It will be a night and day difference when you put some timing in it...

Now you are just getting me exited. I am looking for a 23p prop as I mentioned earlier I spun the rubber hub stiletto 23. If I am looking at a mirage is the 27 powerquest a boat that would benefit from the additional cup of the M+ or should I be looking at a vengeance. I noticed the vengeance is only 13.5 in dia, is that too small for a single eng 27' at 5200lbs

That is small block small boat prop diameter.

Here’s a discussion amongst other 27ft Powerquest owners and what they are running for power and props:
Prop for a 270

My opinion... I'd get it to 12.4 AFR and bump the timing to 32° min. Do AFR first with plug checks and then increase timing 2° at a time. I ran 36° with my 9.7:1 steel Merlin headed engines. (fully wedged and ported) Out of the box the heads are a little better than stock, the split on the cam should be 8°-10° as S30 mentioned. I do believe you have a nice little build but it needs a little tweeking. I don't believe you will/would have ever seen more than few MPH going from a built 502 to another modest built 502.

Edit, i was wrong about prev motor. Doh!
Here is the op’s previous 502 build:

Timing advise 502 carb DUI distributor

1) With that much duration @050 and 9.5:1 I definitely agree with 6 to 8 degrees advanced on the cam (106 ICL). If you install it closer to "straight up" you'll want to prop it for 5500-5600 rpm. You DO need to KNOW where your cam is installed. And you CANT KNOW unless the guy who installed it used a properly calibrated degree wheel.

2) what head gasket thickness are you running? If your pistons are .010" below deck, then you would be best served with .030"-.035" compressed height on the head gaskets because that puts you in the optimum "quench clearance" zone (.040-.045 is best for a 502, .036 to .040 is best for a 454). The reason this is important is because squish turbulence really suppresses detonation and let's you run higher compression and combustion pressures before you have to worry about detonation. If you have the typical. 039 thick Fel Pro Marine gaskets, then that puts you at .049 which is still well within the efficient squish zone.

3) BBC combustion chamber on pump gas with flat top or low dome pistons is baselined for most effective power production at 36 degrees. But for that to happen a lot of stuff has to match up.
Let's say you have a 7.5:1 BBC and 98 octane gas. The low compression ratio keeps cylinder pressures low during combustion. Advancing the timing gives the flame more time to burn, bringing the pressures up and delivering more torque/power. The high octane burns slower than low octane, so it needs lit sooner as well. This bad combination may need to see as much as 40-42 degrees advance before the power peaks. But it is due to a bad combination.

A 9.5:1 502 (like it has been said, you have to know the bore diameter, piston to deck measurement. piston volume factor in Cc, combustion chamber Cc as measured after polishing, head gasket compressed thickness, fire ring diameter, actual stroke (some regrind cranks can be offset ground for a tiny increase in stroke). Knowing all of that is the only way to know the compression ratio)- anyhow a 9.5:1 502 with .049 quench clearance and a 240/244 cam at 106 ish ICL should honestly handle the full 36 degrees of advance on 91 octane, and MAYBE 89 octane.

A/F Ratio?
E10 (gasoline w 10% ethanol) best power between 12.0 and 12.7
100% gas. between 12.5 and 13.2
I would set my ECU to 12.2 WOT.

I would leave a lower pitch prop on it until you are making enough power to spin it over 5600. I'm guessing your setup will make best power at 5400 (unless I am overestimating the flow of your intake man).

Use brand new plugs. Don't run it intentionally rich, as this will mess w your plug readings.
if you want to creep up on the timing, you need fresh white insulators on the plugs and dont soot them up by running too rich.
Run it out hard, pull the stick back.
Shut it down. Pull whatever plug is easiest to yank.
Look at the plug insulator and see if you can detect any metallic speckling - this will be your indication of detonation.
If you dont see speckling, and youre getting more power with more ignition advance. Then keep adding advance. If you get past 36 degrees and move to 38 degrees and you don't get detonation then you know you're sitting pretty.
Next step is to drop octane (temporary tank) and retry. You may discover that you can run 89 octane at 38 degrees.
So THEN you have your parameters.
Whatever octane you can run at 36 degrees is your sweet spot.

my bullet custom grind 238/245 at .050 has a an icl of 110.have done many 454's and 502's at 109-110 and all the motors rip.but if you want the motor to peter out around 5000-5200 all the power to you.merc uses 106 so their motors don't rev up,built in rev limiter.

From s30

Thanks everyone, I am waiting on a couple of more props to come in and a nice day and off from work and a friend to go out and help, may be a while but I will post results.