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Good morning, gentlemen - The shaking is consistent with a tired pump. It's a 4-cylinder unit, and when one or more cylinders give up you get the jerky lift. A lot like a 4-cyl engine missing on one or more cylinders. It's less pronounced at the upper end because the internal feedback reduces the pump output volume until it's shut off at full up. Each cylinder delivers less oil, so each pulse is smaller and the result less visible. Causes: blown o-rings, defective valves/seats, worn-out pistons/rings if fitted/cyls, in that order. I've tried the valve replacement kits with no success; a complete rebuild is better. Parts under $200 US for most models. The draft control system uses slight movement of the rod through the top spring to tweak the lift arms a bit. Same as if you move the position lever 1/16" or so either way (when plowing) to reduce/increase depth. I've never found it particularly useful, but then I'm not a fussy plowman! For the most part it's more of a nuisance. The top spring develops play, allowing the top link bracket to move in and out a bit. This can cause really jumpy load holding part way up, if the load allows the top link to flex. If you have experienced a jumpy hand clutch like the Farmall Regular, you'll get the picture! Adjustment is almost always a pain. The rubber boot deteriorates - usually 20 years ago! - and the resulting water around the parts sets them up nicely. That water moves on into the transmission, so boot replacement is a really good idea. The whole spring assembly has to be removed to replace the boot. After removing the roll pin securing the through rod, it may unscrew from the bracket. More often it has to be cut with a hacksaw, lots of heat to free the threads, and rewelded. Lots of grease on reassembly, tightening to eliminate all play on the rod, but spring can still be rotated. Replace roll pin. Grease or silicone to plug the hole where the rubber plug used to be. Replace in transmission, tighten retaining collar to just remove play. Too much, and play starts again. Sloppiness on the 2 socket head bolts holding the top link bracket does not affect the internal linkage. The oil weight should not make a lot of difference to the jumpy behaviour. Lighter oil might result in less lift power if the pump is really worn. Hope this helps.
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