I'm looking at a 1998 Formula 353, it has 500 HP carb motors with 420 hours on them. When should they be freshened up and what needs to be done? Is there anything else I should be looking for? Thanks for any advice.

About 70 hours ago I would have done a top end job at least.
Becareful, valve springs are known to fail and lifters sometimes.
You will be in need of total rebuilds very shortly.

I bought a 95 336 with HP500's with around 350 hours. One engine had just been replaced and the other had a less than great leak-down test. I ran it for a year with no issues and had the one refreshed a year ago with about 400 hours and had a valve job, new springs, rings, bearings, and ended up needing a new cam because of bad wear on a few lobes. Apparently the new springs are better than the originals, but with 420 hours and the original seals intact, I'd say you're on borrowed time.

I've seen them run a lot longer than that. Do a leak down test. That will tell you the engines condition more clearly than the amount of hours. I have seen a bigger problem with water reversion on the 500HP causing failures. They should have internal exhaust flappers.

Isn't refresh time more determined more by level of use/abuse and frequency of regular maintenance than just hours?

I've thought that with excelelnt routine maint and cruising use (vs racing) a gas marine engine will last at least 1000 to 1200 hours. I know diesels are in the 2000 hour or more range.

On these high performance engines the valve train is the weak link. Valve springs and lifters take a beating with the high lifts and rapid opening rates. Roller lifters wear out faster with the high spring rates required to keep the valvetrain under control at high r's. 400 hrs is pushing it. I would do a valve job and replace lifters and springs at this point. The pistons and rings should be good but do a leakdown test to be sure. Bearing should be the last thing to go. I lost a roller lifter a couple of years ago. Bits of metal splashed on the cyl walls and imbedded themselves in the piston skirts. The cyl walls were scored requiring a complete rebuild.

When I was selling used boats I used to get that same question many times.
I told them the engines were getting tired if they can't pull the RPM they did before with the same propellers.
Valve jobs usually do the trick as long as you have had good oil pressure.
The bottom ends of Chevy marine engines are pretty stout unless the owner ignores all the warning signs and doesn't do the right maintenance.

i had hp. 500s carbs went 1030 hrs. changed value springs at each300 hrs and that it . ran the po po out of it. some go and some dont , when in dout ,, gas it

Ack!!
Everything I know and read says that HP500 carbed motors need new valve springs at the 200 hour interval!!
There are some who had broken valve springs at this point, and others who went far passed this point.

I can tell you that I bought a 1999 HP500 carbed motor with 180 hours on it, and redid the top end.
The top end was in good shape, but several valve springs were very weak, already. None had broken yet.

Also, early versions of these motors had low rise Gil tailpipes, much more prone to reversion. The later ones had taller tailpipes, which pretty much fixed this...

What you all have talked about is amazing, with this many hours on these blue beasts. I hope to have mine last a very long time!
Kent

I have (2) sets of ISKY Valve springs (8205 Plus Endurance), titanium retainers and CompCams 10* superlocks for the HP500's - brand new still in the box if anyone is interested. Will let them go cheap.