Strange Engine Noise at Low MP

[co2bruce]

Been noticing a strange "puffing noise" coming from the engine when pulling back manifold pressure in the pattern. With gear down and MP at 12 - 14 inches is when I notice it. Plane starts fine and flies fine. I do not notice anything strange at normal - WOT. Almost sounds like a miss or something. We checked the plugs and they all look fine, and all CHT and EGT looks normal. Any ideas would be appreciated.

[ArtVandelay]

I guess 

I would try pulling back the power with the gear up, and lowering the gear without reducing power...to further isolate the problem.

[Hank]

Is it anything like the gurgling my truck does when I let off on the gas? You know, the sound of power ready and waiting to be used? Doesn't do it loafing along, just during the deceleration part. Sometimes even my C does it on short final when I pull to Idle.

Edited January 26, 2017 by Hank

[jlunseth]

I don't know what a "puffing" sound is in an engine.  Does not compute.  It sounds like the engine is just running too rich, which can happen at low power.  Has this always happened or is it new?  If new, did you happen to have the mixture reset for any reason?  You might just try lean the engine when this starts, and see what effect that has.

Edited February 2, 2017 by jlunseth

[carusoam]

I'm going with the too rich scenario... excess fuel dumping into the exhaust...

The puffing sound is fuel burning trying to expand the inside of the muffler.

While trying to isolate the challenge work the mixture knob in a different way...  don't go full rich to see if this is the issue.

PP thoughts and M20C experience.  Have not had this issue with the IO550...

Best regards,

-a-

[cnoe]

What's your fuel flow at (sea-level) takeoff? Mine's 0.5 too rich but still doesn't "puff", at least not since graduating college.



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[co2bruce]

Here's the problem

[MB65E]

It sounds like it's just backfiring slightly. 

Try going slower. And not letting the airplane & propeller drive the engine. 

I always try to pretend I'm flying a geared engine. I try and always have the propeller loaded by the engine and not the other way.

Some times faster descents at idle with the prop forward will cause slight a backfire. 

-Matt

[co2bruce]

Right Mag was 3 degrees off, cleaned plugs, and flying the pattern a little leaner. Seems fine. Thanks for all the input