up a slope, and see if it slips rather than powering ahead. If it does, turn it around and reverse up the same slope. If it will go up the hill backwards better than it does forwards, it is highly likely that you have insufficient belt tension, which is usually easily fixed. The Greenfield wheel drive system, which in general I like, has one important design weakness. Its automatic belt tensioning system cannot operate when it is in forward drive, but works well when driving backwards. This is because the spring-loaded tensioner-pulley experiences twice the belt tension pulling it in the belt-loosening direction, whenever you are driving forward. On the other hand it is free to tighten the belt whenever you are reversing. To make the system work, Greenfield fitted a ratchet system to the tensioner, so the tensioning pulley cannot ever move in the belt-loosening direction. Hence when necessary it adjusts the ratchet on the tensioning pulley up a notch when you reverse, then holds it in that position until more wear takes place, and you happen to use reverse, whereupon it tightens another notch. The problem is that the ratchet mechanism is a bit tin-pot, and it also can only move in complete notches, so not-infrequently belt slip occurs in the forward direction. The experienced Greenfield operator then removes the seat and applies a small lever to the tensioner pulley, pushing it forward toward the front of the tractor. A small click may be heard from the ratchet, indicating that it has taken up a notch. The belt slip then disappears until the next time.
The alternative, and more dire, explanation of slip in the wheel drive system is not belt slip, but wear in the forward clutch. Where the belt slip problem is fixable in a couple of minutes, fixing a worn forward clutch is more in the couple of hours category. We have several detailed threads describing how to remove and re-line one of the clutches, and we can discuss this further when you report back. Initially though, we need to know whether the engine is changing speed when the surging occurs. If it is, we can talk about adjusting the engine's mixture. If the engine is not changing speed, I suggest you try the hill-climbing forward and backward test, and see whether the tensioning pulley's ratchet will take up by a notch. If neither of those explanations is applicable, we can talk in more detail about servicing the clutches.
