⚠️ SAFETY WARNING: Always work in a well-ventilated area. Gasoline is highly flammable. Disconnect the spark plug wire before working on the carburetor.

Understanding Mikuni Flat Slide Components

The Mikuni VM28 flat slide carburetor uses three main circuits to deliver fuel:

• Pilot Circuit: Controls idle to 1/4 throttle (pilot jet and air screw)
• Needle Circuit: Controls 1/4 to 3/4 throttle (jet needle and needle jet)
• Main Circuit: Controls 3/4 to full throttle (main jet)

Tools Required

• Flathead screwdriver
• Phillips screwdriver
• Jet wrench or small flathead screwdriver
• Needle nose pliers
• Clean rags
• Carburetor cleaner
• Jet kit (various sizes)

Step 1: Remove the Carburetor

1. Turn off the fuel petcock
2. Disconnect the fuel line from the carburetor
3. Loosen the intake manifold clamp
4. Remove the throttle cable by loosening the locknut and sliding the cable out
5. Carefully remove the carburetor from the engine

Step 2: Disassemble the Carburetor

1. Remove the top cap by unscrewing the two Phillips screws
2. Carefully pull out the slide assembly with the needle attached
3. Remove the float bowl by unscrewing the four screws on the bottom
4. Note the current jet sizes for reference

Step 3: Main Jet Replacement

1. Locate the main jet in the center of the float bowl area (brass hex jet)
2. Use a small wrench or socket to remove the main jet by turning counterclockwise
3. Note the current size (stamped on the jet, e.g., 120, 125, 130)
4. Install the new main jet and tighten gently (do not overtighten)

Main Jet Sizing Guide:

• Stock GX200: 105-110
• Mild performance build: 115-125
• High performance build: 130-145
• Each jet size change = approximately 2% fuel flow change

Step 4: Pilot Jet Replacement

1. Locate the pilot jet (smaller brass jet near the main jet with screwdriver slot)
2. Remove using a small flathead screwdriver
3. Note the current size (typically 15-25 range)
4. Install the new pilot jet and tighten gently

Pilot Jet Sizing Guide:

• Stock: 17.5-20
• Modified engines: 20-25
• Adjust based on air screw response (see tuning section)

Step 5: Jet Needle Adjustment

1. Remove the slide from the top cap assembly
2. Push down on the needle clip and remove the jet needle
3. Note the current clip position (5 grooves total, clip in middle = position 3)
4. Move the clip to adjust fuel mixture:
   • Raising the clip (lower groove): Raises the needle = richer mixture
   • Lowering the clip (higher groove): Lowers the needle = leaner mixture
5. Reinstall the needle and slide assembly

Needle Position Guide:

• Position 1 (top groove): Leanest
• Position 3 (middle groove): Standard
• Position 5 (bottom groove): Richest
• Start at position 3 and adjust based on performance

Step 6: Reassembly

1. Clean all carburetor passages with carburetor cleaner
2. Reinstall the float bowl with new gasket if needed
3. Install the slide assembly (ensure the cutaway faces the intake side)
4. Secure the top cap
5. Reinstall the carburetor on the engine
6. Reconnect the throttle cable and fuel line

Step 7: Initial Tuning

1. Air Screw Adjustment:
   • Gently screw in the air screw until it seats (do not force)
   • Back out 1.5 turns as a starting point
   • This controls the pilot circuit air/fuel mixture
2. Idle Speed:
   • Set idle speed screw for 1,500-1,800 RPM

Step 8: Fine Tuning on the Track

Pilot Circuit (Idle to 1/4 throttle):

• Too Rich: Black smoke, poor idle, plug fouling → Reduce pilot jet or turn air screw out
• Too Lean: Backfiring, hanging idle, hesitation → Increase pilot jet or turn air screw in
• Optimal air screw setting: 1-2.5 turns out

Needle Circuit (1/4 to 3/4 throttle):

• Too Rich: Bogging, black smoke → Lower needle (clip up)
• Too Lean: Hesitation, surging, overheating → Raise needle (clip down)
• Make one clip position change at a time

Main Circuit (3/4 to full throttle):

• Too Rich: Loss of top-end power, black smoke, wet plug → Reduce main jet size
• Too Lean: Overheating, pinging, loss of power, white plug → Increase main jet size
• Change main jet in 2.5-5 size increments

Reading Spark Plugs

• Perfect: Light tan/brown color on insulator
• Too Rich: Black, sooty, wet appearance
• Too Lean: White or light gray, may show heat damage
• Always check plugs after full-throttle runs

Environmental Factors

Jetting must be adjusted for:

• Altitude: Higher altitude = leaner jetting needed (reduce 1 main jet size per 2,000 ft elevation)
• Temperature: Hotter weather = slightly leaner, colder = slightly richer
• Humidity: High humidity = slightly richer jetting may be needed
• Air Filter: Cleaner filter = more air = may need richer jetting

Baseline Jetting Recommendations for GX200

Stock Engine (Sea Level, 70°F):

• Main Jet: 105-110
• Pilot Jet: 17.5-20
• Needle: Position 3
• Air Screw: 1.5 turns out

Modified Engine with Exhaust/Air Filter (Sea Level, 70°F):

• Main Jet: 120-130
• Pilot Jet: 20-22.5
• Needle: Position 3-4
• Air Screw: 1.5-2 turns out

High Performance Build (Sea Level, 70°F):

• Main Jet: 135-145
• Pilot Jet: 22.5-25
• Needle: Position 4-5
• Air Screw: 1.5-2 turns out

Troubleshooting Common Issues

Engine won't idle:

• Check pilot jet for clogs
• Verify air screw is 1-2.5 turns out
• May need larger pilot jet

Hesitation off idle:

• Pilot jet too small or air screw too far out
• Slide cutaway may be too large

Bogging at mid-throttle:

• Needle too rich (lower the clip)
• Main jet too large

Loss of top-end power:

• Main jet too large (too rich) or too small (too lean)
• Check spark plug color to determine which

Pro Tips

• Always start rich and work leaner for safety
• Make one change at a time and test
• Keep detailed notes of jet combinations and weather conditions
• Buy a jet kit with multiple sizes (saves time and money)
• Check jetting after any engine modifications
• Re-jet when moving to different tracks at different elevations
• Clean carburetor regularly to maintain consistent performance
• Use fresh, high-quality fuel

⚠️ IMPORTANT: Running too lean can cause severe engine damage including piston seizure. When in doubt, err on the side of slightly rich. Always monitor engine temperature and spark plug condition.

This guide provides general jetting information. Always consult your engine builder or carburetor manufacturer for specific recommendations for your setup.