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Home> Continental & Lycoming TroubleShoot
Rough engine. Misfire. vibration in Lycoming or Continental aircraft engineQuick check: Most common culprit is the aircraft magneto. One item that is often overlooked when checking for a rough engine are the ignition harness contact springs (cigarette) that connect to the spark plug. Aircraft operators who routinely fly at high altitude such as photo survey report that they routinely replace these springs regardless of apparent condition and this solves many of their "miss at altitude" or poor magneto check problems.
See also rough idle. Pay particular attention to determine it is engine roughness or vibration, particularly when there are no engine instruments indicating trouble. Roughness comes and goes whereas vibration is steady but may change or go away at a different rpm setting. In some cases, roughness will smooth out at a lower altitude, which tends to indicate an ignition leak, usually in the harness or magneto distributor cap. A loose fit between the pancake grommet of the harness and the distributor block towers allows the magneto to flash to ground between the tower and grommet. This would be more prevalent at altitude.
Now onto the listing of other items than can cause a rough engine!
1. Spark plug misfire. Spark plug electrodes should not be worn beyond 50% of original thickness. Note, it's been proven in the test cell and on helicopters that worn spark plugs lower engine horsepower. 2 Misfire caused by insufficient spark plug gap. 3. Fouled spark plug/ Broken spark plug. May be caused by bad ignition coil. Note, a spark plug in a cylinder that is not firing will become dirty and oily. The spark plug us fouled but this may not be the cause of the cylinder not firing. A spark plug with broken ceramic may fire intermittently. Unison RHB series spark plug may be broken by overtorque without any outside evidence. 4. Magneto misfire. Inspect magneto distributor block and cover plate for signs of carbon arc tracking or arc-over to surrounding magneto structure. Check internal magneto cleanliness noting any signs of oil seal leakage. Check point gap and magneto timing. Fresh (unionized) air inside the magneto acts as an electrical insulator. If the magneto distributor block is dirty or starting to break-down electrically, arching first occurs at altitude where the air is thinner and there is less air to provide insulation inside the magneto (assuming an un-pressurized magneto). If the engine is running fine and then is slightly rough at altitude but then clears up when you descend then it could be caused by internal magneto arching. Replace distributor block. You can isolate engine roughness to the ignition system by running the engine in cruise configuration on one magneto and then the other.
Internal magneto arching and roughness can also be caused by teeth having been broken off of the distributor gear. Engines equipped with Bendix D2000 or D3000 magnetos, especially those that operate at higher altitudes, the distributor finger builds up quite a bit of crust and will cause rough engine and poor magneto checks. The distributor block and cap should be inspected regularly. 5. Ignition lead misfire. Can be caused by a faulty lead, faulty connection at spark plug, or electrode pin not making contact with conductor causing terminal well flashover. Spark plug ceramic should not be chipped or cracked in firing end or terminal well end. Check ignition leads with a High Tension Lead Tester. 6. Faulty magneto coil. See Testing the Magneto Coil. When magneto coils go bad, they usually break down when hot. The engine runs smoothly until a half hour into flight before it gets rough. The next day, when the coil has cooled down, the engine operates fine. The magneto will test fine on a test bench. If you suspect this problem, heat the magneto in an oven to 140 degrees F for an hour and then run magneto. The coil checks good with a coil tester when cold but break downs if you heat it up. For best magneto performance the air temperature in the engine compartment at or near the magnetos should not exceed 125 degrees F. It should not be permitted to exceed 160oF. An ohm test of a magneto coil may pass a pad coil.
One item that is often overlooked when checking for a rough engine are the ignition harness contact springs (cigarette) that connect to the spark plug. Aircraft operators who routinely fly at high altitude such as photo survey report that they routinely replace these springs regardless of apparent condition and this solves many of their "miss at altitude" or poor magneto check problems.
7. Carburetor ice. see crbice.htm 8. Propeller out of track or unbalanced. Out of balance propellers or harmonic engine vibrations often result in cracks in spinners and engine baffling. 9. Broken valve spring. Broken valve springs cause rough engine problems more when the oil is cold (first take-off of the day) then when the oil is warm. It takes more spring pressure to counteract a lifter filled with cold viscous oil then a lifter filled with warm oil. Always replace the hydraulic unit on the valve with the broken spring. A broken valve spring may not be visible. Use a blunt instrument and move the valve spring. If it is broken it the coils will separate. For more information about broken valve springs see: eng202.htm 9A. Just as a broken valve spring can cause valve float so can insufficient valve clearance or the use of heavy arms, such as roller rocker arms, in combination with tight valve clearance and/or hydraulic lifter with a low leakage rate. I have had reports of rough engine at high rpm's on cold oil when using roller rocker arms. It appears that the increased mass (weight) of the arms can cause valve float at higher rpm's and are particularly sensitive to lifter bleed-down rates and dry tappet clearance. If you suspect this problem you might try identifying the cylinder with the problem and switching hydraulic lifters with another cylinder and see if the problem follows the lifter. Moving to a cylinder with increased valve clearance may solve the problem. 10. Turbocharged engine and accompanied by poor cutoff. Air-bleed hole(s) clogged. 11. Pressurized Baron. Cabin vibration increases as wing loading increases. Full fuel in the tip noticeably increases vibration levels. 12. Stuck valve. See Stuck Valve for more information. 13. Shorted magneto coil. For information on testing the magneto coil see: eng174.htm 14. 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. 15. Distributor finger in Slick magneto loose. At first the magneto will run rough at altitude. Later, as the finger loosens even further the engine will always run rough. 16. F-33 Bonanza with STC three-bladed propeller. Sonic vibration, can be felt in the rudder peddles. Incorrectly tuned counterweights. 17. Lycoming O-235. Excessive combustion chamber deposits. Refer to Lycoming Service Instruction 1418.
