How do I keep the curve round when setting the threads farther apart? I am using Blender 2.79b on a Mac.

I followed the steps in this tutorial to successfully make screw threads (a spiral): https://www.youtube.com/watch?v=g4NHORCniJY 

There, in Screw modifier we can enter for "Screw" a low number for a sharp turn with threads close together, or a larger number to push the thread turns farther apart.

When I use a much larger number, the thread, which is based on a circle, is not circular throughout the thread. It flattens like a ribbon. I want those long, snakelike threads to remain the same diameter throughout for a full 360-degree turn. 

How do I keep the object round throughout the turn when at a higher number?

Here are the steps in the video:


Delete cube.
Tool Shelf> Create> Circle.
R to rotate, X for x-axis, 9,0 for 90 deg, and Enter.
3D View Header> View> Ortho view.
3D View Header> View> Front view.
S to scale, .,5, Enter.
Drag red arrow to right from centerline.
Ctrl+A and select Location.
Properties Editor> Modifier icon.
Add Modifier> Screw.
Screw Modifier: Set Axis to Y. You see the edge of the object.
Rotate the screen down with middle mouse button to see over the top.
Screw Modifier: Set Steps to 32. Smooths the surface.
Screw Modifier: Set Screw to 3.5 for a sharp turn and larger number for a longer turn, with threads farther away.
Screw Modifier: Set Iterations. Incrementing this adds more threads.
When you increase the Screw setting and stretch the screw, you increase the slope of the helix. Unfortunately, the modifier doesn't automatically tilt the profile correspondingly, so the profile isn't at right-angles to the slope. To correct this:


In the header, or Tab, go into Edit Mode
In the header, or  . on the keyboard, set the Pivot to 'Median Point'
Ensure the eye and little cube are checked in the Screw Modifier, so you can see what you're doing while editing.
A select all of  the mesh of your profile circle
Rotate (RX) the mesh about its median point in global X until it's at right-angles to the slope of the screw thread.