A smoke machine is a great tool to have, because sometimes a small vacuum leak is alot more tougher to find than one would imagine.
This is a smoke machine I made for finding automotive vacuum and exhaust leaks. The whole project cost under $40. Be careful if you try building a home made smoke machine. The mineral oil I used was not explosively flammable, but it will definitely catch fire!
Evaluation of Swedish made 18g smoke cartridges for electric planes.