Short of following the manual's directions for testing the unit, or removing the unit for cleaning or replacement, you might try putting some fuel system cleaning agent in a tank full of gas and running it through. This might clean it up.
I had a variety of problems that were mitigated after running a tank with fuel system cleaner added. Many people on this forum recommend a cleaner with Techron. Give it a try before you plunge into removal of the thing!
If you use Techron, the recommended procedure is to change oil and filter after you use up the doctored tank of fuel.
By the way, one of the best things we have found to spruce up 16V performance is to remove the ignition reference resistor R16--gives you about 6 degrees more ignition advance. Just listen for any pinging under load! Also replacing the center muffler with a small glasspack muffler results in a nice exhaust tone without adding ticket-gathering noise. Also I found adding a Jacobs Pro Street Ignition and wires made for better starting, smoother idling, and around 8% more horsepower--your experience may vary.