Top speed vs state of tune

[RichV]

My 89 American Skier Advance with a Indmar 351 ran 39 flat out the last time I had it in the water per the gps on my phone.   I have no reason to belive it is out of tune.  What speed should a well tuned Adance run with the stock prop?

[backfoot100]

What RPM was it turning at that speed?

[RichV]

The tach shows 4200 rpms at full throttle.  I have a cast 13x13 prop.

[backfoot100]

4200 is on the low side. I think stock 351's should run at 4400 by factory specs and 4600 doesn't hurt. I'm pretty sure that stock props were 13x12 but not sure. A 13x12 would get you back to 4400. The top speed still seems low to me.
When was the last time a tune up was done?

[RichV]

I am not sure what my engine has for a cam, it may be a passenger car or truck cam.  The short block may have been replaced with a junk yard refuge. 

[backfoot100]

What makes you think the short block was replaced?
A factory 351 Advance should easily run 44-45. 4400rpm should get you 44mph or very close to it. 4200 RPM should get in the 42mph range. You may want to verify your RPM with a mechanical shop tach too. The factory tach can be inaccurate.

[RichV]

My engine is painted blue instead of Indmar silver.  The previous owner told me that ithe original had cracked and was repared by a  mechanic by replacing the engine.  When I replaced the valve stem seals, the casting numbers on the head were correct fot the year of the boat.  The block looks not so smooth under the blue paint, the heads look better.

[Mike Harry]

I think you would have an issue hitting the high 30's with an automotive style cam. Has it ever reached a higher speed or RPM's for you? Its hard to tell with what little you are describing.

Are you positive that the secondary is opening at all? Opening fully? Seems like the speed you are describing would be right around the sweet spot where 2 opens to 4. Could very well be a timing issue also............. but if its not detonating or pinging or hard to start seems a little less likely.