The K&B .21 DF is an especially high-performance engine designed for ducted fan models. It is an improved version of our previous, highly successful engine, known throughout the world for its power and proven reliability, and will provide countless hours of sport or competition performance if properly handled and maintained.


.2124 Cu. In. (3.48cc)


.650 (16.51mm)


.640 (16.26mm)


2,000 - 25,000

Power Output

1.25 HP @ 25,000

9.5 oz


PLEASE READ ALL SAFETY INSTRUCTIONS! Failure to read, understand and follow these instructions could result in personal injury and/or property damage to yourself or others.

Take a few moments to familiarize yourself with the various parts of the engine. DO NOT DISASSEMBLE YOUR ENGINE! Doing so will void your warranty. No exceptions!



GLOW PLUG: This engine is designed to use the Long Reach Glow Plug (K&B Part No. 7311).

FUEL: We recommend K&B 515DF (15% nitro, 22% oil by volume). K&B 515DF will give your engine more economy and longer life.

Due to the nature and design features of the “ABC” type engine, the piston and sleeve have been setup to feel a little tight at the top of the stroke. Once the engine has been started, the sleeve will pull away from the piston leaving the exact clearance necessary for maximum performance without seizing. It’s for this reason that an “ABC” type engine does not require prolonged break-in periods. However, we do recommend that you run the engine at a slightly rich needle valve setting for the first two runs. These runs should be made with the engine installed in your airplane, or on a bench test stand, using a 8” x 6p propeller. Typically, the engine should be ready for continuous full throttle operation after fifteen (15) or twenty (20) minutes.

TUNED-PIPE: A pressurized fuel system must be used to boost the fuel to the needle valve. Install a pressure fitting on the tuned pipe, just behind the apex where the cones join. The recommended tuned pipe is the MAC’s Product No. 1821. Direct a length of fuel line from the pressure fitting to the vent on the fuel tank. In turn, the fuel pickup line goes to the needle valve.

  • Never place your hand in front of the fan for any reason while the engine is running.
  • Keep rags, articles of loose clothing, or anything that might be ingested, well clear of the duct intake.
  • Always start the engine at an idle throttle setting.
  • Wear eye and ear protection.

Assemble the needle valve onto the carburetor and turn it clockwise to completely close it. Turn until it won’t go any further but do not force it. Then, turn it counter-clockwise 4 to 4-1/2 turns, if you are running near sea level, 3-1/2 to 4 turns at higher altitudes. As these are only starting settings, you might have to re-adjust the needle valve to make it the correct setting for your area. Once you have arrived at a setting, you can lock the needle valve in place as the carburetor has a built-in collet lock. Turn the black acorn nut clockwise to lock the needle valve in place, counter-clockwise to loosen. It takes a 1/4 inch wrench. Only tighten it enough so that the needle valve cannot be moved. Do not force it.

CAUTION: If the engine is run at a lean needle valve setting during the break-in period, the following may result:

1.    Piston and sleeve will overheat and score.
2.    The connecting rod, crankshaft, and wrist pin will overheat from lack of lubricant and seize, causing the lower con-rod bushing to spin in the con-rod, or in extreme cases, the bottom of the con-rod will break, causing damage to the crankcase, piston, and sleeve.


MAKE PERIODIC CHECKS. After every flight, remove the glow plug and check the coil condition. If it is broken or badly distorted, replace it. The glow plug life will be longer when you achieve a good needle valve setting.

After every four flights, check tightness of the engine head and mounting screws, as well as fan and spinner security.

After every twelve flights, check the tuned pipe coupler, and tightness of the clamps.



If the engine surges, or goes lean, check the following leaks:

1. Tuned Pipe Coupler
2. Pressure Line
3. Fuel Tank
4. Engine Front and/or Back Plate
5. Check tightness of Head Bolts
6. Check tightness of Glow Plug

If the engine won’t transition (accelerate from idle to full throttle cleanly):

1. Check Pressure Line for leak

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MECOA/K&B -- P.O. Box 5 -- Sierra Madre, CA 91025 U.S.A.