Just a heads up. This problem has NOT been resolved. 2 New Engines. Both dyno'd within 1% of each other. All new ignitions. All plugs show about the same color. Drives are fresh and good. Alignment was off but fixed with no affect. Tested the rotational resistance with a torque wrench and both engines are the same in and out of gear. What a headache!!!! Any more thoughts?? What am I missing?? Just so crazy the RPM is still the same off. 300 rom lower at 3000 and 500 rpm lower at 6000

I slept since this was posted last

I did say Outdrive gear ratio as just throwing it out before in my post

verify accurate tach output readings ? - swap tachs to see what happens
leakdown tests ?
ignition related ? Ignition set up
timing and or timing advance ?
do you have knock sensors ?
fuel pressure thru out rpms ?
Fuel vacuum testing ?
fuel flow / delivery problem ?
Valve spring pressure difference ?
cam timing ?
Wideband tuning / readings ?

I would run the engine that is down on RPM's with a portable fuel supply and see what happens - 6 gallon outboard motor fuel tank and new fuel line with a primer bulb - just for starters

These engines are new right off the dyno. I will be getting actual AFR in boat readings next time out but plugs show good fuel. I have o2 bungs in the headers. I will also swap tachs but if you go off the harmonizing affect the tachs are correct. But I will still verify those things. Both these new motors have timing locked at 34 degrees. No knock sensors. Still dominator carbs. Throttle levers and cables good. All blades are opening the exact same from closed to full throttle checking at various points. Both drives are fresh with 1.36:1 gearing. Props are new and done by bblades.

For the time it takes, can you swap out props to see if the rpm variance disappears? Definitely not doubting the props from Bret but its something to consider as you go through the checklist.

Are your two drives in the same physical height as shown on your indicators?

This is my 3rd set of props. All show the same difference. Yes both drives measure identical

Tested tachs yesterday. They are accurate. I swapped them and then confirmed the readings with digital timing light. The port engine is down and even noticeable by a few hundred rpms in neutral at ramp with the throttles in the same position. Again the carbs are both opening the exact same amount. Only thing I haven't swapped is drives. I guess swap them? Both turned easy by hand when I had them off... Wonder if a drive could do this? Alignment and new gimbals just done a month ago FYI... Still so confused!!!

If this was in my hands I would do a manifold vacuum test and compare to each engine and possible a engine power balance test and then I would swap the drives and go run it in the water. total time about 5 to 6 hours of work to find something.

Like I said from the start gear ratio's possible the problem ???. I have seen this happen close to 15 times over the years so not that common but yes it does happen. .

Whatever the issue is its noticeable even in neutral is the odd part. Open carbs same amount and the starboard free rev rpm is higher than port by 2-300 at almost any throttle position.

Oh sorry I missed that about being off in NEUTRAL so that would mean the outdrives do not have anything to do with that.

Intake manifold vacuum source for testing really is good testing if you understand all the readings and how to do it correctly.

Power balance test

compression testing

Leakdown testing

fuel vacuum testing / fuel delivery source - from start to end (somewhere in the fuel system)

and other stuff I posted before.

lets say you are running your engine both at the exact same throttle position and the engine that is 300 or whatever rpm LOWER - spray some carb cleaner or fuel down the carb - did the engine pick up RPM's or lost more RPM's - I would do this test as well but be careful you do not get a back fire. I would try this at idle to and of course higher rpm's ( spraying fuel or gas down the carb do not just DUMP a bunch down all at once to flood it out - I like carb cleaner that I can control the spray and pattern of the it ) just sying