When I get up on plane with the throtle all the way down trim all the way up I can only get 5000 rpm and 58 mph. Let off the throttle (for a split second) mash it back down and it will go up to 5500 rpm and 69 mph. What could be causing this? Any ideas?

Sounds like it might be in guardian mode.

Idk usually that shuts it down to nearly nothing, it's wiered

That motor should be able to turn up to 6400 rpm. Something is not right. I would take it in and have it checked out. Sounds like a fuel restriction problem or reeds.

Sounds like another Merc with ignition coil problems. Yes these motors have reed trouble.

It has a bad rattle at like 5000 rpm, I assumed it was reed rattle which I read was a common problem, but was normal.

It only rattles for a few seconds then quits.

You need to get it apart and check the reeds. Had a 02 XB last year no power, that had bad reeds, fixed that, had injectors cleaned-flowed, Ran like a champ.

Take it to Town and Country Marine in Hallsville, TX. Joe Ed is the best merc mechanic around here in east texas.

I've already talked to Joe about it, and he drove the boat a few months back with the rattle. I don't know what causes it. Guess I just need to take it in and say fix it.

This is just a thought:

http://www.bbcboards.net/mercury-3-liter...t-pictures.html

Yeah I'm actually more concerned about the trotle problem more than the rattling. Cant figure out why I can't pass 5500 rpm

I don't think I should have to run a 24 pitch with a 250

I'd troubleshoot it from easy to difficult by checking checking your throttle cable, Hot Foot(if equipped) return spring and the engine throttle linkage(under the cowl) for evidence of binding, not returning to idle stops and bent or damaged linkage rods.