Carburetor Troubleshooting Guide

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.

Experience has also taught us that in addition to the problems listed below, that many unusual carburetor (and EFI Throttle Body) running issues are caused by air leaks at the intake manifold gaskets. The air leaks that can develop can be engine temperature sensitive, air temperature sensitive, RPM sensitive or a combination of any of these. This makes the normal procedures for finding a manifold leak almost useless except in the most obvious of leaks. Intake manifold gaskets that have been on a bike for most than 24-36 months are very likely to be leaking. We recommend replacing these as a matter of routine maintenance. But the general symptoms of a manifold leak can be slow degrading of performance over time, intermittent issues that resemble 'lean run issues', occasional hard starting hot or cold that may improve with using the choke. The key to determining a manifold gasket issue is usually an intermittent issue with running or a problem that seems to be aggrivated by heat or cold. The cost and time to replace a set of intake manifold gaskets is small compared to the time and aggrivation of a poor running bike that can come with a small air leak. In our experience, replacing the intake manifold gaskets has resolved many nagging issues that seem to have baffled experienced mechanics.

Common issues such as those listed below are addressed in the troubleshooting tables.

  • Fuel Overflow
  • Poor Idling
  • Poor Fuel Economy
  • Poor Acceleration
  • Hard Starting
  • Poor Road Performance
  • Poor High Speed Performance 

 

CV and General Carb Troubleshooting Guide
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.

4. Fuel overflow.

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.

 

Vacuum Piston Assembly Troubleshooting for Keihin CV Carburetors
Condition Check For This Problem Fix the problem by:
PISTON DOES NOT RISE PROPERLY 1. Piston atmosphere vent blocked.

2. Diaphragm cap loose, damaged or leaking.

3. Spring binding.

4. Diaphragm pinched at lip groove.

5. Torn diaphragm.

6. Piston binding.

7. Piston vacuum passage plugged.

1. Clear vent.

2. Tighten or replace cap.

3. Correct or replace spring.

4. Reposition diaphragm lip.

5. Replace piston diaphragm assembly.

6. Clean piston slides and body or replace piston.

7. Clean and clear passage.

PISTON DOES NOT CLOSE PROPERLY 1. Spring damaged.

2. Piston binding.

3. Piston diaphragm ring dirty or damaged.

1. Replace spring.

2. Clean piston slides and body or replace piston.

3. Clean or replace piston.

 

 

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