Had problems with a bad impellor & overheating issues earlier this season. Now those issues are resolved the boat only revs to 4000 rpms & has no boost? It doesn't make any abnormal sounds like something is mechanically wrong , its just lazy? The blower is a Weiand 256 on a 509bbc. Can the blower to manifold seal be compromised causing no boost? I really don't know where or what to check?

Did it ever build boost? What was it before?

After an overheat, always do a compression test 1st.

Overheating can cause some issues. Did you make note what kind of temps you were getting and how long did you run hot. Comp test first as mentioned. Could be as simple as a head gasket to warped cylinders, exhaust valve and others. Assuming it’s running on all 8? Do a comp test and leak down of possible and let us know the results. Hope it’s something simple..... Also change your oil if you haven’t already. Does it look milky on the dipstick after you replaced the impeller?

I did do a compression check & the compression was lower than I expected but all cylinders were close to one another. A Holley tech today told me the head gaskets could be blown leaking into all cylinders. Any blown head gasket I've ever seen in the past is usually between two cylinders not all the the cylinders? I never had a boost gauge on it when it ran good so I don't know what it was producing before. Now that it has a boost guage it swings from 10" vacuum to zero & no boost. @ my elevation & pulleys I would expect to see 3.5lbs. No milky oil. The exhaust manifolds were very hot well into the 300's & the the motor was closer to 240.

I hate to replace the head gaskets before ruling everything else out, My thought is the blower is loosing boost & the drag of the blower is lowering the RPMs? If the intake manifold gaskets or blower to intake manifold gaskets failed would i hear the leak? The motor sounds fine it just has no power or boost.

btw: is this serp or cog belt ? if serp maybe slipping

16rib serp

Had the same thing happen a few times on my 525SC. Replaced blower belt and was good for another couple seasons.

do a leak down test,see if anything is compromised before throwing time and parts at this.

as griff said check the blower belt for slipage.