norns ur car is usin carb fuel system? if it is u need to go 2 a workshop with a gas analyzer to analyze the carbon concentration from ur exhaust, wit this tool then only ur mechanic can adjust the fuel mixture accurately if a mechanic tune just by hearing the engine sound then he's a fake mechanic he doesnt know what he's doing usually for 4g13 wit carb the best carbon concentration is 1.5% wit that percentage ur car will be more economical on fuel and better performance ur idle speed should be around 850+-50 will be perfect.If norns have nt done ur tune for a long time it could be possible ur car runnin ur fuel mixture too rich which will result in 2 much carbon composite in ur engine so better do it fast.