Great big huge massive gigantic colossally large vacuum leak?

My old Datsun Roadster had dual side drafts (nearly interchangable with the Volvo's) and were mostly infallible. You should get at least a pop or two while cranking. I'd try to make things as rich as possible. Back the needles out a long way and see what you get. Make sure its choked real good and *try* to flood the engine - enough to smell it - just to prove that gas can get in there.

Good luck.