pinging

I just bought a 1997 Festiva GLXI 1.5L. 84000kms.

I got a service and tune today. I told the mechanic about the pinging,
especially under load.

No worries he says, I'll just adjust the timing. He loosens the
distributor and turns it while watching the timing mark and nothing
happens. He earthed the correct terminal according to his manual.
He doesn't know whats going on with the timing. He says the pinging may
be computer related.
So I still have the problem, any suggestions?

Thanks in advance.

PS I know I need a new mechanic.