1993 Volkswagen Golf Timing problem or carb problem

I recently took my car to the mechs after my top was blown cause by water pipe bursting had evrything replaced and fixed by the mechanic. Problem is my car lacks power and doesnt go 120km and when I switch off the engine its as if the carb is still taking in petrol causing it to shake. The mechanic said it was the pre-ignition switch from the carb and when I took it to the carb specialist he said my timing was off. Im confused as to whome should I believe. What could be the cause is it the timing or the carb because the carb was fine before the blown gasket.