This is hardly my idea, but I do take credit for my adaptation of it. I googled some articles years ago when I needed to clean the injectors in my Volvo 740 turbo. I made some slight changes to the method, and here it is.
This could be handy for any backyard mechanic with limited access to fuel injector cleaning stations. http://www.youtube.com/watch?v=2y_YSkJmXhM
While you are there, check out some of my tunes by my salsa band!
Let me know what you'all think of my method.
He is using the vacuum to backwash the injectors with cleaner- the vacuum pulls the cleaner up through the tip of the injector and out the port that attaches to the fuel rail.
So- from what I saw, attach the vacuum hose to the fuel rail port and lay the injector tip in a cup full of cleaner. The injector must be open for the cleaner to flow through so you will have to supply a few short 12 volt bursts to allow the cleaner to flow through.
Erik is right-
Backwashing with the vacuum through the spray end of the injector. I suppose this could be done from either end; backwashing makes more sense to me as you are more likely to get any small chunks that made it past the filter but not the valve itself.
I did not run the vacuum because it was super loud and you couldn't hear what I was saying anyway.
Tjts1 is also right, about the dangerous part. I wasn't too concerned about dc spark starting a fire; most people should probably opt for the switch assembly bit. I don't know what the flash point of injector cleaner is, but I suspect it's like oil not like gas. The fumes are the more dangerous part, I suspect as well.
As far as a 9v battery, I don't know if you'd have enough amps to keep it open or not. Besides, a 9v would produce just as much spark as my battery did in the way I was doing it.
The link was great and I daresay I like it better than my idea. Less damage to the vacuum if things go wrong! Also, you could do a before and after spray to see what you accomplished.