[nooner]

I have a 1998 z214 with a 4.3 L and at 4800 rpm it's top speedis 36 mph, any ideas ?

[mikeyt]

Yep you should be in the 40-45mph range. Are you confirming your speed with a GPS? Is the prop in good condition? Is the boat hull clean? Is the motor in good tune?

[GTSJK]

I have a 204 with a 4.3L and with just myself in the boat get ~ 45mph top speed.

I did have problems a fewyears back where my speed was slowly going down. Found out my exhaust flaps came loose and got ingested into the exhaust and was restricting exhaust and thus speed. Ended up getting a rash of serious backfiring and knew something was up.
Got them taken out and replaced (alot of labor to do this) and she runs fine now.