Over several more iterations of pressure testing, I was able to get consistent results. The only way to get completely repeatable results is to have the cylinder on the up-stroke. That seems to be the position where the rings seal the best (with the engine cold, of course). With the clyinder at TDC or in the down-stroke, sometimes something happens during that test that changes the results (the rings flip to the ascending state, maybe), but whether or not it does this depends on factors that I can figure out. Now that I have a consistent procedure, sometime I'll do the test again with the engine warm.
Over the past week I got the engine put back together, and yesterday I took time to do a few more things before getting ready to run test the engine:
I installed quick-release decklid hinge pins. For those thinking about doing this, it's very important to read the instructions. The quick-release pins are larger than the stock ones. You need to ream out the hinges with a 1/4" drill bit for it to fit. I found that wiggling the drill around after drilling the hole made the ends of the pass more open and the pins went in much easier.
I also went ahead and installed new hoses on the right side of the engine. That's standard 2-inch cold-air duct from the auto parts store with a spiral metal reinforcement. I got tired of the Aluminum foil ones not fitting and bending every time I touched them.
And I re-set the timing and then ran the engine and attempted to do a full tune-up procedure. Here's ready to test:
Setting up the carb was ultimately unsatisfactory. The engine definitely ran smoother, and the exhaust was noticably cooler than it has been. However, I think there's a chance I might have a leak in the intake system somewhere, which causes the engine to run erratically and makes it hard to adjust. I have some work to do today, but with a little luck I'll be able to test that this afternoon.
A gasoline engine is an Otto Cycle engine, which depends on being able to compress the air/fuel mixure and when that mixure has burned (and thus gotten hot) being able to mechanically harness the expansion of the hot gasses. The better the combustion chamber is at sealing, the better and more efficiently the engine will run.
So a very important diagnostic test of the health of a gasoline engine is to "pressure test" the cylinders. One way to do this is to employ a "leak-down" tester:
High pressure air comes in from the left, and the right side is connected to a hose with a special hose end that fits into the spark plug hole in the cylinder (instead of the spark plug). The gauge on the left reads the input pressure, the gauge on the right essentially tells you how "good" the cylinder is at holding that pressure. I recently discovered that the valves on the right side (cylinders #1 and #2) of my beetle had drifted drastically out of spec in roughly 1000 miles, which typically indicates trouble. I bought the above pressure tester on ebay and tested all four cylinders on Friday. The testing indicated the same story, cylinders 1 and 2 are not happy, cylinders 3 and 4 are fine. Here is the reading on one of the good cylinders:
So I will definitely need to pull the engine and at least look at the combustion chamber side of the right hand cylinder head to try to diagnose what's going on. Keep in mind, since I was just figuring out how to do this, I did tests tests with the engine cold, which is not the correct way. Now that I roughly know what I'm doing, I will take the car out for a drive and test the cylinders again with the engine warm.