Troubleshooting Mikuni CV Carburetors Motorcycle

Updated February 21, 2017

The Mikuni CV motorcycle carburettor is not a complex accessory. CV is a term used to describe constant velocity, which is an internal vacuum system that maintains a constant pressure in the carburettor fuel bowl. The throttle regulates the flow of gasoline to the engine and the pressure generates a more rapid response than typical carburettors. The Mikuni CV is not difficult to troubleshoot with the aid of an assistant and a few hand tools. The procedure is done with the carburettor in place on the engine.

Begin with the motorcycle engine cool and balanced on the kickstand. Remove the air filter cover and air filter from the front of the carburettor. Depending on the particular cover, there will be a bolt or screw in the centre of the cover that secures the cover to the carburettor. Pull the cover and air filter off and set them aside.

Start the motorcycle and let the engine idle for two minutes. If the idle becomes rough, the fuel mixture setting is OK and you can replace the filter and cover. If the engine idles smoothly without the filter and cover, the fuel mixture is too rich and needs adjusting. Proceed to the next step.

Locate the pilot air screw that is just below the throttle linkage. The screw has a slot in the head and a small spring on the threaded shaft of the screw. Set a screwdriver nearby.

Let the bike continue to idle and replace the air filter and cover. This is not dangerous and should be relatively easy to do with the engine idling smoothly. The idle will be rough again with the filter and cover in place.

Use the screwdriver and turn the pilot air screw counterclockwise one-half turn. If the idle is still rough, continue to turn the screw at quarter turn intervals counterclockwise. Somewhere in this stage of the procedure the engine should idle smoothly. If it doesn't, there's a problem with the main jet inside the fuel bowl.

Have an assistant hold the throttle position at 1500rpm. Loosen the vacuum chamber cap on top of the carburettor by turning it one full rotation counterclockwise with adjustable pliers. Do not remove the cover at this point. Set the pliers aside.

Hold slight downward pressure on the top of the cap with one hand as you unscrew it from the top of the carburettor with the other hand. The downward pressure will prevent the slide return spring below the cap from springing out.

Continue to hold downward pressure and loosen the cap completely. Lift the cap just off the top of the carburettor. If the engine begins to stall out, screw the cap back on a full turn by hand. If the engine speed picks up again, the vacuum diaphragm is OK. If the engine doesn't pick up, the diaphragm and diaphragm gasket need to be replaced.

Obtain a carburettor rebuild kit and spray carburettor cleaner if one or both problems were determined. Refer to the instructions for the kit and clean the carburettor. Replace the main jet inside the carburettor bowl. Replace the vacuum diaphragm and diaphragm gasket. Complete the rebuild as described in the instructions.

Things You'll Need

  • Metric socket and ratchet
  • Screwdriver
  • Large adjustable pliers
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About the Author

William Machin began work in construction at the age of 15, while still in high school. In 35 years, he's gained expertise in all phases of residential construction, retrofit and remodeling. His hobbies include horses, motorcycles, road racing and sport fishing. He studied architecture at Taft Junior College.