Idle Screw Adjustment

Murray’s Carburetor Kits are delivered calibrated and in sync. Before shipment, they are tested and adjusted on a live motor.  Each set is set to a baseline idle, so before adjusting, please call or email us so our team can ensure that adjustment is necessary.  Our contact information can be found at the bottom of our web pages.

Here are a few guidelines for idle speed on Murray’s Carburetor Kits:

  1. The idle screw is the brass screw with an exposed spring. (Pictured below)
  2. Turning the screw clockwise raises the idle, and turning it counterclockwise lowers the rpm.
  3. Idle Speed should be between 1350 and 1400 rpm.
  4. Any adjustment to our dual carburetor setup should be exactly the same on both sides.  Ex.  If the idle screw is turned in 1/4 turn on the left carb, then the right carb should also be turned a 1/4 turn.  If this adjustment isn’t performed precisely, the idle will be out of sync and cause poor running conditions.
  5. Make small adjustments!  A little is a lot on Mikuni VM Series Carburetor idle screws.
  6. If you’re adjusting the idle down, you need to go past the point at which you want to start and tighten the screw to the desired increment.  This is because the idle screw is under tension by the spring.  When loosening, the tension is released, and the screw needs to settle back in.  So, always adjust up to your desired idle screw location.  Our guitar player friends will understand this is like tuning a guitar or bass down.

Mikuni VM Idle Screw circled in green.

Showing the location of a Mikuni VM idle screw.

 

How To Get Idle Back In Sync

Many customers forget or don’t know that Murray has calibrated their carburetors on a live motor before shipping.  If your cable is routed incorrectly it can cause issues that seem like the bike is running rich or idling too high.  Many people decide to self adjust, and this often leads to an idle sync issue.  If this is you, we have a couple of options for you .  One, you’re welcome to send the carburetors back, as they come with lifetime servicing, and we will clean and calibrate them free of charge.  Two, you can attempt to do this yourself using the directions below.  If done properly your idle will be close to in sync.  

Step 1.  You will need a caliper like the one shown below.

Vernier caliper for measuring Mikuni VM idle protrusion.

Step 2.  Set your caliper to 13mm and lock it into place.

Vernier caliper locked at 13mm to measure idle screws on a Murray's Carburetor Kit.

Step 3.  Use the caliper as a gauge to set your idle screws to the exact same protrusion.  Use the picture below to see how you should measure.

Using caliper as a gauge to set idle screw height on a Murray's Carburetor Kit.

*Please note that this procedure will only get you to close and not perfection.  For a complete calibration give us a call or email us using the contact information below.*