Question for Paul McBride, the General Aviation News engines expert: I recently overhauled the Lycoming O-360-A3A engine on a Piper PA-28-180.
It runs smooth on cruise and RPM above 1,200, and the mag checks are very good.
However, whenever I start the plane and reduce RPM to 1,000, it runs very rough. It idles at about 650 rpm and rough as well. At start up and low RPM, the RPM fluctuates quite a bit before it eventually stabilizes.
My engine monitor at this low RPM also warns about CHT differential exceeding 100° between cylinders (I think between cylinders 1 and 4).
Once I increase RPM, everything goes away. The problem is not as bad after the engine is hot (like after mag checks or end of flight).
I greatly appreciate your help. Thank you.
Romeo
Paul’s Answer: Romeo, your question may sound complicated, but I’m going to try to take the easy way out.
After reading about your situation and doing some soul searching, I think I may have the answer.
My first thought from your description is that the carburetor is running lean at the lower operating RPMs. Once you get above 1,200 RPM, you’re running on the main metering jet in the carburetor and things seem to be okay at that point.
Just for fun, I’d like you to start the engine and see if it acts as you explained. The next thing I’d suggest is for you to manually lean the mixture while at idle RPM. If the carb is operating normally, you should see a 25 to 50 rpm rise just before the engine quits. If you don’t see the RPM rise that could indicate a lean mixture.
Next, I’d like you to pull carb heat on and see if that has any effect or makes the engine run smoother.
If the engine runs smoother, that will indicate that the carb is lean at the lower RPM settings. You should be able to adjust the idle mixture screw on the carburetor.
Another thing that leads me to think this way is the fact that the engine runs better after it is hot, which may, again point out that the mixture is lean.
Hopefully, after following my suggestions you’ll be able to correct the situation.
With a poor idle and a ‘cold’ cylinder, I’d suggest looking at an induction leak.
If it has a new carburetor, check that the correct jets are installed and that the float level is correct.
There is also the possibility of a sticking exhaust valve. There is no info if these are new or overhauled cylinders.?