Hi, Avocadofarmer.
Never having had my head inside a #29 CCU or laid eyes on a manual for same, I don't KNOW if the adjustments are the same as the #25 but I'd pretty near bet that they are. IF SO:
1. You need to tighten up the brake spring, the one that lies along the top of the the 'works' just under the top of the case. I suspect there will be a castellated but otherwise round nut on the front end of that spring. A little effort with a pry bar or a largish screwdriver, maybe a bit of 'loose-juice', possibly even some 'not-the-queen's English' oughta 'gitter dun'. In extreme cases, a few good hits with a hammer and a punch may be needed to convince the little sucker that you mean business. (Once upon a time, I believe there would have been a collett wrench for this task. If you are a truly dedicated afficionado, you could maybe make yourself such a tool.)
It possibly doesn't need a whole lot of adjustment if, as you say, it will currently hold unless disturbed. This can be done with the engine running and the blade on the ground but try to make sure that no 'gremlins' are climbing around the machine at the time, especially on the control deck.
2. You then need to go through the whole 'set the neutral position, then adjust the clutch' procedure as in the manual.
On edit - forgot to add, the left drum that keeps on spinning probably does so 'cos the brake is not engaging for some reason. Even though the clutch plates may not have pressure on them, there is usually enough friction between them to drive the drum if the brake is not applied and there is no rope to stop it. You DO need to investigate and remedy before connecting anything to the drum.
Hope this helps.
Thanks Deas, i'll be in there later today. I'll let you know how it comes out. if I'm nice, maybe i can convince the wife to come out and take some pics when i start ripping. Got the rippers from Willie a while back and can't wait to polish the rust off of them.