Carburetor adjustment.

[Jb7054]

I’m not too familiar with carburetors but I’m trying to fix surging on my lawn tractor. It’s a B&S 21hp single cylinder with a Nikki carb. I’ve made some adjustments but I’d like to understand what the screws I’m turning do. On the carb there are two screws. One comes off at an angle with a plastic end on it. It doesn’t turn much but doesn’t seem to have much of an effect on the engine as it’s running. Above that is a screw with a spring. It seems to limit how far the throttle can open. The more I tightened that screw the less erratic the engine ran. With that screw completely tight the surging is just about gone but I feel like that’s basically choking the engine.

 

[bertsmobile1]

Surging on a governed engine is always caused by the engine not getting enough fuel
So stop twiddleing screws you have no idea about and clean the carb as shown here outdoor power Equipment site

 

[Jb7054]

I originally sprayed it with carb cleaner without removing it. Now I disassembled every part. It’s cleaner than it ever was. Inside and out. Once I reinstall it I will still have to adjust the throttle screw to get the idle right. Is there a common number of turns for these?

 

[keakar]

I originally sprayed it with carb cleaner without removing it. Now I disassembled every part. It’s cleaner than it ever was. Inside and out. Once I reinstall it I will still have to adjust the throttle screw to get the idle right. Is there a common number of turns for these?

the screw with the cap on it is usually set at turned in until it bottoms out and stops and then back it off 1 1/4" complete turns out

the one pressing on the tab you want to set it just touching the pin plus a half turn more, adjust if its too high but wait until after engine has warmed up before you do a final adjust to set the minimum slowest idle speed so engine wont ever die on you if you set the throttle too low

 

[StarTech]

There is not a preset number turns on the minimum idle speed screw. Basically you want to set the idle to 1750 to 1900 rpm using a tach. With idle speed set if you still have surging it can be a loose main jet. The tiny o-ring shrinks with age on these main jets. And for the inexperience you can easily loose this jet while the carburetor is apart. Plus whenever you have Nikki apart the bowl gasket set should be replaced. If installing a new gasket it need to be inspected with a magnifier for any nicks or other damage. I have received defective gaskets from Briggs in the past.

Also some engines just need the idle speed set a little higher. Setting too low will also cause surging as the governor closes the throttle plate too much and completely shuts off the fuel/air mixture.

Note using carburetor cleaner on rubber parts is asking for problems. Here all I ever use on carburetors is soap and water in an ultrasonic cleaner but years ago I take them fully apart and use a dip carburetor cleaner. Back then it was a lot of extra work and I had to replace a lot of paper gaskets.