LYCOMING 0-360 ROUGH RUNNING ENGINE

Hopefully, this post will help others with Lycoming 0-360 engine issues that cause a rough running engine.  We have been through the mill on this one so I thought I would post a sequence of possible issues for a rough running Lycoming aircraft engine.

First, check to make sure your engine primer is closed and locked.  If the primer is not locked it can allow fuel to pull through the system.  Each time the cylinder draws on the intake stroke the primer will leak extra fuel into the cylinder.  This results in two symptoms.  One, the engine will run rough.  Two, the engine does not want to die when you pull the mixture back all of the way.  This is because the cylinder with the primer is sucking gas into the cylinder with each stroke.

Second, check for intake leaks.  Places prone to leak are the gasket between the cylinder head and the intake tube flange.  These are very inexpensive.  Also, check the short tube that runs into the engine oil pan intake.  If these tubes are loose they leak air into the intake.  A special aircraft tool is required to tighten these.  Lycoming has one that costs around $8,000 and you can rent it.  I also found rental is available at other places.  Our local engine shop loaned us their Lycoming tool.  It is very easy to use and reseals these intake tubes correctly.  As an added bit of protection we put a bead of Permatex Ultra Copper around each tube where it enters the engine just to make sure there was no leak.

Third, check for a broken spring on the cylinder.  It is difficult sometimes to see.  Our engine had a broken exhaust valve spring.  We did not find it until we pulled the cylinder and took it to an engine shop to get the valves re-seated.  The engine shop called and said the spring was broken.  According to the mechanic this is not very common, but does happen once in a while.

The above assumes you already know your mags are good and you spark plugs are good.

Hope this helps.  If it does, please post a response.