Wholeheartedly endorse this sentiment.

If you have a bone stock motor, give the D-Jet a good going-over. It's a fairly simple system, with only a few sensors and a few controls. There is onl a minor mechanical component to it (the fuel pump and pressure regulator) - the rest of it is all electrical, and thus doesn't respond well the mechanically minded people tinkering with it. What I find works well is to google up the proper readings for all the various sensors, get a multimeter, and spend a while checking them all. I prefer to look at a wiring diagram and check the sensor readings at the computer end of the wiring, by checking across the proper sets of contacts in the big multiplug. That way you test the wiring loom as well as the sensor. Get a reading that doesn't make sense, check it again at the sensor to narrow it down to the sensor or the wiring.

It can also be of great help to have a working D-Jet car in the vicinity, so you can swap component one by one, to see if the problem moves with one of them.

If you have a modified motor (D-Jet isn't very adaptable, at least not in any proper way) or you are *really* frustrated with the whole mess and want to just pitch it in the trash can, replace the 35 year old D-Jet controller with Megasquirt, and re-wire the system with new wiring. Get a bung welded on the exhaust pipe and use an O2 sensor for some feedback. Megasquirt is a DIY system, basically a completely open and configurable FI computer that can be configured to run just about any sort of even fire gas engine with injectors. It can reuse *most* of the hardware D-Jet had (it has it's own MAP sensor, so you can pitch the troublesome hand-grenade D-Jat MAP). And it has been done before by other Volvo B20 owners, so you can get well sorted fuel maps from someone else and won't have to come up with them on your own. Cost would probably be similar or even less than a pair of rebuilt SU's, because you already have almost all the fuel injection hardware sitting right there on the car already.

As cute as a pair of SU carburettors are, fuel injection simply works better. More precise metering of fuel, better atomization, more power (those long uninterrupted intake runners), better mpg (well, if stoiched via feedback).
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I'm JohnMc, and I approved this message.