You can test for plugging with an ordinary vacuum gauge. Take a reading at idle, then hold rpm at 2,500. The needle will drop when you first open the throttle, then stabilize. If it then starts to fall, suspect excessive back-pressure.

But don't stop there because this isn't a one-test, go/nogo kind of situation. So, check back-pressure directly. With air injection, you can disconnect the check valve from the distribution manifold, and plug in a pressure gauge (you want one with a low scale). Or, remove the O2 sensor and take your reading at its hole in the manifold. Since specs for the normal pressure range vary considerably, look it up (if you can find it). As a rule of thumb, however, we'll say that anything over 1.25 psi at idle, or more than 3 psi at 2,000 rpm, means you've got blockage somewhere.

Speaking of direct tests, you might want to think about opening a connection or temporarily removing the oxygen sensor to relieve pressure. How's it run now? And you may as well take another vacuum reading if you can stand the noise and fumes.

With the engine fully warmed up and running, you can use a surface thermometer or a pyrometer to compare the temperature of the pipe just ahead of the converter with that of the outlet pipe. If the outlet temperature is at least 100 deg. F. higher than the inlet temperature, the catalyst is working after all, its most important job is to cause HC and CO to burn up, and that process makes heat. If both readings are nearly the same, or the outlet is cooler than the inlet, the rapid oxidation reaction isn't happening. Don't jump to an unfortunate conclusion just yet though because the problem may be lack of air injection (a subject I'll get to momentarily) rather than a contaminated catalyst.