If your engine wasn't pinking before, then your mixture could be too lean or your timing is out.
If the bike ran well in the past, then your carb could be showing signs of wear e.g. warped flange.
The condition of the plug should give you some idea as to the mixture.
My bike started pinking especially fully freighted and up hills. It doesn't now. However, I put in a new piston and rings, put on a brand new carb, fitted a new HT lead, Mag pick up and unsupressed plug cap and de-coked the head.
I think the biggest improvement was the new carb. The bike starts first or second kick, hot or cold and doesn't pink.
The condition of the Mag and the carb seems to make huge differences to "performance" and starting on old bikes.
Try a new plug with the correct gap first, but with my luck the solution tends not to be a 5 minute job!
Regards
Pat
Solved !!! In the end I had to replace the Mag pick up, timing was slightly out, changed the head gasket and torqued it down properly as instructed on this forum and switched to Shell unleaded instead of Morrisons.
Its really rewarding after all that work to have a bike that feels sorted and bumbles along quite happily now.
