ASV RC-100 Fuel Issue? Clicking/ rough idle at low RPM - smooths out at high thrott

My buddy has an RC-100.. THe diesel fuel cap is half-on and/or broken and the machine runs like crap at low idle, it makes a hard clicking noise and idles rough... WHen you raise the throttle it smooths out and runs much better.

Any thoughts?

ASV RC-100 Fuel Issue? Clicking/ rough idle at low RPM - smooths out at high thrott