Light knock at idle

gilitos

gilitos

Senior Member
I've got a light knock, mostly noticeable at idle, on a rather fresh rebuild Maybe 3000 miles or so) by a reputable local shop that does lots of A's and V-8's. Seems worse when cold. Doesn't seem to depend much (if at all) on the load, although retarding the spark may quiet it a bit. For example if I'm pulling up a hill and really put my foot in it, the knock doesn't increase. I noticed in another post a reference to rods that weren't straight. Any other ideas? Thanks!

Giles in Cincinnati
 
Had the same symptoms, turned out to be excess clearance on the center main. The tip off to me is the knock increases as timing is advanced. I would first check the center main clearance. It is not bad yet and can be fixed from underneath, remove maybe a shim or two. Put a jack under the crankshaft and jack it up a bit to take out any sag in the crank with the cap removed.
 
Keep us informed on developments. I've had the same problem for the past three years, starting from the day my new rebuild was installed. Mine sort of comes and goes, as the car just sits and idles in the driveway. Doesn't seem to be getting any worse over a few kilomiles, including drive to Williamsburg meet right after installation. The rebuilder was at that meet so I could have him take a listen--had no ideas. Others have suggested a startling variety of possibilities, none of which seem to fit the data.

If you've got a timing light and mark on your crank pulley, here's a cute trick that can help narrow possibilities, taken from http://remanufactured.com/Engine_Noise_Diagnosis_101.htm :

Diagnose Noises with a timing light?​

Valve train noises occur at half of crankshaft speed so even if your ear can't tell whether the noise is happening at 700 rpm (raps per minute) or only 350 rpm, your eyes can. Hook the timing light to any one cylinder and watch the flash illuminate the timing mark. Stare at it for a while and see if the flash jives with the knock. If it does, then it is more likely to be rocker arms, pushrods, lifters, camshaft, cam bearings, timing chain and gears. If the noise seems twice as fast it is probably in the crank, mains, rods, rod bearings, wristpins and pistons.​

Even if you don't currently use a timing light to set your timing, I recommend buying a cheapie to play with; White-Out typo correction stuff makes a nice timing mark on crank pulley, but any white paint would work. My knock is apparently in the valve train system, so it's nice to know I don't have to fret about bearing shims and such.​


Steve​
 
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You might check the timing gear nut for tightness,first.
Remove the timing pin. Start the engine. At idle, carefully (Watch out for the fan) insert the pin, and gently press it against the spinning timing gear. If the knock changes, or goes away, the timing gear nut is loose. Remove the cover, tighten the nut and you're done.
 
Yeah, I worry about center and/or rear main, because that means two babbit pours in a row have failed. My previous rebuild lasted about 3000 miles before the rear and center bearings cracked up, necessitating the last rebuild. And I'm gentle on my motor.

The timing light idea is great and I can't wait to try it. And I'll check the timing gear as well. I'll post results.
 
P.S. notice how all our avatars are shot from the same angle? What gives? Is this the equivalent of a movie star's "good side"?
 
maybe its just to much advance. if i pull my lever to much i get knocking at idle but at the right spot its quiet. thats pretty much where i keep it while driving also, except pulling a hill.
 
By chance do you have installed on your engine a two piece crankshaft pully? and is it good and tight?

Just hoping that it is something super simple to fix...
 
Cam Plunger knocking,replace spring and plunger.
I have found these with the spring missing or not working anymore due to dirt/sludge.
Wick
 
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