Model Engine Company of America
Most of the time the problem of erratic erratic running is caused by debris in the carburetor. Many times we receive engines back only to find foreign matter has been introduced into the fuel passage of the carburetor.

This is an actual engine that was returned to us because it was overheating and running erratically for a few minutes before it would die. After removing the main needle and receptacle this is what we found, honest.  This was so apparent we couldn't believe it either, that's why we took the picture. This is either a sliver of fuel line apparently when it is pushed over the brass tubing on the fuel tank or some silicone sealant. If you use silicone sealant don't use it on nipples or other parts of the carburetors. Naturally the customer sent the engine back for warranty, but how can this be consider this the factory's fault?
Anytime debris is in a fuel passage it moves around causing the mixture to change as it plugs and unplugs the fuel flow, hence rich, lean, correct settings, but no constant setting.

On other types of carburetor designs, like this K&B mixture disc, you must check inside the spray bar as many times something is plugging up the metering slot that controls the fuel flow for idle and midrange. Symptoms are not being able to obtain idle settings, not getting a rich enough high speed setting, and erratic settings as described above.


Another culprit of erratic running is Muffler Pressure.
We found that sometimes the muffler pressure nipple can become plugged up with oil residue from the exhaust. When this happens the fuel tank receives pressure then not so the result is the fuel mixture is never the same. One time the fuel is being supplied under pressure then not or even to the point no fuel because no vent or pressure is present in the tank. Try running the engine with the pressure nipple disconnected. If the engine performs differently this may be the problem. Try changing brand fuels or replacing the nipple with one that is designed with a stand off available from MECOA.

 

The above information is provided as a guide. Since MECOA/K&B has no way of determining the ability of the individual using and understanding this information, we assume absolutely NO RESPONSIBILITY for any damage to person or property from the use of this information.
 

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