Having trouble keeping the engine running at optimum rpm. Can't seem to adjust the carb to get a constant rpm. I am seeing bubbles in the fuel line to the carb. Is this usual? I could not find any leaks in the tank or lines. The tank is full of fuel. New OS 46 engine and it ran fine for the first half dozen tanks of fuel.

Thanks, Laurence

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

spot on advice there!

another problem which is more common with truck engines is leaking bearings, backplates or other places, take the fuel line off your tank, plug up the exhaust port and carb and put the engine into a bowl of water (hold the fuel line up above the water) then blow through the fuel line and watch for bubbles, if you see bubbles theres an air leak

works for me every time

I would try padding the tank first since it's the easiest thing to do.

Actually, the best way to do it is to remove the tank and run the engine. If the bubbles disappear, foaming is the problem.

If the bubbles persist, you've got an air leak

The plane is a Nexstar 46 trainer ARF. Engine; OS .46 AX. The fuel tank has three lines and the tank rests in a saddle made of a rubber type material though not very thick. I am sure foam would do a better job of insulating it from the frame. However the neck of the tank makes direct contact with the firewall, through which it extends.
Removing the tank and running the engine does make sense and I will replace the lines as well.

If you find damage where the line goes on the brass tubing, file or sand any sharp spots you find on the end of the brass. Always check this when insalling tanks.