912 ULS no more than ~4800 RPM

No doubt difficult to recall - did you notice any change in engine sound/feeling when you went to full throttle?

If not, the problem may be your tacho, giving an incorrect reading.

If it passes the preflight A/B ignition test, and it runs smooth as you say, then spark plugs and caps don’t seem a likely cause of reduced max power. Assuming compression is good, an air/fuel problem seems more likely. Maybe the air filter as others have suggested, but you would not think that would happen suddenly. 

Hey Sean!
The engine was smooth. apart of the 4.8k When I was Taxing out at idle and I tried to put power, the engine almost quit. I had to move the throttle up, down to keep the engine running. 

Hello Jeff

I'm changing the spark plugs because it's time to do it. I'm changing the caps because I took one out to check the spark plug and when I tried to put it back it didn't make good contact so, I'm going to change the 8 caps too. A/B was good.

agree, I'm not a A&P or iRMT but I think is air/fuel problem

Air / fuel problems do not usually occur on their own - something else is usually the precursor.

Restricted / excessive fuel flow - possible partially blocked filter, fuel line  "kinked", choke mechanism staying on, broken return spring, lever slipping on cable, etc.

Air - almost always a dirty filter, collapsed / blocked air inlet tube