Engine troubleshooting anyone?
 

The 3.6 in my 74 (bought used and it's from a 92 964 with adjustable valves) has had a bit of popping through the exhaust, particularly during deceleration. I also noticed a slight amount of valve noise. Yesterday, the valve noise got louder and the popping more significant. I isolated the noise to cylinder 3 and the popping to the left exhaust pipe. I'm running PMOs, so I tested vacuum with my unisyn and all cylinders were pulling an equal amount, except #3 which had half the flow. I'm hoping this is just a valve adjustment issue (too tight or too loose). I'm pulling the valve covers today, but any other thoughts?

The popping from the exhaust, on deceleration, is most probably a vacuum leak.
You confirmed when mentioning that your getting half the flow on #3 jug.
You can check the compression on that cylinder and compare it with the others, just to make sure the valves are seating well.

Exhaust popping is unburnt fuel escaping through the pipes.

Did you attempt adjustment of the carb for #3? Perhaps a bad or missing bypass screw seal.

Go for the air leak test to start with as that could be your problem ,are the carbs set up right (excluding your problem)?,hope its not a bad valve:(.
Could be a dry or rolled "o"ring afterall.
If so do a hot compression test targeting that cylinder (mainly),hope you don't come to this,cheers.

If the problem is progressive, which it sound like it is, I doubt it is a missing part causing it. I'd be inclined to check the valve adjustment on the #3. Sounds to me like it loosened up...