Might be ign timing. I had never checked my timing until I found my centrifugal weights rusted and bound up (at full adv). Before I pulled the distributors, I checked the timing and was startled to find timing locked at ~42 degs Plugs showed no signs of detonation. Repaired the distributors and set timing to 36 degs at 3k. Lost abt 8 mph. Started a thread and another member mentioned that some of the old school big blocks like a lot of timing. I believe the heads have a lot to do with how much timing you can get away with. I never did put the timing back to 42 cause the boat already scares the crap out of me when it climbs up on the pad.

Exactly. Did you CC the heads before installing? 190 PSI is high for stock marine.

Does it lose RPM if you hold it wide open?

What octane fuel?

What ignition system are you running?

I got back out on the water and did some testing this past week, with the timing set at 36* I ran 63ish mph with a light load (just me and my dog in the boat) which is the best I have seen in this boat. It pulled 5000 rpm. but my over the phone tuner guy is saying it should be running closer to 70.

I found a little more info on the heads. I swapped out the cracked gen VI 088 heads for the 990 heads. the 990 heads are the rect port heads that came on the mark IV engines. the heads were decked (not sure how much), and had light cleanup porting work done to them. new ARP studs and new oem replacement rocker arms were used.. also, the machinist had a tag on the heads that said " new HiRev Springs"

I run 91 octane.
Ill have to double check the name on the ignition but its the one where it wants you to set base timing by using a jumper wire? I've just been setting the distributor with a timing light at 3500 rpm.
my tuner guy said crank the timing to 38 but im a little hesitant because I think there are other things here holding me back.

Also, it is clear that the boat is running very rich at low speeds. when I hit the throttle and that accelerator pump dumps fuel I can see it in the exhaust. my idle adjustment screws are at 1/2 turn out. while the boat was idling in the water I closed the screws and the engine stumbled a bit. I set the screws back to a half turn out and the idle picked up.my guy said that they should not be closed that much and that I have a N/S or internal leak going on.
this is at early morning Air temps at 90*+ here in havasu

before my next time out I will pull the carter afb carb and see if I can find an issue, but my guy told me to buy a holley 4780 800cfm if I have the money

A question that I have come up with is about springs. My gen vi engine came with a roller cam. could the springs be a problem? not sure what HiRev Springs means

did not cc the heads, it seems to hold the 5000rpm pretty solid. I can pull plugs when I pull carb also.

HI Rev springs are a valve spring. They make them for hydraulic roller cams. I assume you still have the hydraulic roller cam in there. Accelerator pumps dump a lot of fuel. Also, your idle circuit should have nothing to do with anything once the throttle is cracked.

It appears that you are where you were prior to rebuild. You might look at reading plugs and jetting out on the lake.

This is incorrect. Have witnessed and repaired lean pop misfires when planing due to idle screws misadusted (when engine cold) and also lean pop misfires when idle circuit clogged with crap.

I've cured many idle/low speed/accelerating/ etc a Holley by removing idle misxture screws, placing tip of red stra on carb cleaner can onto idle air bleed and spraying until comes out of idle screw hole well. Then stick the red straw slightly into idle screw hole and spraying until comes out of idle air bleed hole okay. Do this a few times back and forth. Reinstall screws and set idle mixture.

That ignition is set by warming the engine then shut it down. Ground the purple & white ignition shunt wire. Start the engine back up an set the idle to 775 or less rpm. Set timing to 22 degrees below what you want all-in to be. Readjust idle if needed. Then set your idle mix screws for either highest idle or most vacuum. Depending on if you have a vacuum gauge handy. Then readjust idle to 700, 725 or so. Shut off engine then remove ground from purple/white wire.
When you restart, the ignition will correct and smooth out your idle, and it will adjust timing on the go for you. Your idle and other rpm hunting should go away.

Thunderbolt V ignition can't just be set to an all-in value cause it makes adjustments based on acceleration vs steady speed. It constantly advances til it hears knock then backs up a bit. They call it mean best timing.