since 1940

Rough Idle or Poor idle in Lycoming or Continental aircraft engine

 1. Mixture to rich or lean. If mixture is too lean the engine stumbles on power application. Lean engine if mixture is too rich. Engine will smooth out and pick up rpm as it is leaned. After mixture is set, re-adjust idle speed. A rich mixture may backfire (more technically,  inlet charge combustion). Rich idle, black smoke, and fuel dripping out carburetor is a sign that the fuel level in the float bowl is too high or fuel pressure to the carburetor is too high. Possibly the carburetor float is sinking.

 2. Faulty ignition system. Fouled spark plug. Check magneto drop and condition of plugs and leads. Check for worn spark plug leads shorting out on a metal part.

 3. Plugged nozzles. Usually accompanied by indicated high fuel flow. A partially plugged nozzle will allow enough fuel to flow at idle for combustion. As power is increased the cylinder gradually leans out until detonation or lean misfire occurs. Exhaust gas temperature rises and does not respond to mixture control until idle cut off is reached. The reason for this is the blockage now becomes the primary restriction and is independent of mixture control position. Nozzles can be flow-checked in containers of equal size to locate plugged or partially plugged nozzle. May be able to locate problem by feeling for cold cylinder. Inspect with magnifying glass before cleaning.

 4. Induction air leak. Induction air leak at one of the following locations: Hoses, Cracked intake pipes, loose flange bolts, loose plugs in intake port, loose intake tubes in Lycoming engines, punctured flex tubing connecting the carburetor to air ducting. Slight air leak in induction system you are usually able to adjust initial idle but rough in 1,000-1,500 rpm range. Large leaks into induction system, usually unable to throttle engine down below 800-900 rpm. On Lycoming engines the intake tubes become loose where they are swaged into the oil sump. These tubes must be re-swaged with a Lycoming swedging tool. silicone sealant will not work. Check the sniffle valve on fuel injected engines for leakage (see below).

Make sure all plugs are installed in cylinder

Fuel selector shaft O-ring leaking allowing air to be sucked into fuel system when boost pump is on. Reported on a Beech Bonanza K. Engine starts but won't run, low boost pressure. May be more pronounced on R or L tank than both.

 5. Fuel drain valve not seating properly. These valves, called sniffle valves can cause the cylinders to run lean. To check the valve, operate the engine and place your finger over the valve. If you feel any suction or if the roughness goes away, the valve should be replaced.

 6. Engine mount. Engine mount contacting airframe member can transmit vibratory impulses into airframe. Check for sagging engine mounts.

 7. Mount bushings improperly installed.

J-9613-12 engine mount bushings

 8. Internal injector leak (Lycoming RSA fuel injection) Usually unable to adjust injector at idle or wont hold adjustment. Check by disconnecting induction system at injector inlet to observe impact tubes. Put throttle in full forward position and mixture in full rich position, cap fuel line to flow divider, turn on boost pump. If fuel is observed coming out of impact tubes, then injector has internal leak and must be replaced.

 9. Fuel vaporizing in lines and distributor. Occurs at high ambient temperatures and prolonged operation at low or idle rpm.

 10. Air-bleed hole in fuel injector nozzle blocked. Check nozzle screen and shroud for deformation.

 11. Sticky valve in fuel flow divider.

 12. Uneven cylinder compression.

 13. Improper fuel pressure.

 14. Sticky valves. Occurs most frequently in the hot summer months. Frequently occurs on the first start of the day when the engine is cold. Remove rocker gasket. Turn engine backwards until valve is fully closed. Using both thumbs, depress valve springs. Valve should open and close without any feel of dragging. Valve movement should be smooth.

 15. Insufficient spark plug gap. Cracked spark plug insulator. Remove the spark plug lead and check for any black
lines in the terminal well of the spark plug. A black line is usually caused by a cracked insulator allowing the spark
to short to ground.

 16. Mixed nozzle set in Continental Fuel injection engines. Make sure all nozzles are same size (letter).

 18. Lean at idle. Check induction system for leaks. Use a vacuum cleaner and soap and water solution. If you've
just installed a new cylinder check to make sure that the primer plug on the cylinder's intake port has an Allen plug
installed.

