[Reiver]

A guess.
On the 930-16 USA model the lamda controls the enrichment (on a 930-10 euro the wur does).
Yours, you stated, was unplugged so the box that controls the lamda is supposed to go to an auto default setting without input.
What was your AFR/CO levels as if too lean you can get a backfire on start...another item to check is your wur/pressure device.
The lambda control, wur, fuel head work together with the cold start device and throttle plate to deliver the proper fuel at the proper time...if any one is grossly out of spec you can have a bf.
All of these systems will kind of compensate for each other a bit depending upon how you have your CO/AFR set but only to a point.
Once your air box is airtight check your CO when you can run the car....has the wur ever been rebuilt? If you have a solid spray pattern your fuel head is prob ok.
You should see fuel in the intake area if you have been trying to unsuccessfully start the car...that's part of the cold start/diffuser function. Before the diffuser was put in place the cold start element just dribbled fuel into the airbox/runner area.
My 930-10 ran great even with a very out of spec wur....because I'd enriched the afr to compensate...if the afr got lean it would lightly backfire.
I had the wur rebuilt by Larry Fletcher at CISflowtech (guru and 1/2 with any bosch CIS element) and the car runs better but now modifies afr properly on demand...this is a euro but since your lambda is not functioning I suspect you may have a lean situation with a similar result.
Like I said...a guess.