Brian,
Keep it simple - Have you checked the condition of the air cleaner - Black smoke is a sign of a rich mixture eg too much fuel.
Does this Mountfield have a Briggs and Stratton engine with a primer bulb on the right hand side?
If it does then there is a good chance that the diaphragm is failing between the carb body and the tank. The only fix is a new diaphragm. Fitting is quite straight forward if you are careful and put everything back as you found it. This is a job that I intend to do a tutorial on over on my forum very soon. (Briggs and Stratton part number 795083)
If the mower has a Mountfield engine fitted then problem could be with the choke linkage, the carburetor it's self or with the governor spring set up.
If you are in any doubt, don't muck about with the carb or governor on you engine, a professional will fix it in a fraction of the time and won't risk your investment.
Admin - www.gardenmachinerypro.co.uk
May 2012
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