There are 3 types of 2nd gear clutches.
The open type (you can see the spring) and the closed types.
Also the 2nd clutch come with a yellow and blue dot on it(stiffer spring) or with only one dot(yellow)
I got the same problem after flipping the clutch that there was a strange grinding sound onces a while.
Checked it, and it was the play on the ax-sprocket.
I made a shimring for it and it was gone.
Use a normal 1st gear clutch and a flipped 2 dotted 2nd gear clutch.
Use thick motoroil, 20-40 sprockets and with a fast 50cc is runs fine.
A 70cc is running better by the way..more torque..