Champion Generator not even close to rated output

You might want to consider a new carburetor. They cost under 20 dollars:
Click here for carburetor

Another thing to mention is your extension cord probably has 15 amp rated ends and you are trying to draw 25 amps. It will eventually overheat the cord ends, plug strip and 5-20r receptacle. Maybe you should try two separate extension cords, if the load is going to be continuous.