Z/S sound in lapwing theory

[jhonasn]

Maybe that's only a perception mine because in my native language we differentiate clearly this, but, why in lapwing words like: desire, because, base, spousal where the S sound like Z we use S in the stroke stead off Z?
First I thought that's because in english all S sound like Z but we have words like: beside, insert, generosity, suppressant, versatile it that's S actually sound like S.

PS.: I'm developing a portuguese theory inspired in lapwing, so this question will help me in my theory too.

[aerickt]

Hey, sorry for late reply, but the rule is to normally use S or -S for all sounds spelled with an s. We would only use STKPW or -Z if the sound is spelled with a z, or for disambiguation purposes (e.g. rice RAOEUS and rise RAOEUZ).