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Home> Maintaining
your Continental and Lycoming engine
Aircraft Maintenance Articles
Troubleshooting a rough engine
Please don't fly an aircraft when the engine can be hard to solve since there are many possible causes. First check the health of the engine. Here are some things to help you focus on the most likely cause:
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troubleshooting a rough engine
Ignition Problems: 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.
Ignition problems can be turned on or off by switching from one magneto coil's primary wire going from each magneto to turn on and off intermittently resulting in engine roughness occurs more often with a cold engine
power. Check for a broken or weak valve spring. Compare the spring tension from one valve to another as you push on the rocker arm to pop open the valve. A broken valve spring may be hard to spot so you should move the spring with a wooden dowel to expose any break.
A weak or broken valve spring allows the hydraulic lifter to pump-up with oil and holds the valve open. Valve spring tension pushes oil out of the lifter making sure that it doesn't pump-up. When the oil is cold and viscous, the lifter doesn't leak much oil and a weak spring may not push hard enough for the oil to leak out, causing the valve to not fully close on the valve seat. Later, when the oil warms up and thins out, the lifter leaks more oil and the valves closes normally. This is one reason why valve problems are temperature related. Check dry tappet clearance. This is the clearance between the rocker arm and the valve tip with a dry hydraulic lifter. At the same time inspect the hydraulic lifter for proper operation.
Too little tappet clearance can cause camshaft lobe distress as combustion
forces pushing on the valve face, instead of being transferred to the valve seat
are transferred to the camshaft lobe On A series Continental engines the
hydraulic lifters on one side of the engine can pump up and hold the valve open
if the engine was originally equipped with an oil cooler and then the oil cooler
was removed without removing the 22130 restrictor from the crankcase.. 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.  Valve
spring will collapse if it gets too hot. It gets too for 2 possible reasons: 1.
No oil to rocker cover, or 2. Leaking exhaust valve causes valve to overheat.
Intake Problems
If the engine is rough at idle and a cylinder has just been replaced then check to make sure the
Allen plug is installed in the cylinder's intake port.
Fuel Delivery Problems
The best thing to do is swap fuel components.
When all else fails
When the problem defies all known physical solutions I have in desperation considered hiring a priest.
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lying on a feather bed. Parachute Sense US NAVY 1944
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