18. Beech Model 76 Dutchess. Over-rich at off-idle. Install new carburetor idle tube per Lycoming Service Instruction 1390. 19. Lycoming fuel injected engines. Rough idle requiring excessive leaning to obtain smooth operation. Check for fuel stains in intake tubes. If you find fuel stains it could be coming from a seal leak in the Bendix fuel servo or from a leak in the expansion tube located inside the plenum. 20. Water ingestion into Bendix S1200 series magnetos on Continental 470 and 520 series engines. See Bendix Service Bulletin 611 for addition of moisture drains. 21. Bendix S1200 series magnetos with older green distributor blocks, loose oilite bushing causing distributor gear to become mistimed. Refer Bendix Service Bulletin 613. 22. Damaged or missing air inlet straightening grids (H-35 Bonanza). 23. Buckled or punctured AMC air blast tubes (PS "pressure" carburetors). 24. Induction system leaks. Be sure to check and make sure primer plug is installed in the cylinder's intake manifold 25. Leaking primer solenoids. 26. Water in carburetor air section (PSD-5C). 27. Improper "E" gap magneto setting. Common problem. As the magneto cam wears it causes engine timing to drift. The mechanic then "bumps" the magneto to re-establish proper engine timing. Each time the magneto is "bumped" the magneto's distributor finger is moved further from the optimum position. Eventually the finger is positioned between electrodes such that at altitude the spark can jump from the distributor finger to either one of two electrodes causing the magneto to fire the incorrect spark plug. Remove the magneto and check for proper "E" gap timing. 28. Defective magneto capacitor (condenser) or bad condenser ground. A capacitor that has lost its capacitance causes arching between the magneto points. This causes overheating of the points and erosion of point surfaces. The plastic cam follower mounted to the points may melt from the heat of arching. Thus by examining the points you can determine of the capacitor is working or not. To check the capacitor see: eng185.htm . Capacitor (P/N 10-382681) coming apart. The swaged on top section was loose and in imminent danger of separation.
29. Intermittent short to magneto "P" lead or "P" lead grounding spring. This turns on and off one magneto causing slight power changes but not as severe as a misfire in a cylinder. Disconnect the "P" lead from the magneto and with the ignition switch in the "ON" position for that magneto, check for continuity between the "P" lead conductor and ground. There shouldn't be any. Have someone jiggle the "P" lead as the short is most likely intermittent at a chafe point. On the Bendix S20/S200 series magneto the portion of the "P" lead wire going to the capacitor inside the magneto cover can be chafed. This occurs when the wire isn't routed properly. Remove the magneto cover and check the wire going to the capacitor for chafing. Check the ignition switch. On early Bendix S4-20,21,25, S6-21,22,23,25 magnetos, vibration may be shifting the breaker grounding spring. Bendix eliminated the grounding spring in 1957 ref. SB236 Lycoming SB404. 30. Engines equipped with HA6 carburetor - excessive retention screw wear - Refer to Precision Service Bulletin MSA-6 or latest version. 31. Engines equipped with Continental C-75, C-85, C-90, C145 O-200, O-300 see Precision Service Bulletin MSA-2, MSA-7, or MSA-8 carburetor nozzle replacement. 32. 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 . 33. 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.
34. Turbocharged aircraft using mechanical wastegate's. Check that the cable going to the wastegate is not stuck or sticking. 35. Carburetor bowl retaining screws loose allowing air leak into carburetor bowl. May also cause fuel leakage when the engine is not operating. 36. Loss fuel line connecting causing air to enter carburetor. 37. Too much tappet clearance on Lycoming 235 series engines. See AD80-25-02 or Service Bulletin 453C 38. Continental GO-300 rough in cold weather conditions - intake manifold diffuser ring not installed per Continental Service Bulletin M61-2. 39. Fuel Injector blockage. Contamination partially blocking outlet on fuel distribution valve. 40. GTSIO Series
Engines 41. Beechcraft Bonanza with E series engines and using the old disc style magneto drive assemblies. This is a real old problem first noted in CAA bulletin No. 107 dated February, 20, 1956 and quotes Continental Service Bulletin M53-3 dated December 22, 1953. Neoprene drive disc material is torn and the drive lugs are not engaging the full depth of the slots resulting in approximately 55 degrees variation in timing of one magneto. Evidently a change was made to the disc drive and the thick gasket didn't allow full engagement. For details get a copy of as M53-3 recommends a different magneto flange mounting gasket depending on if you have the new or old style disc style flange. 42. Sagging engine Lord mounts. They sometimes allow the engine to move a bit and it can actually touch the mount causing an incredible vibration that is one to one with RPM. 43. Lycoming engines with Bendix D2000/D3000 magnetos. P lead terminal connector shorting out against inside of magneto capacitor. This can cause a whole range of symptoms from complete magneto shutdown to intermittent engine vibration as one magneto vibrates on and off.
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Spark
plug resister
like this, then
suspect a bad spark plug or harness contact spring. A new terminal button
looks like this 
10-391584
Bendix Distributor block
10-357164-1
Bendix Magneto Coil
Slick magneto coil
with small crack in side. When first found the crack had black soot around the
crack from burning inside the coil. Because the crack is on the side of the coil
it is very hard to see without removing the coil from the magneto.
Lycoming
O235 engine
K-3984
Slick magneto capacitor. 
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.
