hey i got a 89 corolla sr5 4af. when i bought the car i changed every fluid and filter and the plugs. when i changed the plugs they were really white every cylinder. so i changed them and in 2 months i decided to check them. they were white again, so i went and bought a lower heat range plug and that still didnt do it when i checked those in 2 weeks. i changed the timing as well to stock 5 degrees. ( i live in british columbia) i checked the codes today and the reader said no codes stored. so im thinking that it might be a vaccum leak or intake manifold leak? or it might be an o2 sensor bad? maybe carb? it runs fine but the plugs are white assuming that its lean. any help is appreciated

Iain