How To: Idle mixture screw adjustment
Watch a step by step tutorial on how to adjust the idle mixture screw on a typical carburetor. Whether your a professional mechanic or a novice this video will show you all you need to know to properly set the idle mixture. The idle mixture screw is only one facet of overall carburetor jetting, however it is one of the first places to start when diagnosing a poorly running bike (motorcycle) or ATV (quad). Follow the tips in this video to be on your way to being a jetting pro! Now this video shows a generalized procedure for adjusting the idle speed air/fuel mixture. All carburetors are basically the same but with some subtle design differences. Use this video to help identify what type of mixture screw you have. A carburetor will have an air screw or a fuel screw, one or the other but never both. This video uses some general guidelines and specifications, but always refer to a service manual for specific adjustment baseline specifications. If you need help with carburetor tuning or need help understanding carb operation leave a comment below. Click here for some service manuals and wiring diagrams (updated frequently)
If your having troubles getting your bike running for after it has been sitting for an extended period of time check out the other pages that can help you Get a neglected bike running again AND How to clean the fuel system on a motorcycle or ATV. These things should be done FIRST!
Pilot jets which are sometimes called slow jets are the fuel metering system in modern carburetors that determine how your engine will run at lower, off idle, throttle openings.
The Pilot jet only controls the air/fuel mixture at idle and low, off idle, throttle openings. The screw adjustment (explained in the video) is used to fine tune this adjustment at idle speed only. It has very little or no effect on off idle and larger throttle openings. There are several elements to any carburetor important in achieving this and these are the throttle slide and its cutaway angle, the throttle needle that is held into the throttle slide by a circlip, the slow or Pilot jets and then the main jet. The main jet is screwed into a brass component located in the float bowl of the carburetor and is simple to remove by draining the float bowl carefully and using a screwdriver or a hex wrench to remove the old main jet and exchanging for the new chosen size.
Main jets control top end or larger throttle opening performance, Pilot jets and needle position control the slow and mid range performance.
The pilot or slow jet is also accessed from inside the float bowl area but is a small diameter brass jet with a screwdriver slot just in front of the larger brass main jet. Pilot jets can be removed and changed for a chosen size after determining what is wrong with the engines performance beforehand.
When changing exhausts and filters to a different to stock design or when you ride in a significant temperature change , altitude or humidity difference a pilot jet change may be needed. Below is a graphic that will help explain the different jetting effects related to the throttle slide opening and how each adjustable part in the carburetor affects the jetting at a given throttle opening. Studying it may help you diagnose a jetting problem not related to the idle mixture.