Load Sensitive Engine Noise

My engine recently developed a ticking noise that I cannot figure out that sounds a lot like valves ticking. I have checked and double checked valve clearance and cam chain adjuster, inspected and rebuilt the oil pump, removed the sump and cleaned out and inspected the screen, checked for oil reaching the top end by running it with the valve covers off, blown compressed air through the oiling system, checked the cam chain front bolts.

When I first start it it is very quiet (I have elephant foot adjusters). It makes the noise after it is warmed up at part throttle between about 3500 RPM and 4500 RPM and is very sensitive to throttle position getting loudest at slightly less throttle than is needed to maintain speed in 5th gear on a level road.

If I am on or off the throttle I do not hear the noise. When it is hot I do hear a slight knocking sound at idle that usually stops after idling a minute or two.

Any help would be greatly appreciated since I'm stumped.

Sounds to me like a loose wrist pin in one of the pistons, or piston skirt slap. When power is applied or removed, the piston is either forced down by combustion, or the inertia of the con rod pushing up and down from coasting, the extra clearance is minimal because the forces above and below the piston are highly reactive. At neutral/coasting throttle those forces are vastly reduced. Also when cold, the oil is thicker and when the aluminum piston heats up the clearance/tolerances increase, so it will loosen up more when hot.
If possible use a mechanics stethiscope and point it at various points on the motor to hopefully isolate the sound.

Another trick is to put you and the bike in a stone dead quiet place, put a tightly fitting wrench or lockgrip pliers on the rotor nut, rotate to TDC, and, like a '30s safecracker, rotate the crank fore/aft several degrees, listening and feeling for any unnatural 'clicks'. Do this also at 90° after, BDC, and 90° before. If you get any of those 'clicks', you've confirmed conrod and/or wristpin problem. If you don't get those sounds/feel, then it doesn't rule that out, but could still be conrod and piston, just not that bad...

Thanks for the suggestions NONclow and TwoMany. I was thinking it was the rod small end but wanted to get some more unbiased opinions. I keep hoping it is the piston so I don't have to split the cases and either fix that crank or rephase my spare.

Going to try your suggestions this evening.

So I bought a mechanics stethoscope, it helped some but because of the way sound is transmitted through the metal it was still difficult to pinpoint. It did help to confirm the trouble was on the left side.

I also tried TwoMany's suggestion and was able to make a noise in the left cylinder at about 90 degrees from TDC by rotating the crank back and forth just a bit. Could also hear the noise coming out of the spark plug hole on that side pretty well confirming the problem.

So, this evening I tore it down and found the problem, a bad big end bearing on the left side. The max spec for side to side movement of the rod small end is 2mm, the left one measures almost 3mm and the right one is within spec. Also noticed that when the left one is rotated through the range of the down stroke, if I put some sideways pressure on the rod as I rotated the crank I could feel some roughness.

Anyway, thanks again for the suggestions. It was good to be able to confirm the problem before ripping it all apart. I just wish this would have shown up last fall so I had all winter to fix it.

Sorry to hear about the bad crank, pretty rare on these things.

Welcome to the 1930's safecracker's club...

I am concerned that this failed rather early at only 38,000 miles since the crank was completely rebuilt with all new bearings and rods. I have a spare stock crank that I think has 80,000 on it and the rod bottom bearings are perfect. I do run it up to and maybe a bit past the redline quite a bit but I have had this bike since new and have always done that.

I have a couple of thoughts on this, these are aftermarket rods and rod bearings, perhaps they are not as good as OEM despite the claims? Also, it does now have larger and heavier JE pistons at 700cc to deal with.

Once I split the cases I am going to take a good look at the oiling system too but there is no evidence of problems elsewhere, cam, bores and pistons look beautiful.

Another 'ding' on aftermarket parts. Even though they may be able to replicate the dimensions, I've always been leery of the metallurgy and heat treatment. An interesting article I read long ago stated that maximum bearing life would be achieved when the bearings were 1 Rc point (Rockwell 'C') harder than the races (properly matched sets are important).

If you do decide to split it, perhaps you could put an indexing mark on the crankpin before splitting. Then, if you find fatigue brinneling on part of the pin, might be able to determine where (in the crank cycle) overloading is occurring. ...

Any suggestions on replacement rod(s)?