Paul wrote
I have a Mountfield mower with a Honda GCV135 engine which starts fine
and seems to run fine but then cuts out after a minute or so, it
restarts ok but cuts out again after a minute.

A friend looked at it and checked the plug and said it seems to be
running rich does anyone have any suggestions or know how to set the
mixture on this engine.

That engine has a paper air filter element, how long since you changed it?
Take a look at...
http://www.honda-engines-eu.com/en/welcome.html

which has some information and no doubt an e-mail to them would get you an
answer if it's a known problem like an electrical cut-out fault.
Alternatively they have a good search facility for service facilities.


--
Regards
Bob
In Runnymede, 17 miles West of London