Loud Rapping / Slapping Noise

Moderator: Larry Jowdy

Loud Rapping / Slapping Noise

Postby TSchommer » Thu Aug 02, 2018 4:29 pm

I have recently developed an unusual noise, that I can’t seem to pinpoint. It seems like an exhaust (bad muffler) issue, but there are some things that don’t add up. First, it sounds, runs and revs just fine when stationary, so the sound only happens under load, it also seems more tied to the actual throttle position rather than the tachometer / rev counter. I uploaded a video, where you can hear it at 0:44 sec (lower rev), and then again at higher revs at around 1:07 (I think I’m merging on to a highway. I think the sound always happens, but it used to only come on at full throttle, now it seems much louder, and it happens much earlier in the power band.

https://youtu.be/n2jMxwivZgI

Any thoughts?
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Re: Loud Rapping / Slapping Noise

Postby EEricson » Thu Aug 02, 2018 6:24 pm

Piston slap?
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Re: Loud Rapping / Slapping Noise

Postby DannyP » Fri Aug 03, 2018 5:24 am

It's either pinging(or pinking as the Brits call it). Also called detonation. Easy to cure, if it's caused by carboned-up cylinders. Remove carb tops. Rev engine to about 3000 and trickle water into one carb throat at a time. Watch the nice white cloud you get out of the exhaust. Once cleaned of carbon, the cylinder pressure drops due to combustion chamber being clean(lowers compression ratio back to where it should be). This should cure it for a while. If not, possibly timing too far advanced(actually check this first).

It also could be a loose or badly positioned fan hitting the shroud. Or possibly a loose flywheel. Fan is easiest to check, especially on a Spyder. Flywheel obviously requires pulling the motor.
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