Scott 'Gunny' Perdue wrote:
I agree with your observation on a sinking float.... as I said it only occurs at idle, not at higher power settings. Adjusting the idle mixture setting has always cured the problem. A leaking intake system... there's a thought worth checking out.... but on the other end, at higher power settings wouldn't it have trouble making full MP? It's supercharged and it makes MP at takeoff power.
Inspector... are you saying that you crack the propane valve slightly so that the engine will suck in the propane and speed up because of that? The carb came with the engine at overhaul 250ish hours ago. It has always run a little rich on the idle side.
gunny
After a little more thought...
If you were to drill an 1/8th inch hole in an intake pipe, I would expect a lean mixture in that pipe at idle since you would be sucking in air. At MAP above atmospheric, you would blow a small amount of your intake charge out, but it would not create a lean condition, just a slight loss in MAP. I'm not convinced that would even be noticeable as an MAP drop since there is some tolerance in the field barometric pressure/MAP RPM check.
Now if the idle mixture had been set to compensate for the lean condition in some cylinders to get smooth running, you might actually be masking your lean problem and calling it a too rich condition. The "good" cylinders could actually be the too rich ones while your bad ones could be the lean ones.
What you may be fighting is too great a variation of mixture in one cylinder vs. another.
If you could idle the engine for 10 or 15 minutes in this too rich condition, do you think you could tell which cylinders are actually running rich? If they all get evenly sooty, variation isn't your problem, that would point to an induction leak at the carburetor or an internal carb problem.
The intake pipes do have rubber seals AFAIK, so they may have perished due to calendar time.
That's my theoretical thoughts, maybe Rich can tell us if his experience could discount any of that.
And I still haven't given up on the leaky primer thought. If the sooty plugs were the ones with the primer lines, that would be quite a coincidence.
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I mentioned possible porosity issues because after 800 years of vibration, stripping, rebuilding, occasionally a small crack or porous spot turns up and causes a leak of either air or fuel caused by a speck of casting sand finally becoming disloged. See if you can borrow a known good carb and see if the problem goes away, if it does you're out the cost of another carb but also have a cool door stop since yours would now be unairworthy, if it doesn't then the troubleshooting continues. Troubleshoot in small, simple steps. You'd be amazed at how many flights have been delayed or cancelled over a burned out light bulb and the mechanics wanting to go after the main pivot washer only to discover much later, it WAS the lightbulb. Just because the carb came with the engine don't make it guaranteed as 'good to go'. 