 19. Check hydraulic plungers for proper bleed down. May also be caused by cold weather or using a high viscosity engine oil where the hydraulic lifters pump up and keep the valve open. Bleed down is determined by:

• Type of lifter; both Lycoming and Continental have used different bleed-down rates to improve engine idle. One example is Lycoming Service Letter L102
• Valve spring pressure - a broken or weak valve spring allows the lifter to pump up
• Oil Viscosity has a direct effect on bleed down rates

 20. Check valve rocker clearance. Especially important on solid tappet engines ( Lycoming O-235).

 21. Fuel vaporizing in fuel lines or distributor. Encountered only under high ambient temperature conditions or following prolonged operation at low idle rpm's.

 22. Damaged or missing air inlet straightening grids. A few airplanes use (Piper Super Cub being one) use air straighteners in the airbox to reduce air turbulence.

 23. Leaking primer solenoid

 24. Leaking carburetor discharge nozzle.

 25. (Turbocharged Lycoming engines). Fuel pump diaphragm leaking fuel out vent line into induction system. See Lycoming Service Bulletin 497 for restrictor kit.

 26. Primary venturi in carburetor loose. A loose venturi can be detected by removing the carburetor air box and carefully checking for movement of the primary venturi with your finger. The venturi should be replaced before next flight if there's any movement.

 27. Continental fuel injected engines. Frequent adjustment of the fuel pump pressure to correct rough idling and to obtain proper fuel flow indicates that the fuel pump through bolts are loose.

 28. Float shaft and bracket worn on carburetor flooding carburetor.

 29. O-200A engines, one piece venturi can cause engine richness, roughness, or hesitation. Precision Airmotive has an improved main fuel nozzle that corrects this problem. Reference Precision service bulletin MSA-7 dated 10/11/94 or latest
edition.

30. Improper internal timing on Slick magneto. On Slick 4200/6200 and 4300/6300 series magnetos check to see that the timing pin (Slick T-118) was inserted into the proper holes in the distributor block. If the magneto is pinned incorrectly the contact points can still be synchronized to the engine firing point, but the distributor finger is positioned over the wrong tower. This can lead to a confusing condition where the contact points seem to be synchronized, but the engine runs poorly on the subject magneto.

31. Ignition leads routed to incorrect cylinder. This may happen on Lycoming engines where Lycoming has alternative lead routing. If you install a new harness and use the same routing as the old harness it may not be correct.

32. Continental fuel injected engines rough idle can be caused by low un-metered fuel pressure. This will also cause the engine to quite in takeoff roll.

33. Continental fuel injected engines rough idle can be caused by high un-metered fuel pressure. Engine will run fine when it is first started or cold and then run rough when it warms up. 

34. Sniffle valve stuck open. The sniffle valve is used on turbocharged engines and is installed in the cylinder drain system and is normally open only when the engine is at rest.

 

75444 Lycoming sniffle valve	LW-13807 Lycoming 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.

35. Loose fuel line connection causing air to enter carburetor.

36. Aircraft equipped with Pressure Carburetors such as PS5C will exhibit overly rich mixture, poor idle, black smoke when the diaphragms are shot and the unit needs overhaul.

37.Recently, a Cessna 172R made an off-airport landing, as a result of not being able to initiate an in-flight engine restart following an engine failure during a power-off stall demonstration.

Other model 172R and 172S aircraft were examined and found to exhibit engine idle speed and fuel flow settings that were as much out of adjustment or worse than the one involved in the off-airport landing.

Reports concerning 182S and T206H models indicate this problem may involve all Cessna aircraft that have been manufactured since the resumption of production of the models identified above. Cessna has information in the applicable maintenance manuals that adequately describes the procedures to check and set the engine idle, fuel flow, and speed settings on these aircraft. Also, Cessna may soon revise the "ground idle test" and the in-flight engine restart procedures applicable to these aircraft. 

Cessna may also issue additional service and operational information to help identify, what appears to be a significant shift in the idle fuel flow. The fuel servos used on these aircraft are suspect for the problems identified here and historically require seasonal or periodic minor adjustments to the idle fuel flow and speed settings. However, the shift in engine idle fuel flow and idle speed currently being reported are considerably beyond the seasonal/periodic adjustments normally required.

The fuel servos used on these aircraft are suspect for the problems identified here and historically require seasonal or periodic minor adjustments to the idle fuel flow and speed settings. However, the shift in engine idle fuel flow and idle speed currently being reported are considerably beyond the seasonal/periodic adjustments normally required.

Continental C-85, O-200, O-300 Missing Lock-O-seal on intake spider studs. Will cause roughness between 1,000 and 1,700 RPM. Rough idle.

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