Help: 4 stroke 50hp Mercury not running well

[blakevw]

I need some help figuring out an issue with my outboard.

when the engine is in neutral it revs up fine, seems to get top rpm (I don't have a tac), but when in gear the rpms are maybe half what they should be. When the boat planes (I have to move forward in the boat to get it to plane) the RPMs are higher, but still not 6500.

I have:
Changed the plugs
fuel filter (this helped with idling)
switched to the spare prop
run a full can of seafoam
drained the gas
topped up the engine oil

The problem started up a week ago, I was sure it was the spark plugs, but nope.

Any suggestions?

Here is a video of the motor full out: https://www.youtube.com/watch?v=Arnav_fKc5I

Thanks in advance

 

[Texasmark]

2 carbs I guess? (If fuel injected forget this.) Might pull the cowl and I guess there is a plastic sound box covering the venturis of the carbs....pull that off too so that you have a clear shot at the venturis. Back to the store for a can of Sea Foam Aerosol this time and shoot it into each venturi as the problem is happening and wait for a reaction before you squirt the other carb. If you are having carb problems, the engine will respond to the SF spray for a few seconds as it burns it off and then go back to doing what it was doing. Do that and come back with the results. Running on muffs or at idle is nothing compared to under load on a boat and especially during the hole shot (coming onto plane).