Get rid of the chain tensioner and shorten the chain. I can see from your photo why you're having chain problems. Sprocket alignment is critical. Chain tension is too. You want 1/2" to 3/4" of slack. Any more or less can cause you trouble.
As for stopping with the engine running, you have to pull (squeez) the clutch lever in when your speed drops to below about 10mph. That is the 'disengaged' position for the clutch. When you take off from a dead stop you'll need to pedal assist up to about 10mph before releaseing (engaging) the clutch.
Safety warning: Don't rely on the clutch locking button on the handlebar lever and leave the bike running unattended. Those lock buttons aren't always reliable and can release allowing the bike to move.
A slow idle speed can be adjusted by screwing in the idle speed screw. In is for a faster idle, out is to slow the idle speed. It is not a mixture screw simply a mechanical way to block the throttle from closing completely.
Tom
As for stopping with the engine running, you have to pull (squeez) the clutch lever in when your speed drops to below about 10mph. That is the 'disengaged' position for the clutch. When you take off from a dead stop you'll need to pedal assist up to about 10mph before releaseing (engaging) the clutch.
Safety warning: Don't rely on the clutch locking button on the handlebar lever and leave the bike running unattended. Those lock buttons aren't always reliable and can release allowing the bike to move.
A slow idle speed can be adjusted by screwing in the idle speed screw. In is for a faster idle, out is to slow the idle speed. It is not a mixture screw simply a mechanical way to block the throttle from closing completely.
Tom