Hard starting accompanied by having to depress accelerator to keep engine running points to compression issue likely related to valve adjustment as mentioned. Glow plug(s) failure(s) will cause hard starting, but typically engine will begin to run smoothly within one to two minutes after starting. Air and fuel filter problems can affect starting and driving conditions (reduced high speed performance and intermittent cutout, due to loss of air and fuel delivery). Leaking hand pump usually causes hard starting but engine will begin to run smoothly sooner than 10 to 15 minutes after startup. As stated above, if engine is significantly more difficult to start when air temperature is lower, this points to valve adjustment needing attention. Telltale symptom of valves too tight is engine runs on less than all cylinders while warming up. Eventually last cylinder affected begins to fire and engine runs smoothly (notice too very black exhaust smoke initially due to unburned fuel, smoke eventually is reduced when affected cylinder(s) begin firing correctly). The typical case of tight valves needing adjustment is an engine that starts better during "summer weather" and becomes much more difficult to start during "winter weather" because ambient air temperature is lower. Remember too: unburned fuel in cylinders washes down walls reducing lubrication leading to increased wear.