Engine wont rev past 2500

I have a 2001 johnson 30 hp 2 stroke it runs fine in the low rpms but as soon as you rev it past 2500 rpm it starts thumping and wont rev. I pulled the fuel filter of and there was alot of gunk in it i cleaned it out it was one of the screen types.when i first start it runs fine for a minute or 2 then it starts thumping and wont rev.when i tilted the gas tank up to the side where the intake is. it ran fine for about 45 minutes. however that stop working is it bad gas or somthing else i cleaned the tank with some water still no affect the spark plugs look fine. What should i do. is the bowl of the carb gunked up.

What do you mean by "thumping"?

Does the engine feel like it's hitting a submerged object over and over as you advance the throttle?

it sounds like its missfiring.I cleared the tank out and filtered out all of the gunk. it still does the same thing. When the tankl is tilted up it will run fine somtime it will go for up to 10 mineuts then as soon as the tank falls down it stops working right could it be timing advance.