Briggs and Stratton 12.5hp - engine surging

Now I'm not too familar with engines at all so please pardon all statements that make no sense....just let me know about them and I'll try to clear up what i mean.

Ok, with the disclaimer done now we can get down to business. After getting my riding mower out this spring it's been not running so well... There's a consistent up and down revving of the engine at idle and when running at the normal throttle position. With the choke engaged it evens out, but the throttle position required to keep it running smooth changes whether the blade is engaged or not. And when getting heavy load (going through tall grass or lots of clippings) it'll almost stall.
I've taken apart the carburetor (it was fairly clean), cleaned out the fuel line and gas tank, replaced the spark plug and still have the same issue.

I may have messed with some things i shouldn't have when taking apart the carburetor. All I know at this point is i can get the surging to stop by adjusting the top no load screw (screwing it in), but this seems like its making the engine work too hard (its kind of hard for me to tell though because the after market muffler that someone has rigged onto the engine came off earlier this spring). Examining the throttle plate, how much rotation should there be from full open to where it engages the top no load screw? Should the throttle plate be able to fully close?

Any thoughts or directions on what i should be doing would be very appreciated!
Thanks.

 

The throttle should be able to close pretty much all the way. Adjusting that screw is masking the problem. You have a restriction in the carb still, it needs more cleaning.

 

Thanks cheese. I'll give it another cleaning.