Idle is set too low
Low-speed needle is set to rich
Low-speed needle is set to lean
Clutch is binding
Glow plug is loose
Glow plug is fouled
Piston and sleeve are worn
Damaged bearings in the clutch bell
Adjust idle-speed screw clockwise
Lean out low-speed mixture
Richen low-speed needle
Take clutch apart and fix the problem
Tighten glow plug
Replace glow plug
Seal engine with high temp sealer
Clean or replace bearings