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So I have tried a couple different props .motor runs great .strong but with each prop it tops out at 4k rpm. Could this be a linkage adjustment ? With a 17 pitch it runs @28 MPH @4k rpm ......with the 15 pitch its about 24 MPH @ 4k ??? I'm thinking its a linkage or rev limiter issue ??

Sure it's not a bad or mis-calibrated tach? You need to get the rpm reported by the ECU to see what's really going on. The ETEC engine cable is $50 bucks on ebay and a simple Garmin 421 chartplotter will read it, nothing else necessary.

Or if your a cheapskate like me take the prop off and hook up your garden hose to lower unit cooling intakes and increase throttle, if its the props it will easily hit the rev limiter, if its the tach(which I believe Mastreb is correct) it will stop a 4K but you will still hear it increasing in rpm speed with your ears.

Agree with all the above, the fact that the 15 pitch doesn't raise the revs logically indicates that either the tacho is wrong and it is cutting out at normal 5600? or there is a fault causing it to cut out early but either way if you don't want to go to the dealer I would favour Rays method as it takes out any electronic bias, sometimes simple is best.
Would have thought the top speed of the 90HP would be higher, how does that compare to other users top speed?

Agree with others; sounds like a bad tach. With my medium/heavy loaded a 13 3/4" x 15 pitch hits 5000 rpm. I dropped down to 13 3/4" x 13 pitch and rev to 5200+. Top end is only 24 mph but I finally have REVERSE that will give you whiplash . BTW I'm running the pontoon series which has a slightly lower gear ratio.