Mikuni Carb Information

The Following information has been gathered from working with the Mikuni VM series Carbs. on the 1/2 VW and the Verner 3V

  1. Float Level
    1. Set to factory specs or close to. Jet-R-us sells a neat tool so help set the jet precisely.
    2. If float set to maintain to high a fuel level fuel can dribble out of carb intake during flight.
    3. If float level set to low engine can starve at full power, as on take off, it will start loosing power.
  2. Those little lines connected to exterior of Carb. One of those lines is for overflow when carb float level is way to high and to provide air vent for float bowl. Placement of this line can affect fuel mixture if in a low pressure area or a high pressure area. The other line is an air vent line and except for fuel coming out of it, placement of line termination can also have effect on carb operation, lean or rich.
  3. ADJUSTING AIR SCREW: small brass screw on an angle at carb.
    1. Adjust for highest idle rpm with a warm engine, do not use to set rpm.
    2. if air screw is out more than 2 turns at highest rpm a small pilot jet should be fitted. At about 3 turns out spring ends and it could come out in use.
    3. if air screw is in less than 1/2 turn a larger pilot jet should be fitted.
    4. Air screw is a fine-tuning adjustment designed to allow carb to be slightly adjusted for variations in atmospheric conditions.
    5. Turning in richens mixture, turning out leans mixture.
    6. Lean setting will cause engine to surge at very low rpms, bog out or cut-out when throttle is opened.
    7. Rich setting will result in hard starting, plug fowling at low rpm’s, sputtering as throttle is cracked open.
  4. IDLE SCREW:     large screw with spring on side of carb
    1.  This screw adjusts idle rpm, set so that any clockwise movement of idle screw increases rpm.
  5. THE AFORE MENTIONED SETTINGS WILL PRIMARILY AFFECT IDLE AND PERFORMANCE THRU 1/4 THROTTLE.
  6. SLIDE VALVE NEEDLE CLIP POSITION in the 1/4 to 3/4″ throttle position.
    1. Raising the clip will lean out mixture
    2.  Lowering the clip will enrichen mixture
  7. MAIN JET SIZE AFFECTS 3/4 TO FULL THROTTLE PERFORMANCE:
  8. FIRST TIME SET UP:
    1. Start with large main jet, very large
    2. Start with pilot system, then mid range and then main jet.
    3. Get bottom and mid range perfect, (up to 3/4) throttle.
    4. Then do main jet, this way you know the needle/needle jet is not the 3/4 to full throttle fuel restriction.
    5. THROTTLE POSITION, POSITION OF SLIDE, IS WHAT IS IMPORTANT AND TO BE MEASURED WITH THESE SETTINGS, NOT ENGINE POWER.
    6. Having an EGT is very important to set up carb. Need for a very accurate and fast acting EGT cannot be understated.
  9. Misc. info.
    1. I found a situation where engine would not go to idle when throttle closed. Causes for this can be Spring closing slide was not pushing it down as result of air flow thru carb and pressures created.  Use stronger spring or I stretched my spring about 3/4″ inch. Another possible cause is to large a pilot jet. This would be evidenced by engine that goes to idle slowly when throttle is closed.
    2. Make sure you have a throttle stop on throttle cable at wide open throttle so lever hits stop just before cable bottoms out. I pulled end off throttle cable before adding a stop.

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