Hmmm, where to start. The trouble shooting method he suggested is how to check for a vacuum leak. There really is no specific “seal”; however the entire intake system is sealed from the atmosphere (just keeping it simple guys!). If the carb cleaner gets pulled into the system through the leak the engine will speed up, hence revealing the location of your “vacuum leak”.

Anyway back to your problem, my personal guess is you have a failing fuel injector.

Is your idle high or low? Or normal?
Is the idle steady, or does it cycle?