There's a lot of misunderstanding about the regulator on these engines. There is no pump, and the regulator does not regulate the pressure coming from the tank. Rather, it regulates the flow of fuel into the engine, and prevents the engine from being flooded when running at a low or idle speed. As I understand it, here's how it works. As the engine rotates, crankcase pressure is sent from the bottom section of the regulator back to the tank, through a check valve that forces the tank to hold the pressure. Crankcase pressure actually fluctuates from "positive" to "negative" as the piston moves up and down in the cylinder, so the check valve causes the pressure flowing towards the tank to be held and trapped in the tank while the piston is moving the other way. The result is a tank that has a constant positive pressure inside it, which in turn forces the fuel out the clunk towards the engine under pressure. As long as the engine is running at a mid to high RPM range, it is burning the fuel as fast as the tank can deliver it, and everything is fine. But at low speeds, especially at idle, the flow, or volume, of fuel would be too much and would flood the engine, so the regulator comes into play by reducing the flow. The adjustment screw on top balances between the crankcase pressure coming from the bottom, which is ported into the case, and the spring-loaded valve on the other side of the diaphragm. What you are trying to do is find the proper tension on the valve so that at low speeds the valve stops the flow and the engine draws fuel by vacuum just like any other engine, but then at higher speeds the fuel can flow at at greater rate based on the demands of the engine. At high speeds the crankcase pressure below forces the valve to open and allow fuel flow, at lower speeds the spring overcomes the crankcase pressure and shuts the flow off, preventing flooding.

Now to answer your question, I think it is normal for you to be able to draw air through the fuel lines. The carb has to be able to draw fuel at idle since it isn't being pumped into the carb below about 2000 RPMs. When you close the needle valve, that stops the air flow, as it should. The way to find out if you have a leak is to run the motor for a few seconds, and then leave all the lines alone for about 15 minutes. Next, remove the fuel dot from the vent line and listen for a hiss. If you don't hear one, then get some dishwashing liquid and run the engine again, but this time start painting the lines and the regulator with the soap. You should see the bubbles when you find the leak. And an air leak will only be seen on the pressure side, as a leak coming from the clunk side of the tank will result in fuel dripping or spraying from a fuel line.

This is all subject to correction by one of the YS experts who lurk around this forum. I may not have all the facts exactly right, but you can read some of this info on Troy Newman's (a real YS expert!) website at this link:

http://www.patternflyer.com/troy/?page_id=13

Hope this helps!

David