Why is my diesel engine difficult to start and smoking?… I very rarely use it !

Diesel engines of all kinds only run at full efficiency when they are at their designed working temperature and delivering close to, or at their maximum load. Auxiliary yacht engines often don’t get to be used in this way. This has been found to be particularly true of small single cylinder engines. I believe this is because they proportionately create more noise and vibration installed in yachts up to 25ft or so, and conversely the small yacht’s sails can be more readily hoisted and set. The result is these engines are often run for only a few minutes a day manoeuvring out of a berth or off a mooring.
Internal glazing and carbon build-up is caused by short periods of running or prolonged periods running at low loads. Such conditions cause low cylinder pressures and consequently poor piston ring sealing since this relies on the gas pressure to force them against the oil film on the bores to form the seal. Additionally iron blocks expand less than aluminium pistons so at less than designed working temperatures the pistons are a relatively slack fit in the bores. Resultant low cylinder pressures cause poor combustion. This poor combustion leads to soot formation and unburnt fuel residues which clogs and gums piston rings and valves. This causes a further drop in sealing efficiency and exacerbates the low cylinder pressures.
Glazing occurs when hot combustion gases blow past sealing piston rings, causing the lubricating oil on the cylinder walls to burn creating an enamel-like glaze. This polishes the bore and removes the honing marks which hold oil to effect the seal. The glaze can only be removed by stripping the engine and using a honing tool or ‘glaze buster’ on the bores.
The exhaust valves are also vulnerable to soot and unburnt fuel residues. They are subject to high combustion gas temperatures and the valve head is only cooled by contact with the valve seat when the valve is closed. Any residue build up on the valve seat will cause the valve to leak and reduce compression. Eventually the residue acts as an efficient insulator and prevents the valve head from being sufficiently cooled by contact with the valve seat. The valve head reaches a critical temperature and burns away or a piece breaks off.
Diesel engines should run at full working temperature and at least around 75% of their maximum rated load. Short periods of low load running are allowable providing the engine is brought up to full load, or close to full load on a regular basis. The problem of soot or carbon residue and glazing can be alleviated by the use of white diesel instead of red. This is generally a better quality and has a higher Cetane rating producing a cleaner, more complete combustion. It also has a lower sulphur content and added detergents which are claimed to remove deposits.