Lycoming O-360 Runs Rough

Although this is not directly related to our tool kits and tools, it is worth sharing.  I did a lot of research on the web to see if anyone else had experienced this problem.  I found a couple, but many that I found never posted a solution.  Here is one possibility.

Symptoms:  The Lycoming O-360-A4A had been running fine.  After a 1-hour cross country flight, when descending from 7,500 msl, power pulled back to about 1,700 RPM the engine felt very rough.  Leaning the engine significantly seemed to help, but remained rough.  After landing the engine felt very rough to shaking severely.  A run-up seemed normal.  Mag check good on both mags.  Above 1,750 RPM the engine seemed to be ok, but below that there was no way to make it run smooth.  Since it ran ok at high RPM the airplane was flown back to home base.  Same symptoms on descent and approach at home.  Still very rough on the ground.

Troubleshooting:  The Lycoming O-360 is relatively easy to work on and access everything.  We ran a compression check, compression in all 4 cylinders was good.  Spark plugs looked perfect, except #1 cylinder plugs were black.  Bottom plug had alot of lead deposits.  Installed 2 new plugs.  Engine was still just as rough.  Above 1,750 RPM it smoothed out.  Noticed by touch that #1 cylinder was barely warm when other cylinders were too hot to touch.

Fix:  Checked the intake tube on #1.  The gasket between the intake tube and the cylinder head was 90% missing.  Only about 10% of the gasket was around one bolt on the intake flange.  Installed a new flange gasket ($1.57).  Engine was still rough.  Pulled the spark plugs, they were soaked.  Cleaned and dried the plugs.  Engine ran perfect.  The intake gasket was the culprit.

I don't know how common this is, but I hope this blog post will help other owners trying to troubleshoot a similar problem on the Lycoming O-360 engine.

Please, share your troubleshooting experiences here too, if you have solutions others may need to know someday.