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DrillDown Icon Engine Carburetor - Will not shut off.
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Engine Carburetor - Will not shut off. How to resolve.

Question: My engine will not shut off. What do I do?

Answer: Generally you should not adjust the factory settings until the engine has been run for 4-6 tanks of fuel at least AND you should only adjust the factory settings if the engine does not idle well or does not transition to higher RPM well.

All engines idle poorly and transition poorly on the first tank of fuel, a bit better on the 2nd, 3rd and 4th tanks and progress to normal operational reliability by about the 6th tank of fuel.

If you find that after this breakin period, your engine will not shut off  review the documentation first. All of it! Then read it again for good measure.

Now put your engine on an engine test stand and set the center line of the test stand fuel tank about 1/4" below the needle valve. Check your tubing for leaks and secure attachment. Fill the tank with good quality fuel as described in the instruction manual. 10% nitro maximum. 18% oil minimum with Oil being a mix of synthetic and castor.

Do not start your engine yet.

Look into the carburetor throat and ensure that the carb barrel can rotate fully so that there is no opening remaining. If you find a small opening remaining, adjust the idle stop screw (not the mixture... the stop screw) so that you can rotate the throttle to fully closed.

Loosen the carburetor cinch bar by backing off the retaining nut a turn or so. Gently wiggle the carburetor and pull it out of the engine. Inspect the O-ring around the flange of the carburetor to ensure that the O-ring is not damaged or nicked. Carefully reinsert the carburetor back into the engine and tighten the cinch bar nut securely while pressing the carburetor firmly into the engine. It is important to have a tight seal between the carburetor and the engine... the O-ring provides this seal provided it is firmly seated and is not damaged.

Now start your engine. Provided that the engine has at least 6 tanks of fuel through it and it is mounted on a test stand with the fuel tank set up properly, you should be able to close down the throttle barrel and the engine should stop in 5-10 seconds or sooner. If your engine continues to run for longer than that, the engine is drawing air from somewhere such as:  

  1. From the fuel feed (air in the lines caused by a leak or foaming fuel due to vibration), or
  2. Through a poor seal around the carburetor flange, or
  3. Around a poor fitting carburetor barrel due to wear, or
  4. Through the front bearing due to an alignment problem usually caused by the crankshaft being bent after a crash or drop.

To resolve which is which:

  1. Check the fuel system again. Change the tank and the tubing. Cushion the tank on foam sheeting and ensure the fuel is not foaming.
  2. Check the O-ring seal again.
  3. Very very carefully, plug the inlet to the carburetor throat while the engine is idling.  If the engine stops the carb barrel is leaking. You will need a new carb barrel and body or more likely it would be best to install a new carburetor.  
  4. Remove the prop and thrust washer. Cut a very thin sheet of gasket paper or laminated playing card material and insert it as a gasket around the crankshaft between the crankcase and the thrust washer. Reapply the thrust washer, prop, prop washer and prop nut. Tighten securely. The idea here is to create a temporary gasket across the face of the front bearing to restrict any suction induced air flow. Start the engine, idle down. If the engine will stop with the gasket in place, air is being sucked into the engine from the front of the engine when the gasket is not present. This will require a new crankshaft or possibly a new crankcase or both. It may be more cost effective to purchase a new short block. Professional service is recommended. 
Article ID: 5254