Knock when rolling on throttle???

[DLD1]

I have an unusual knock in the engine when gently rolling on the throttle. It only happens when starting from a dead stop or when cruising at a constant slow speed and then gently rolling the throttle on. I've adjusted the valves, cam chain tensioner, checked the timing advance rod, all of which seem fine. It sounds very much like a rod knocking (which I've heard several times before, I used to race cars), but I've never had a rod knock that went away under hard throttle. Also, there are no other symptoms, the engine runs well, and there are no unusual vibrations. I'm a little confused by it, so I thought I would put it out there and see if anyone had experienced this with their XS. I'd like to avoid teardown if I can, but don't want a hole in the case where a rod exited either.

Thanks in advance,

DLD1

 

[jamesgs4]

clutch chatter maybe?

 

[DLD1]

I intend to check the clutch, although it happens when the the clutch is already released.

 

[Sundie]

Piston slap. For whatever reason, it goes away at higher rpms on the xs650. Or maybe you just can't hear it as well.

 

[CDNTX650]

super loose drive chain?

 

[DLD1]

Thanks for the suggestions. I'm pretty sure the chain is not loose, but I'll double check it, and as far as piston slap is concerned, I suppose it's a possibility, but the pistons and bores were checked not that long ago and were within tolerances. I suppose something could have let go or changed. I'll definitely keep that in mind if I decide to tear it down for inspection.

DLD1

 

[jayel]

when starting from a dead stop or when cruising at a constant slow speed and then gently rolling the throttle on.DLD1

these are ball bearing lower ends they don't really like being lugged

 

[XSLeo]

Low octane gas.

 

[DLD1]

XSLeo,


I had not thought of that. It's certainly a possibility.

 

[maxwell]

what is piston slap and what does it mean?

 

[DLD1]

Piston slap is generally when the piston skirt area is worn enough to allow the piston to rock back and forth in the bore, causing it to "slap" the cylinder wall, making a knocking sound. As far as my particular engine is concerned, it shouldn't be a problem unless a piston skirt broke off, because I checked the pistons and cylinders and all were within factory tolerances when installed.

For anyone interested, I've drained the oil through a cheesecloth to check for particles, and removed the sump plate. No particles or other debris at all, so at least nothing has been disintegrating. I also reached up to the big end of the rods, and they seem to be nice and tight. Next step is to check the clutch. If I find nothing there, the engine comes out of the frame.

DLD1

 

[pamcopete]

DLD1,

An exhaust leak where the headers are clamped to the head can sound very metallic and occurs when rolling on the throttle.

 

[DLD1]

DLD1,

An exhaust leak where the headers are clamped to the head can sound very metallic and occurs when rolling on the throttle.

Thanks, Pete. I haven't experienced that one. I'll check that before I go pulling the engine.

DLD1

 

[maxwell]

hmm.. i'll check mine out tomorrow too. i hope that's it lol

 

[DLD1]

DLD1,

An exhaust leak where the headers are clamped to the head can sound very metallic and occurs when rolling on the throttle.

Pete,
Thanks for the tip. Unfortunately I checked it out this evening and it was not the problem. Actually things have changed since I first posted. Now the knock is notable at idle, and at higher rpm as well, not just at roll-on. I'm going to check the clutch because it's easily accessable without pulling the engine, but I don't expect to find anything. After that it's pull the engine and tear into it to find the culprit.

Thanks to everyone for the suggestions.

DLD1

 

[DLD1]

To anyone interested in the results of my troubleshooting, I decided pulling the engine and tearing it down was the only reasonable solution after investigating all the possibilities I, and you all, could think of. Upon pulling the head and cylinders off, I found the right side rod had little bronze material left in it. I re inserted the piston pin and found substantial slop, definitely enough to cause the knock. I'm not sure what the cause was, other than just age. I haven't found any other issues so far. Now I'm faced with having to replace rods, which brings up the possibilty of a rephase. I've always wanted to rephase, but have never been able to justify the expense (still can't), but it seems like the perfect opportunity to do so. Anyone out there know of good reasons not to do it?

DLD1