I have made hundreds of single and double flares with the tool shown and even with one which included rollers and tho they worked I was never happy with them.
3 years ago I purchased an Eastwood flaring tool, there are others too probably by the same manufacturer, and could never be happier. It handles up to 3/8" tubing and every end is perfect with 2 pulls of the handle and a click of the tool head. It can also put a "bulb" end with a little practice. Yes it was over $100 and since my son has done his complete brake and fuel system and I have done a rear end change and disc brakes I can say it was worth every penny. I'd show a photo but it's at his garage right now and are ez to find on the Internet.
Over the years learning to do things on my own and purchasing good quality tools that will last a guy time me a life time has not always been possible but was done as the need arised. I don't a shop or make any profit but sure am happy when thing go right because the correct tool was there to use.
We started using nicopp and so far been pleased.

56 Vic, B'Ville 200 MPH Club Member, So Cal.

I made it with copper since I had nothing else

thanks for the comment

Are there various diameters in 3/8 ...? I have measured the connection of the carburetor and it gives me 15.6mm and another that I have supposedly also 3/8 gives me 16.6mm, is that normal?

I believe there are 4 dies in the kit. Front the fuel to the carbs we used 5/16” and I believe the brakes are 3/16”.

56 Vic, B'Ville 200 MPH Club Member, So Cal.

I once did domestic air conditioning. Lots of flaring of copper tube, but only single flares.
The refrigerants ran at many times the pressure of fuel lines and they would never leak if done right. Aircon copper tube is seamless.
 There are a few tricks to not getting the flare splitting. When using the pipe cutter, don't cut right through. Cut just enough that after it work hardens you can snap the tubing off. Doing that then doesn't press the end of the tubing in as far and less deburring is required. It is the deburring that makes the ends too thin and they split when flaring. The only size that was difficult to get correct was 1/4 and some times I would have to cut it off and repeat the flare.
In saying that. It is illegal here in Australia to use copper for fuel line for the very reason Ted mentioned. If there is any movement it will work harden and could eventually crack. It could take years, but it is a risk. If it is a short run to two fixed points on the engine you might be safe. But if it connected to rubber hose or is a long run where it can vibrate under its own weight then I would not take a chance.

Thanks for the feedback. I also did many air conditioning installations, in fact the tube I use is 3/8 seamless air conditioning. But I'll try to find a malleable steel tube to replace it

Greetings to all. If there is no setback it is Friday I will start the engine, I am already on my nerves. Any last comment will be welcome

NICE

Cake lid air cleaner looks good on that Y

56 Vic, B'Ville 200 MPH Club Member, So Cal.