Ask Paul: What causes this popping sound?

Q: I am test flying my new RV-9. It has a new Superior O-320, dual p-mags and an MTV11 2-blade MT prop. When it has been running at cruise power for a few minutes or more (say 2,300 rpm x 23 inches or more) and then I reduce the throttle, I get a popping/crackling sound from the engine. If I reduce the rmp before pulling the throttle, it doesn’t seem to do it so much. This typically happens when joining downwind to land, but other times too if I pull the throttle.

I have tried this with the manifold pressure line to the p-mags connected (timing advances) and disconnected (timing fixed).

Have you any ideas what may be happening and if it is a problem? The CHT and EGT seem fine (although #3 CHT is higher than the rest at about 400 in the cruise).

JOHN BONE, via email

A: John, I’m going to go out on a limb here and take a guess that the engine, under certain conditions, may be running too lean and the sound you hear is the little back-fire in the stacks. Does it happen just after landing while you taxi off the active runway? It’s not unusual, especially on some Continental engines, for this popping to occur during that scenario.

I’d be interested in knowing the rpm rise when the engine is shut down at idle (600-700 rpm) by moving the mixture control lever slowly to the idle cut-off position. Typically you should see a minimum of a 25 rpm rise before the engine quits. If it’s less than that, the mixture may be a bit lean and should be adjusted accordingly.

Does turning the electric fuel boost pump on or off have any affect? Another thing to check is the manifold pressure at idle rpm. It should be 10 inches or less. If it’s above 10 inches, it could mean there is an induction leak somewhere.

These are just a few of the things that come to mind. I think checking some of these things, in addition to doing a good visual inspection of the engine may lead to the solution.

One place I would check closely is the #3 cylinder. The 400° CHT temperature you mentioned on the #3 cylinder is well within limits, so it may be just fine and, because of its location, it’s not unusual for that cylinder to run just a little higher CHT than the others.

I’d still give a good look for any sign of an induction leak, which may be indicated by a slight fuel stain on the intake pipe near the flange if the gasket is deformed or the flange itself is cracked.


Paul McBride, an expert on engines, retired after almost 40 years with Lycoming. Send your questions to:


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