No orifice? Not sure how that would work.
I just had a quick stooge round on goggle and there is a few places that give instructions on how to make your own. Some of the suggestions were a bit odd (And a bit dangerous) to say the least but there were a few good ones too. On a couple they suggest filling the fitting between the two gauges with 'metal set' type epoxy then drilling the orifice in this once set. Not a bad idea. Orifice size is critical. .040" seems a consensus but I'd need to confirm. I also recall that the lead in to the orifice and the length of the orifice play a part in calibration. I have source I can look up but I don't have it here. There are also a few people out there advocating 100 psi inlet pressure. Can't say I agree. 80 psi for safety- and it's all you need.
Speaking of safety, it should be said that they are dangerous to the uninitiated. Your introducing compressed air to the cylinder which will spin the engine sharply to BDC if it is not clamped. Seek training and be careful if not familiar.
