since 1940

Fluctuating rpm in Lycoming or Continental aircraft engine. Erratic rpm. Rpm won't stabilize

1. Defective aircraft tachometer. If engine speed varies 25-50 rpm at the tachometer but is otherwise smooth, suspect a defective aircraft tachometer. Lack of lubrication causes the tachometer to gradually bind resulting in small needle fluctuations. Most mechanical tachometers have a oil port on the rear of the instrument where the cable installs. No one ever goes back there and lubricates it.

2. Sludge in governor pilot valve or relief valve.

3. Burr in pilot valve lands.

4. Backlash in propeller governor control system.

5. Excessive oil leakage in engine transfer seal. On Continental 360
series engines the Teflon transfer collar seal may have been burnished (damaged). Pre-oiling the engine after
extended periods of inactivity will help prevent this.

6. High propeller friction.

7. Governor function upset by malfunctioning engine.

8. Air trapped in propeller. Cycle propeller from MIN to MAX pitch several times to purge air.

9. Sticky pilot valve.

10. Bent pilot valve.

11. Excessive internal leakage in governor.

12. Carbureted engine. Float not properly aligned so that it rubs against the sides of the bowl. May only show up in flights as the aircraft banks. Tip the carburetor back and forth and listen closely. You should hear the float moving but shouldn't here it rubbing on the sides of the bowl.

13. Defective magneto coil. Coils usually begin to break down when hot. Ohm test is not adequate for testing coils. Coil tester required.

14. Defective sniffle valve. Sniffle valve is a fuel purge valve that is sometimes used in the induction system and is only open when the engine is shut down. With engine shut down, valve should be open to allow fuel to drain overboard. With engine operating, valve should suck closed. If the valve fails to close, mixture problems will occur, especially in the lower rpm ranges. Excessive lean mixtures at low engine speed will result in rough and erratic idle performance. You can check the valve by placing your finger over the drain tube while the engine is idling; the valve is bad if you feel suction on your finger or the engine performance improves.  Lycoming part number for the sniffle valve is 75444.

75444 sniffle valve. The sniffle valve has a small flapper valve that closes with manifold pressure and is normally open with the engine at rest. This allows any fuel in the intake system to drain.

Copyright 2003 by Sacramento Sky Ranch Inc. All rights reserved.  Prices subject to change without notice. Not responsible for typographical or misprint.
Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided  is not intended to supercede or supplement the F.A.A. approved  maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.