There IS a measureable pressure in the fuel lines. It is very small. You can make far more pressure (or vacuum) with your mouth, btw.
The pressure calculation requires knowing the cross sectional area of the oultet (lets assume the tubing inner diameter), and then extending that area up to the surface of the fluid, forming a column. The weight of the volume inside that column is the head pressure.
For most of our SOHC fours, you can simulate this head pressure with a simple drinking straw, the same height as the distance from carb inlet valve to the fuel top surface (maybe a Foot?), and filling it with water. Note that pressure at the bottom outlet of the straw reduces to zero as the fluid level in the straw reduces. We're talking grams weight here, in the 5-10 range. I estimated a 50 gram force per centimeter input to a converter, and got 0.71 PSI. This would be with a full tank of gas. The number would get smaller as the tank empties.
To answer the original question, no, the Harbor freight tool isn't going to be very helpfull in measuring carb fuel line pressures.
Another thing to observe, is the cab needle valve spring strength. Pretty tiny and easy to compress, yet it is more than enough needed to stop a full tank of gas from entering the carb.
Cheers,
