Popping and crackling noise under cowling

[clipquito]

Does anyone know why there would be a consistent popping and crackling noise under the cowling on taxi -- could not hear it in flight. We just returned from trip in my Clipper from Utah to WI (27.3 hours). Oil, CHT, EGT temps were spot on the entire trip. The popping happened on the last 6 or 7 legs. There is a bit of black sludge on the bottom cowl. Some evidence of exhaust blow-by possibly a little more than usual. Also the CHT probs seem "burnt" where they enter the exhaust stacks. Thank you.

[Gilbert Pierce]

Loose wheel, brake or wheel pant.

[clipquito]

Thank you. At first that is exactly what I thought, but I inspected both wheels and pants and found nothing loose or of concern.

[Vagabondblues]

Assuming your engine is a O-235 in a clipper? The valve lash should be checked regularly for the correct cold clearance. At an idle setting like taxiing the oil pressure is lower causing less splash and spray of oil onto the cam. Reduced oil film (hydrodynamic action) lowers some of the cushion to the lifter which increases the valve lash clearance.

The other thing I would suspect is your idle is set to low. If your idle is lower than 600 rpm the impulse coupling could be catching when you hit a expansion crack in the taxiway.

Todd

[clipquito]

Thanks so much. Actually it is an O320 150hp. Not sure what a valve lash is. .. Idle if anything is a bit too high -- 900 RPM.

[Vagabondblues]

Thanks so much. Actually it is an O320 150hp. Not sure what a valve lash is. .. Idle if anything is a bit too high -- 900 RPM.

The 320 is hydraulic lifters or zero lash in the system at any temperature.

I hear similar noises whenever a tail wheel aircraft is on the taxiway next to my hangar. Up north in WI we have really big expansion cuts for the frost.

[Gilbert Pierce]

The tailwheel hitting the taxiway centerline reflectors is noisy.

[Pacerfgoe]

Give your exhaust a damn good whack with your hand, maybe it's got a loose or broken baffle....

[clipquito]

Love it! Best suggestion yet. Thanks!

[Gilbert Pierce]

The Clipper is supposed to have muffler support rods that go from the two muffler inlet clamps up to two tabs on the engine mount. Those two rods are supported on springs at the mount. Make sure those support rods are secure and there some vertical movement allowed by the springs. See figure 3, Piper drawing 11966