Blackhorse Travel Air

The air bleed - on the carb there are two small screws, one adjusts the closed end-stop, and is usually mounted at the back of the carb, and vertically. The other is usually at the front, mounted horizontal, and across the line of the crankshaft. It blocks or unblocks a 1mm hole that alters the amount of air allowed in when the engine is at tickover, so that you can slightly richen or lean out the mix, without touching the main jets.
It ONLY affects the tickover, because as soon as you start to open the throttle, the barrel allows more mix in, bypassing the air bleed.

Glad to see you're back flying again S, you've been having a rough time just lately.

You have an air leak or a blockage in a fuel line, by the sound of it.
Unplug all the pipes, and have a good blow down them, then put a spare piece of fuel tube on the carb nipple, take the needle valve right out, open the throttle wide, and have a good blow down that, in case there's a bit of muck in the jet.
If that doesn't work, check that the carb screws are tight, where they hold it onto the crank-case, and check the case backplate screws are tight.

Of course, since you have two identical motors you could swop them over! But I'm guessing that's a lot of work!