Trying to track down cause of slight knocking noise. First a description: Hollow knocking similar to a hammer tapping on the side of the heads. It is happening only at idle and using stethoscope the sound seems to be in the heads. 1990 Evo with a recent rebuild 12000 miles ago with S&S big bore (3 5/8) and S&S cases. Used original harley heads to save costs but rebuilt with heavy duty springs and machined for the higher lift. Crane high roller cam 581 with Jims lifters. Did not put high compression pistons in because I did not want to deal with bad gas. The knock seems to be there hot or cold and seems to go away as soon as I get above 1000 RPM. Idle set at around 850 RPM. I know enough about motors for the noise to bug me. Any suggestions? Could it be something with the combustion chamber of the Harley heads with the 3 5/8 big bore kit? I was thinking maybe carbon on parameter of pistons hitting heads because of smaller combustion chamber? Forgot to mention, checked compensator sprocket and primary chain. That area now perfectly quiet. Also checked timing. If it was piston slap wouldn't I hear it more towards the base of the cylinder?

 

I'd say it's probably because of the cam you have. When you start getting into bigger cams it effects the idle quality. A lot of bikes with cams have to run their idles up at 1000 rpm.

 

I agree with Larry. bump the idle a bit..You aint running a Mik are ya?

 

Still have the Harley CV carb installed with some modifications. Seems to be feeding enough gas. Dyno tested at 80.5 hp with 89.1 ft lbs torque and feels steady all the way up to the red line. I will set the idle up. Anyone have suggestion how to sqeek a few more hp out of what I have done please pass it along. I was thinking about replacing the heads with a set of S&S or Elderbrock but kind of pricey with unknown paybacks. Thanks.

 

Sat the idle up and most of the knocking went away. Still have some slight noise, but it seems to be by the exhaust valve. Maybe from heavier springs? Thanks for the suggestions. Does not sound bad enough that it will come apart so I will ride until it get's worse I guess.

 

keep us posted...

 

Although the knock has reduced considerably, there still seems to be some from the rear head. It seems more prevalent around the exhaust port. Using a stethoscope with a probe to isolate. Unfortunately, we are now getting snow so not a lot of riding weather until spring. Thinking about tightening up the lifter adjustment on the exhaust pushrod. If that does not eliminate it then replacing the Harley heads with S&S.

 

If, the valve stem ends with the rocker tips, were almost touching the inside of the cover when new, a very little valve seat wear would let this stem rise and then the affair would, could touch the cover. Just a tap.

Maybe. Just trying to thing what might be a cause.

Easy to check anyway.

 

Good suggestion and like you said easy to check. We are heading for some hibernation weather so I will have some time to spend pulling the rocker covers off. Thanks.

 

ide go with piston slap,noise can travel thru a motor and be hard to tell where its coming from,stethascoe or not.running your idle at 850 your going to go thru rod bearing like their going out of style,run it around 975-1000
kirby
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