EXmark quest model qts708gem4200

Hi , I’m new to this group. I have a 42” eXmark quest 708cc twin. I believe i purchased it in 2014. Just recently I’ve had problems with it running properly. It has power loss at high speed. And seams to be missing. I have done quite a bit to it trying to figure out what the problem is. It came on quite sudden. Changed the fuel filter, plugs are in good shape. Not the air cleaner. So then I took the carb apart. It’s a double barrel Huayi carb. Was pretty clean. But replaced the gaskets, and float needle anyways. That didn’t work either. Took off the flywheel cover and sanded the coil gaps and flywheel pickup. That wasn’t it. Also cleaned the screen on the fuel pump. Is there any thing in the fuel tank that could need cleaned? Anyone have any thoughts on this. Thanks

Take the plug leads off one at a time and see if it makes a difference