This guide covers problems and solutions routinely encountered on stock and mildly modified motorcycle engines. As the horsepower goes up, so does the number of potential situations that impact performance or create problems. When the carburetor is acting up on a performance engine, these basic problems must be eliminated as a source of the problem before going on to performance tuning the carburetor.
Click if you have:
Fuel Overflow
Poor Idling
Poor Fuel Economy
Poor Acceleration
Hard Starting
Poor Road Performance
Poor High Speed Performance
| Condition | Check For This Problem | Fix the problem by: |
| OVERFLOW | 1. Restricted fuel tank vent system. 2. Loose float bowl screws. 3. Damaged float bowl O-ring. 4. Damaged or leaking float assembly. 5. Particle contamination in fuel inlet fitting cavity. 6. Worn or dirty inlet valve or seat. 7. Improper fuel level in float bowl. |
1. Correct restricted hose. Replace vapor vent
valve. 2. Tighten screws. 3. Replace O-ring. 4. Replace float assembly. 5. Clean and clear cavity and fuel supply tract. 6. Clean or replace valve and clean seat. 7. Adjust float tab for correct fuel level. |
| POOR IDLING | 1. Idle speed improperly adjusted. 2. Inlet system air leak (faster idling). 3. Loose low speed jet. 4. Contaminated or plugged low speed system. 5. Enrichener valve not seated or leaking. 6. Leaking accelerator pump. |
1. Adjust operating idle speed. 2. Correct as required. 3. Tighten jet. 4. Clean, clear and correct as required. 5. Adjust, clean or replace. 6. Repair. |
| POOR FUEL ECONOMY | 1. Excessive use of enrichener system. 2. Enrichener valve not seated or leaking. 3. Dirty air cleaner filter element. 4. Restricted fuel tank vent system. 5. High speed riding style. 6. Idle speed improperly adjusted. 7. Loose jets. 8. Fuel level too high. 9. Plugged or restricted bowl vent. 10. Worn or damaged needle or needle jet. 11. Vacuum piston assembly malfunction. 12. Plugged air jets or passages. 13. Excessive accelerator pump output. 14. Idle mixture screw adjusted to far out |
1. Limit system use. 2. Adjust, clean or replace. 3. Clean or replace as required. 4. Correct restricted hose. Replace vapor vent valve. 5. Modify riding habits. 6. Adjust operating idle speed. 7. Tighten jets. 8. Adjust float tab for correct fuel level. 9. Clean and clear passages. 10. Replace needle or needle jet. 11.Check Vacuum Piston Assembly. See troubleshooting chart. 12. Clean, clear and correct as required. 13. Check and clean accelerator pump bypass orifice. 14. Set idle mixture screw to 2 to 3 turns out. |
| POOR ACCELERATION | 1. Throttle cables misaligned. 2. Inlet system air leak. 3. Restricted fuel tank vent system. 4. Restricted fuel supply passages. 5. Plugged bowl vent or overflow. 6. Enrichener valve not seated or leaking. 7. Worn or damaged needle or needle jet. 8. Vacuum piston malfunction. 9. Plugged jets or passages. 10. Fuel level too low. 11. Accelerator pump leaking or no output. |
1. Adjust throttle cables. 2. Correct as required. 3. Correct restricted hose. Replace vapor vent valve. 4. Correct and clear restriction. 5. Clean and clear passages. 6. Adjust, clean or replace. 7. Replace assembly. 8. Check Vacuum Piston Assembly. See troubleshooting chart. 9. Clean and clear as required. 10. Adjust float tab for correct fuel level. 11. Repair as necessary |
| HARD STARTING | 1. Choke (Enrichener system) plugged, not
properly functioning or improperly operated. 2. Inlet system air leak. 3. Restricted fuel supply. No fuel in float bowl. 5. Plugged slow jet or passages 6. On high compression (over 10:1) engines, excessive voltage drop to the ignition coil can cause hard starting. A clear indication that this is the problem is that the engine wants to start after releasing the starter button. |
1. Clean, adjust or replace; or read
Owner’s Manual. 2. VOES vacuum hose not connected. Correct as required. 3. Correct fuel supply or passages. 4. Float level wrong. Fuel needle and seat need replacement. Fuel petcock left on while bike not running. 5. Clean, clear and correct as required. 6. Wire the coil directly to the battery using an air conditioning relay. |
| More HARD STARTING - COLD | 1. Throttle plate not closed | 1. Adjust throttle cables. |
| More HARD STARTING - HOT | 1. Idle mixture screw adjusted to far out. | 1. Set idle mixture screw between 2 and 3 turns out. |
| POOR PERFORMANCE ON ROAD | 1. Idle speed improperly adjusted. 2. Inlet system air leak. 3. Restricted fuel tank vent system. 4. Dirty or damaged air cleaner element. 5. Enrichener valve not seated or leaking. 6. Restricted fuel supply tract. 7. Plugged bowl vent or overflow. 8. Loose or plugged fuel and air jets or passages. 9. Worn or damaged needle or needle jet. 10. Vacuum piston assembly malfunction. 11. Accelerator pump inoperative. |
1. Adjust operating idle speed. 2. Correct as required. 3. Correct restricted hose. Replace vapor vent valve. 4. Clean or replace. 5. Adjust, clean or replace. 6. Correct and clear restriction. 7. Clean and clear passages. 8. Clean, clear and correct as required. 9. Replace assembly. 10. Check Vacuum Piston Assembly. See troubleshooting chart. 11. Repair as required. |
| POOR HIGH-SPEED PERFORMANCE | 1. Inlet system air leak. 2. Enrichener valve not seated or leaking. 3. Restricted fuel tank vent system. 4. Restricted fuel supply tract. 5. Dirty or damaged air cleaner element. 6. Plugged bowl, vent or overflow. 7. Worn or damaged needle or needle jet. 8. Vacuum piston assembly malfunction. 9. Loose or plugged main jets or passages. 10. Improper fuel level. 11. Accelerator pump inoperative. 12. Restrictive exhaust flow. |
1. Eliminate air leak. 2. Adjust, clean or replace. 3. Correct restricted hose. Replace vapor vent valve. 4. Correct and clean restriction. 5. Clean or replace. 6. Clean and clear passages. 7. Replace assembly. 8. Check Vacuum Piston Assembly. See troubleshooting chart. 9. Clean, clear and correct as required. 10. Adjust float level. 11. Repair as required. 12. Loud pipes do not mean better flowing pipes. Exhaust gasket interfering with exhaust flow. |