Symptoms of a Bad Carburetorby TJ Hinton
Persistent problems such as hard starts when cold, lean misfires and poor performance across the throttle range are indicative of a carburetor that needs attention. The rubber components inside can fail and the ports and passages within the carburetor body can become clogged. You can solve most carburetor problems with a good cleaning and the installation of a rebuild kit.
Lean Running Condition
Obstructions such as varnish, two-stroke oil and ethanol gel can restrict or clog the jets in the carburetor bowl, preventing an adequate amount of fuel from reaching the outlet ports. Usually, this manifests itself in the idle jet, and since the idle circuit is active throughout the throttle range, a lean idle circuit will cause sneezing or popping in the intake, as well as overheating and backfiring. Air leaks in the intake after the fuel ports in the venturi can also cause a lean condition, so be sure to check your intake manifold gaskets, and any shaft seals within the carburetor body. Restrictions in the fuel filter or fuel lines can also mimic the symptoms of a clogged carburetor.
Rich Running Condition
Black, puffy exhaust and carbon-fouled plugs are the hallmarks of a rich running condition. Failures in the carburetor float, needle valve or seat can cause unrestricted fuel to enter the bowl. This will cause more fuel to enter the venturi, causing the rich condition, and may be accompanied by fuel leaking from the carburetor atmospheric bowl vent.
Poor Acceleration and Performance
Stumbles and intakes sneezes that only occur when accelerating aggressively are indicative of a damaged accelerator pump diaphragm or clogged accelerator pump passages. The engine momentarily suffers a lean condition when the throttle is opened, and without the shot of fuel from the pump to temporarily enrichen the air-fuel mixture, the engine will misfire. Constant-velocity carburetors can deliver poor mid-range performance if the carburetor slide diaphragm is damaged and leaking. The vacuum leak will prevent the slide from opening fully, and it will artificially limit the operating range of the carburetor.
Hard starting, especially when cold, can be caused by any problem leading to a lean condition. However, if the problem only manifests itself on cold starts, then the problem most likely lies with the choke or enrichener circuit. If you have a vacuum-operated choke, then you may have a vacuum leak in the hose, or a bad vacuum motor. Check mechanical chokes for proper adjustment and linkage operation. Enrichener circuits are a miniature carburetor within the carburetor, complete with its own fuel intake orifice in the bowl, air-bleed passage and discharge port after the throttle plate or slide. Restrictions in the air-bleed passage caused by failed rubber components can prevent the enrichener from working properly, and cause a lean condition with poor idle on cold starts.
TJ Hinton trained as an auto mechanic at Mississippi Gulf Coast Community College and then later graduated from MMI as a certified motorcycle mechanic . He's also worked for 20+ years in home construction, remodeling and repair. His articles appear on InternetAutoGuide.com and TopSpeed.com.