"Michael Walsh" wrote in message
om...
I had my central heating oil boiler serviced last year, it had a
bearing replaced in the motor.

Since then it seems to burning a 'rich mixure', i.e. there is a very
obvious kerosene smell from the exhaust. And the oil fill seems to be
used quicker.

The smell is not forced to be due to a rich mixture. Unlikely though it
sounds a weak mixture also creates smell in the exhaust gases. There is a
fairly narrow band in which the mixture is "correct". In this band the
efficiency peaks, outside it the efficiency falls off fairly rapidly. Rich
mixtures invariably leads to soot formation which manifests itself as
visible smoke and soot deposits around the flue discharge. Weak mixtures
smell sour and the excess air causes the heat produced to be swept through
the boiler and out of the flue too fast to be absorbed by the heat
exchanger. In both non-ideal cases the fuel usage goes up.

There is a mixture screw/valve in the side but when I tried to adjust
it the burner stops and refuses to start again of its own accord
unless I increase the mixture.

Depending on the screw it might be the pump pressure adjuster. Is it on the
burner casting or on the oil pump?


Is it the quantity of fuel or the lack of air? Or am I completely on
the wrong track.

See above. It may be pertinent that the burner motor had a new bearing
fitted as this would involve removal of the fan and cleaning off of the
blades during reassembly. This gives fan delivery performance improvement
and weaker mixture results.


I can't remember the burner model number but if it helps I can look it
up to-night.

It would be a help, together with the make and model of the boiler