Opening throttle won't do anything for the number other make it come up slightly quicker. It pumps what it pumps. If it has been sitting I would run it first then redo comp check. IMO it can not be accurately done on a motor that has been sitting.

It can vary a little from engine to engine depending on internal parts however 5-10%. Test should be done with engine warmed up to near operating temps however when done cold sometimes any known problems may exaggerate and show lessor comp. So some may argue its best to do cold however most mechanicss will perform when warm.

Well...just did the leakdown....all the cylinders at 125psi were at 20%...all 4 cylinders at 95psi were at 70% and the air was escaping through the carb...meaning intake valves.

Hopefully just pulling heads and a valve job would fix things. Again the tests were done cold. I redid the compression tests with two different gauges both 90-95psi on the same 4 cylinders. Maybe the one who posted about the center riser manifolds is correct about slight reversion.

Supposedly the boat still runs good...so we will see. Not a deal breaker if the price is adjusted to compensate for work.

I wouldn't be surprised that with fresh heads and fixing the manifold situation that you'd be good for another 900 hours.

yeap, needs valve job now, as said before, do that and good for a long time as long as was maintained well, about those hours heads need work

unless he has recent receipts, replace the exhaust manifolds...

the 330 merc was what like 7.9:1 static compression? 125psi is probably within range

Thanks for all the input so far.... no receipts for manifolds but saw notes in a log book for rust on the manifolds and a receipt for riser gaskets. Sucks buying boat knowing it needs works..... if the sea trail goes ok... may use it for a short Michigan summer and do the heads next winter.... if it comes pulling motor not fun in the style Bertram (flush deck motor yacht)... motors are mid shop under the salon floor, headliner has be removed and 6x6 panel removed from cabin top and motors hoisted out.

Per mercruiser recommended ranges and test procedures regardless what many might not know or think. Just putting it out for info.

http://www.boatfix.com/merc/bullet/97/97_25.pdf

Well at least it's better knowing going into rather than hitting you unexpectedly and hopefully the sale price will warrant such to be done. Just keep in mind IF it's a salt water boat and given its 5 and 6 on each engine with the lower compression just be prepared for the worse and hope for the best. Reversion seems unlikely in a boat like that with 330's however I just saw a set of heads that came off salt water boat with reversion issues and didn't look pretty. Just a thought but maybe post an ad in wanted section for 330 peanut port heads. there use to be a a lot of take offs and they sit. I had to sets I just gave away with 5 hours on them. Not only that in a weekend you could have old ones off and new ones on. I just have to see you take out a piston or cylinder wall from a potential broken valve. Its your decision of course but at least think about it anyway.