Did you check the valve clearances before you took it apart? a too tight or too lose valve could explain the lean condition as much as a blockage in the carbs could.
Was the high idle when cold or warm? These bikes are cold natured and the procedure for setting the idle in the manual calls for setting the idle when hot. However cb750s are cold natured and the warm idle setting will sometimes cause the bike not to idle when cold and PO might have screwed the throttle stop in a couple of turn to get it to compensate. Hoever it could also have been an air leak at the #2 cylinder which would also explain your lean condition. Too bad you took it all apart chasing an oil leak before you started to diagnose anything. What does the plug look like? if the plug is normal and you are seeing only a white chalky residue in the pipe - could be an exhaust leak.
Also since you have a big valve cb750 (77-78F), and since you already took it apart, might as well do valve seals, guides, and seats. The big valve heads tend to chew up guides around 30k miles.
A chewed up chain adjuster will make the engine noisy, and eventually cause all sorts of other damage but I doubt it would cause a high idle unless the timing chain skipped a tooth, and then you would have all sorts of other running problems. It is also a sign that the PO was kinda lax about maintenance.
Bike 101: before you put on the cool seat, or rip off all the paint, did you give this bike a proper service and tune up? I'm guessing no. We aren't talking anything major here, just basics of simple maintenance that probably would have required 0 parts and def would help you in answering the questions you have now about strange things you are finding in the motor. Some of these things you really should check before purchasing the bike, and you don't really have to do all of them, even 50% of them will put you that much ahead.
the simple maintenance punch list for a new to you and unknown bike:
- check static and dynamic ignition timing
- Check to see if ignition advance unit is working
- check and set valves.
- balance carbs.
- Compression reading
- inspect and evaluate plugs.
- check and grease swingarm bushings
- check brake pads for unevem wear
- check tires for uneven wear.
- check brakes for dragging and leaks.
- check carb manifolds for leaks
- check exhaust for leaks.
Honestly, it is maybe two hours worth of work to do the whole list.
