Troubleshooting Mikuni CV Carburetorsby John Albers
Vehicle Is Difficult to Start
Mikuni CV carburetors can be touchy when it comes to keeping them properly adjusted. If the engine hasn't been run in a while, the jet may be gummed up, preventing an even supply of fuel. Spraying some carburetor cleaner into the jet should fix this. If the engine is difficult to start only when it is still cold, the throttle plate may not close entirely when it should. Adjust the throttle cables until the plate stays closed when the throttle is not engaged. Conversely, if the engine is tough to start only when it's been running for a while, the idle mixture screw in the side of the carburetor block is too far out, producing too rich a ratio of fuel and air leading to the engine. Tighten the screw with a turn or two of a flathead screwdriver.
Engine Idles Roughly
If the engine is idling rough, coughing and sputtering, then it's the low speed portion of the Mikuni CV carburetor. This could include the low speed jet, the inlet system air intake or the enricher valve. The low speed jet is probably not plugged if the engine actually started, but it could be improperly seated within the carburetor, causing an inconstant flow of fuel. Likewise, the inlet air intake could be partially blocked or the air intake hose may be broken. Clear any visible blockage and look for cracks on the hose, replacing it if need be. The enricher valve could be improperly seated as well, in which case it would visibly rattle. It also might be leaking, being wet to the touch. Replace this valve if that is the case. The engine should run clean now.
Vehicle Has Poor Acceleration
First make sure that there is enough fuel in the tank to provide an uninterrupted flow through the carburetor. Try putting a little carburetor cleaner in the fuel mix to clear out any blockages. Next check the throttle cables. If they are misaligned, it would result in poor engine power because the throttle plate isn't opening entirely, starving the engine. Likewise, check for all the problems there would be if the engine were idling roughly. Should nothing be noticeable, the accelerator pump may be broken, in which case the maximum rate that fuel could reach the engine would be reduced. If your car has this problem, take it to a repair shop to ensure the new pump is properly installed.