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Troubleshooting Your Tecumseh Mini Bike Carburetor

The following troubleshooting guide is as recommended by Tecumseh and answers the question most often asked. Just following these steps may eliminate an unnecessary carburetor rebuild. Feel free to print a copy to keep in your work shop.

Hunting and Surging at True Idle

Engine hunting and surging at true idle is caused by a fuel delivery problem or an air leak. True idle is the carburetor setting when the throttle plate linkage is against the idle speed adjusting screw after idle mixture adjustment. Because the throttle plate is held stationary during true idle, hunting and surging must be caused by an improper air-fuel mixture related to an air leak or an obstruction in the idle circuit. At true idle, the governor spring applies no force on the throttle plate and has no effect on the idle characteristics of the engine.

Hunting and Surging at Governed Idle

An engine hunting and surging only at governed idle and equipped with an idle mixture adjustment has a governor system or carburetor problem. The idle mixture must be adjusted correctly. Hold the throttle plate linkage against the idle speed adjusting screw and increase the idle speed to the specified governed idle speed. If the engine operates without hunting or surging, the problem is the governed idle spring or linkage. If the engine continues to hunt and surge, the problem is in the carburetor. After testing, return the engine to the correct idle speed.

Hunting and Surging at Top No Load Speed

Troubleshoot hunting and surging at top no-load speed using the same sequential steps used to isolate a governor system or carburetor problem during true idle and governed idle. Once the idle mixture is adjusted and the engine idles smoothly, increase the engine speed using the idle speed adjusting screw. Hold the throttle plate linkage against the idle speed adjusting screw until the engine reaches the specified top no-load speed. Without any appreciable load, fuel is provided by the idle circuit. If the engine continues to hunt and surge, the carburetor is the probable cause. In this test condition, the governor system has no effect on engine speed. The idle speed adjusting screw control the throttle plate position, which affects engine speed. If the engine operates properly when controlled by the idle speed adjusting screw but hunts and surges when controlled by the governor spring, the governor system is suspect. Check the governor linkage for resistance and binding, and replace the governed idle spring and the main governor spring.

Hunting and Surging Under Load

Hunting and surging under load usually indicates a carburetor or fuel delivery system problem rather than a governor system problem. Fuel fed under load is primarily fed through the main jet and emulsion tube. Most loads are constant enough to maintain the rpm of the engine. The governor system has very little additional effect on the performance of an engine under load except for applications with sizable varying loads such as a minibike go kart or generator.

Poor Performance Under Load

Poor performance under load requires first eliminating compression component problems. If the problem is isolated to the fuel system, the cause is usually debris in the main jet or air bleeds. To isolate the problem component, examine the exhaust gas when the engine is under load. If black smoke is present, there is an excess of fuel. This condition may be caused by an incorrect float level setting, a partially clogged main air bleed, or debris lodged between the needle and the seat. If black smoke is not present, and there is no black residue on the muffler deflector, the main air bleed or main jet is probably obstructed. A primer bulb system or choke can also be used to quickly troubleshoot carburetor problems related to a lean or rich condition. After making carburetor adjustments, slowly close the choke plate when the engine is operating poorly. If performance improves, the air-fuel mixture is too lean. If performance worsens, the air-fuel mixture is too rich. A primer bulb system can also be used to test for a lean or rich condition. Depressing the primer bulb injects extra gas into the carburetor. If the engine air-fuel mixture is too lean, the injection of the extra fuel should improve engine performance. If the engine air-fuel mixture is too rich, the injection of the extra fuel should worsen engine performance.

Most Often Overlooked 

Many engines are now produced utilizing automotive style paper element air filters. Of course these air filter elements should automatically be changed during service. It is worth while to note that if the paper element becomes saturated, either with oil, fuel or water, the porosity of the paper can change dramatically, therefore changing the performance of the carburetor. If you experience poor starting or running of the engine, it is always worth the while renewing the filter elements first of all. This will eliminate any possibility of a damaged air filter causing the problem. If, for instance, the air cleaner element has become soaked in fuel or oil, due perhaps to the rider laying the machine on its side, or water from riding in wet conditions, this would not necessarily be obvious until inspecting the filter. Drying the filter will not remedy the problem. Once the paper element expands from being damp, it will not contract again and the pores in the paper will be permantly damaged.

If you have determined that your carburetor is in need of a cleaning or rebuild, please view our guide titled Rebuild That Carb You will find neat tips and tricks to help you with your carburetor rebuild project.

If you view our Compression Myth guide you will be able to determine whether you have compression issues.

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Happy Trails, Chuck at Valley Farms

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