Laurence,
Seeing tiny bubbles in your fuel line means that air is some how entering your fuel system, and is contributing to your difficulties in carburetor tuning.
Air can get into the fuel line from a small hole or tear in your fuel line. This can be either be in the clunk line inside the tank or between the tank and carb.
You should replace BOTH of these fuel lines with new fuel line. Inspect the new fuel line carefully. Even the smallest tear or hole can cause air to enter your fuel line.

The next source for air getting into your fuel lines is from vibration of the fuel tank. The tank must be isolated from the airframe to prevent this.
To isolate your fuel tank from your airframe, wrap your tank in some foam padding.

These are the most common sources for getting air bubbles in your fuel line.
I would start here with replacing ALL fuel lines, and inspecting these new fuel lines to make sure there are no holes.
Then also properly isolate your fuel tank from the air-frame.

If you do this and still have air bubbles then providing extra information would be most helpful
What plane is this engine setup in?
What engine are you using?

I hope this helps,
Ryan