Atco Ensign problems

18 months ago our lawn mower became ill and would lose power whilst cutting the grass. The mower would tick over ok (albeit with timing issues), but when moving, the engine would just cut out. We were always able to complete a few lengths of the lawn though.
When calling out an engineer to sort the problem (and sharpen the blades) he suggested it may be the carburetor.
Present day!!
I just completed an oil and spark plug change. The mower started 1st time and ticked over; still out of time and with a little misfire. I again completed a few lengths of the lawn, after which the mower cut out; In fact, the mower was then only happy ticking over.
I have adjusted the mixture to make it richer, which allowed me to complete the lawn. The mower is still a bit nervous when getting going, and does not tick over like a happy engine should. As I said earlier, there is also a little misfire or cough/splutter. Would prefer to keep using this mower as we prefer the quality to the nasty B&Q mower we also have.
Is it the carburetor or a settings issue?

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