I've changed the entire ignition system and used fuel injection cleaner in an almost empty gas tank. Nothing helped.
-> Fuel injection cleaner may or may not work. Also depends on which cleaner you used. Of all I've used only Techron has tangible effect. Dismantling the injectors out and manually clean them is BETTER than using any fuel cleaner.


I checked the brickboard FAQs, but there's no discussion as to how often to do this off-car. Any suggestions?
-> Difficult to say as this depends on lots of factor - driving style, fuel choice, local fuel quality, ethanol or not. One thing I know our Bosch pintle type injector is the least affected compared to other type injectors. In your case the spray was still conical only the O rings were affected. So maybe once every 10 years or so - along with O rings change.

Amarin.