This may be a bit off topic but here goes. I am working on an old tractor with a Zenith 10002 carburetor. This particular model carb has a removable venturi. Unfortunately the original venturi was lost, ran fine when had it though. Installed another venturi that was lying around and would run only at idle and slightly above, when try to speed up engine it cuts out and dies. Found another venturi and tried it, would not start at all unless full choke and carb dripping fuel, then would start and immediately die. I don't recall the inside diameter of the original but the one it idles on is about 1/2", the other one is about 3/4". I suspect need a venturi with an inside diameter that is smaller than 1/2". If I understand the theory correctly the purpose of the venturi on this carb is to create a low pressure that draws gas into the intake, smaller ID venturi would result in larger pressure drop thus more gas and allow engine to draw enough gas to run normally. The tractor in question is an Oliver 70.

If anyone has any ideas as to what the problem is or a source of venturi's would like to hear. I tried ordering a rebuilt carb and the venturi it came with is larger than 1/2", didn't measure it to get actual size. It will start and run for a few seconds with full choke on this one but no longer.