Lake lanier -1,071 feet (326 m) above mean sea level.

Cool. Once in a while someone asks about low power and then later we find they are at some crazy high elevation like 8,000ft or something.

Which # cyl's ?

just did a engine like this, in 18' powerquest, stock 260 short block, votec heads, performer RPM intake, 600 edelbrock carb, and had comp grind me a custom roller cam, boat runs awesome, needing to change dist now as on rev limiter easily plus needs to turn 5500 to be at peak power

If you haven't bought pistons yet get yourself some d-dish pistons to complement those heads. Huge upgrade. Also do yourself a favor and check the piston to deck clearance. .040 quench distance is the trick.
From that you can either measure or at least calculate expected compression ratio with your heads and head gaskets.

As far as carburetor, try it before you mess with things. If you are good at research you can dig up the carb specs for same engine as manufactured for Mercruiser. I think they built some that way. But it would take some looking.

A couple of thoughts. Vortec heads run less timing than early style heads. You probably had too much advance. Also, if it had flat top pistons the compression would have been raised to around 10:1, which definitely would require premium fuel. If you are using the stock rods, spend the money for upgraded bolts.

Bob

I put 64 cc heads on a 400 small block. 3 hours running in the river in a 24 sundancer and most of the pistons were missing chunks from the top ring up. stock cast pistons plus 10:1 or more compression plus high throttle marine engine loads... I guess the pistons were just trying to adjust the CR to where it shoulda been! get a vortec roller block. they are cheap and plentiful. forged pistons are absolutely a must. my one problem with vortec heads is the intake bolt pattern. I like the old style with more bolts at a better angle to seal the intake to the heads. a vacuum leak at high throttle is like hitting the oxy lever on a cutting torch.

Welcome!!

I just built a nice little Vortec 355 roller block with Vortec heads. I picked up the heads from a local junkyard and the block I snagged off CL. The heads had the spring pockets cut for traditional springs and the rocker studs have been pinned. I was going to use the Alex's kit, which has great reviews....its a good choice. As for a cam, I am running an Indmar 340 cam (same as 300 Merc) or you can run the GMPP 14097395 cam (IIRC). I am running Speed Pro hyper pistons which should yield me right around 9.5-1 CR. This engine is going into my 18 Donzi, which should make for a fun ride.

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Best of luck