1990ish 520 idle issue

[cgambino1018 28]

Newbie here
Have a 520 with I believe Kohler engine.   The idle is sporadic.  About 3100 to 3700 rpm.  

 

When you look down the carb you can see gas being sprayed in and the stopped.  Also the lower butterfly is opening and closing.  

 

When I put my finger over the gas being sprayed in the idle becomes consist. 

Any advice is appreciated 

Edited May 25 by cgambino1018
Added engine label

[RED-Z06 2,567]

That would be an Onan P220 engine.

 

Some of these had an acceleration pump that shot a bit of gas in on throttle up, which if its surging...happens every time the throttle blade opens.  You can remove that air filter housing and the top of the carb has 4 screws, then the carb top lifts off and you can blow out all the jets and look for trash in the bowl/bottom half. Surging is usually caused by a blocked up pilot jet.

[oliver2-44 10,679]

Reds answer is spot on. 
Lots of friendly help here and some good natured kidding around. 

[Ed Kennell 41,772]

I had to clean my P220 ONAN carb three times before i I found the speck of dirt that caused the surging.

[lynnmor 7,788]

Yes, clean the Nikki carburetor on your 20 horsepower Onan as mentioned above.  If that doesn't fix it, check to see if the 3/8" diameter aluminum welch plug is in place on the muffler side of the carburetor.  The last of the carburetors without an accelerator pump were 1997 and possibly a few 1996 models.  Here is a video so you know what to look for.  If you could supply the serial number on a label under the left side of the seat stuck to the fender, we can determine the year, the label on the engine leads me to believe that you have a newer tractor with the pump.  Report back, there can be a number of other things that can cause your problem.