Carburetor Tuning Guidelines

1 FULL THROTTLE - TOP END

Primary adjustment:

A. Try different main jets

Select Best Main Jet. To get the best, most even top end power (after 7500 rpm), select the main jet that produces the highest top speed. If the bike pulls harder at high rpm when cold and less hard when fully warmed up, the main jet is too large. If the bike doesn’t pull well at high rpm when cold and gets only slightly belier when fully warmed up, the main jet is too small. In order to properly tune the midrange and low rpm carburetion, THE MAIN JET MUST FIRST BE PROPERLY SELECTED after 10 to 15 minutes of hard use!

2. FULL THROTTLE MIDRANGE (5000 to 7000 rpm)

Primary adjustment

A. Set needle clip height

Set needle height for best power between 5000 and 7000 rpm. If the engine pulls better when cool, but soft when at full operating temperature, it is too rich in the midrange and the needle should be lowered.

3. FULL THROTTLE -4000 rpm

Primary adjustment:

A. Set needle clip height

To get best drive ability, set clip height so that engine will accept full throttle in 2nd gear from 3500 to 4000 rpm. Remember, always count from the top of the needle. REMEMBER, THE MAIN JET and FLOAT HEIGHT (especially SuzukiIYam400IGOOI7SOl000!) MUST BE PROPERLY SELECTED FIRST! If the engine has a “dry” flat spot at approximately 3500 to 4000 rpm, raise the needle (lower the clip, i.e. go from #3 to #4 position from top), to start the taper earlier on the needle and richen up that area.

It the engine has a “wet” rhythmic, soggy spot at 4000 rpm that gets worse as the engine heats up, lower the needle (i.e. go from #4 to #3 position from top). This will start the “point of origination of taper” later on the needle and lean out that area. If the engine is left with the needle set too high, the engine may foul plugs on the street and will be “soft” coming off low speed corners on the racetrack.

4. FULL THROTTLE at 2000 rpm

Primary adjustment:

A. Float height (fuel level)

Adjust Floats to raise or lower the Fuel Level. Base settings are usually given if a particular application has a history of fuel level criticalness. The fuel level height in the float bowl affects full throttle/low rpm, also, richness or leanness at cruise/low rpm. For instance, a bike that runs cleanly at small throttle openings when cold, but starts to show signs of richness as it heats up to full operating temperature, will usually be leaned out enough to be correct if the fuel level is LOWERED 1mm. Check out and RESET all: Suzuki (all), Yamaha (all), and Kawasaki (if low speed problems occur). Needless to say, (so why am I saying it’?) FUEL LEVEL IS EXTREMELY IMPORTANT!!!

5 SYNCHRONIZE CARBS

For the smoothest idle and 2nd gear, 4000 rpm, steady state cruise operation. Set mixture screws at recommended settings, as a starting point.

6. PART THROTTLE CRUISE

Primary adjustment:

A. Adjust mixture screws

For smoothest idle, 2nd gear, 4000 rpm, steady state cruise, and 118 throttle high rpm operation.

Pilot fuel mixture screw settings, float level AND pilot jet size are the primary sources of mixture delivery during 4000 rpm steady state cruise operation. If lean surging is encountered, richen mixture

screws (turn out) in 1/2 turn increments. Alternative pilot jets are supplied when normally required. Pilot fuel mixture screw settings, float level, and pilot jet size also affect high-rpm, 0 to 1/8 throttle maneuvers. Too lean, will cause surging problems when the engine is operated at high rpm at small throttle openings! Opening the mixture screws and/or increasing pilot jet size will usually cure the problem. A lean problem decreases as the engine heats up.

If the throttle is lightly blipped” at idle, and the rpm drops below the set idle speed, then rises up to the set idle speed, the low speed mixture screws are probably set too rich: try 1/2turn in, to lean the idle mixture. A rich mixture gets worse as the engine heats up.

If the throttle is lightly “blipped” at idle, and the rpm “hangs up” before dropping to the set idle speed, and there are no intake leaks and the idle speed is set to less than 1000 rpm, the mixture screws are probably too lean: try 1/2 turn out, to richen mixture. Be sure there are no intake leaks and the idle speed is set at less than 1000 rpm.

7. POP QUIZ!!

What if the main jet is properly selected (test #3), Part throttle cruise is properly adjusted (test #2). and you find that the best position for test #4 (full throttle ~4000 rpm) is clip position #2 and the best position for test #5 (full throttle @ 5000 to 7000 rpm) is clip position #4? See Section 8.

8. FINAL

The engine should: If not:
Accelerate strongly and evenly - 7500 rpm to redline. See Section 1
Pull smoothly through 5000 to 7000 rpm. See Section 2

Pull smoothly through 4000 rpm. See Section 3
Accept full throttle smoothly from low rpm. See Section 4
Idle and cruise smoothly. See Sections S and B

9. NOTES

Carb tuning is a combination of science, art, intuition, and a lot of wizardry. The answer to Pop Quiz:

You probably need the next step richer needle to change the 5000 to 7000 rpm without changing the other portions of the needle.