What is this Carb part and how should I adjust it? 2001 Merc 9.9

I rebuilt my first carb on my 2001 Mercury 9.9 bigfoot. Everything went well except I'm not sure what this part is (- see attached picture #37). In the rebuild kit, this part came with it so I changed it. The engine runs great until I put the cap (#38) over the #37. Then the engine has a hard time running smoothly.

Mercury Parts Express doesn't say what this is except it's "NSS" or not sold seperately.

Any idea on what this part does, and how I should adjust it so when I put the cap over it the engine still runs smoothly?

Thanks in advance.

Re: What is this Carb part and how should I adjust it? 2001 Merc 9.9

Is #35 a spring?
Then 37 should be the idle mixture setting and the plug is EPA's attempt to keep owners from messing with it.

It is necessary to remove for proper cleaning, also pay close attention to the very small fuel holes under that top plate, all must be clean and clear.

Idle mixture must be set IN gear while IN the water for the proper load and back-pressure on the motor, adjust to 'best' idle speed PLUS about 1/8 turn out to provide the additional fuel for acceleration. Test acceleration, any hesitation proves a need for 'slightly' more idle fuel.

Install cap only after perfection, or leave it off. Laws require a shop to reinstall the cap.

Re: What is this Carb part and how should I adjust it? 2001 Merc 9.9

35 is a spring so I believe you are correct. Makes sense because there is hesitation when throttling up. I will put her in the water and see how well I can adjust it. Thanks for the good info!

Re: What is this Carb part and how should I adjust it? 2001 Merc 9.9

Is it posible that you left out part #36 when you reinstalled it. i beive that is an O-ring to seal air leakage around the needle valve. If you adjust it right with leakage it will be rich with cap installed. I know this carb very well.