I hope I can go about this scientifically, since there's nothing worse than just saying "it doesn't sound right." I have two issues, a rattling noise when riding, and a knock at start-up. They may or may not be related.

Background:

* Ran very rich for at least 100-200 miles.

11K miles on 1989 XT600, purchased recently from guy in AZ. He had it jetted lean for the high elevation, and since I'm in SF he offered to rejet with the TT main jet (richer). I accepted, and he rode about 400 miles to meet me half-way. The back fender was coated with soot when he arrived, however the spark plug looked OK when inspected (and replaced), though the pointy Iridium plugs are hard to gauge.

* Ran low on oil during the trip. He probably warped the oil filter cover when doing the complementary oil change, because it was weeping oil. When I went to change the oil, the dipstick was dry, though the res still had some oil inside.

The bike runs very strong and does not burn oil.

Symptoms:

What happens...

* Knock for 2-5 seconds on cold start (cold in CA means sitting overnight at 70F). Sounds like a hammering THACK THACK THACK instead of the usual bassy THUMP THUMP from the exhaust.

* Irregular rattle starting ~3000 RPM which disappears into general engine noise if I accelerate hard past 4500RPM.

* Not audible at low (1/8th) throttle in the aforementioned range. As soon as I bring it to 1/4 or 1/2 it will begin to rattle (audibly).

* If I'm cruising along at 30MPH and rev up to 40MPH it will rattle. If I do the same throttle motion with the clutch in, it will not rattle audibly.

* BBs in a tin can from 5000RPM to redline. Will continue "pinging" for a split second after I roll-off the throttle.

* Rattle seems to emanate from the upper portion of the engine, but I haven't had a chance to ride, er, head to head with the bike. It's hard to pick it out at idle (consistent with my rev-test above).

* Most of my observations were done with earplugs in, which made the noises stick out more from the background noise (isolated the exhaust noise, mostly).

* Seems to vibrate more than the XT600 I had running before it blew out 2nd gear.

What I've done:

* Replaced spark plug - when inspected after 10-20 miles of hard riding (not much 5-7K RPM though), it looked normal.

* Replaced oil with 10w40 generic and then 20w40 Yamalube

* Pulled cam chain tensioner and re-tightened. Has plenty of notches left.

* Adjusted valves. Exhaust was a little loose, but still good.

My hypotheses:

Rattle

* Running rich caused carbon to build up on the cylinder head and is producing detonation, explaining both the rattle and the pinging in the 3000-7000rpm range.

Cold Knock

* Pessimistic: Running low on oil f'd up my crankshaft bearings.

* Optimistic: Carbon knock. As described in relation to a car:

* Pessimistic: Carbon build-up on valves is pushing them into the cylinder head, which slaps them shut. Not sure how this would manifest itself, but I thought I'd throw it in as a long shot.

* Optimistic: 20w40 oil takes time to distribute & warm up.

Any other hypotheses? What do you guys suggest I do? Any thoughts, or even a bump for this thread, would be appreciated. I will try to follow up with some sound recordings, but I'm not sure how easy it will be to pick out the noise.

I am on the verge of ripping the cylinder head off and going to town on it and the piston crown with a toothbrush and some solvent.

Thanks,
Mike

Damage from lack of oil kinda depends on 'how low for how long' but its certainly possible. The engine rattling while under load and going quiet when you clutch in sounds like big/small end damage.

Better safe than sorry & stuck in the middle of nowhere. Pull it to bits so you can check everything properly, hammering & thwacking noises cant be good.

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The bike is 17 years old, irrespective of how many miles it has on it that means things can and probably will go wrong. Thouroghly and proffesionaly clean the carburator (not just a spray and pray). Put stock yamaha jetting in it for that model with respect for things like a modified or replaced exhaust. Cam chain tensioners sometimes don't tighten like they should and need to be replaced. How many miles have you ridden it to determine that it doesn't burn oil? I have seen cylinder to piston clearances worn out of tolerances by one weekend riding with improperly serviced air filters. It sounds like pulling the top end down would be cheap insurance to see how things are. With that few miles on it could be a valve stuck from bad gas. This is where the valve sticks in the guide and the piston slaps it and then the valve is slightly bent but still seals and gives a good compression reading but makes noise. These are general ideas but remember repair work is 90% patience.

About 400 miles, progressing from full to below the dipstick. It probably lost 1L to 1.5L of oil. I just RTV'd the sucker when I changed the oil.

All good points. I will pull the top end and see what shows up. As for patience, I seem to spend more time working on my bikes than riding, and I'm starting to wonder which I like more...or at least that's what I tell myself when it's time to rip apart another bike.

OK, I ripped it apart.

Piston Crown



Alternate angle: http://img99.imageshack.us/img99/2482/cimg0423br4.jpg

Cylinder Head & Valves



Alternate angles:

http://img220.imageshack.us/img220/930/cimg0412kh2.jpg

http://img99.imageshack.us/img99/4018/cimg0416lq3.jpg

Cylinder Bore



Alternate angle: http://img220.imageshack.us/img220/245/cimg0429lu5.jpg

I will remove the cylinder tomorrow and play around with the conrod, but this looks like there might be evidence for my theory. I haven't seen many heads & crowns, but having that much visible carbon build-up can't be good. Couldn't/wouldn't bits of it heat up and produce detonation?

As for the bore, I can still see the horizontal marks from when the sleeve was machined, though there's a bit of scuffing on the front part of the bore. Doesn't look too bad, does it? I'll see if I can't get measure the clearance tomorrow.

Now looking at the piston crown, I see an irregular circle in the center, which is actually a slightly raised area. I assume that's normal and not a sign of damage?

Anyway, do these pics say anything to you guys?

With your topend. The amount of carbon deposit is normal. It also shows your bike is not loosing oil via the cilinder / piston gap, as oil tends to dissolve the deposits on the piston crown, which would manifest as a clean edge on the piston crown.

Auke

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Auke
http://aukemarieke.blogspot.com

Bummer, so much for my optimistic theories.

Any tips on diagnosing the small-end and big-end bearings on the conrod, once I pull the jug? Check for play, smoothness?

Looks about normal, cylinder might be worn close to its limit maybe causing a bit of piston slap when cold. If the bike was run withlow oil the camshaft bearing surfaces would show damage first, how do they look?

__________________
Winston
85 xt600Z
02 YZ250

Here you go, with my other XT600 for comparison (similar mileage, different dipshit owner who probably checked valve clearances with his toenail clippings, since they were double what they should have been when I got the bike).





Nice gouges on that one.









Hrm, yay for pits.

In short, my new XT could look a whole lot worse. I don't see any major gouging, scuffing, or wear (then again, maybe I'm the ray charles of motorcycle maintenance). However, bearings are perhaps much more sensitive than cam surfaces, so we'll see what turns up as I dig further (most likely tomorrow).

with camshaft / bearing surfaces. Used, yes, but don't worry. Note that the surfaces in the cover are almost new, as they are not loaded by the mechanism. It's the surfaces in the head itself that are loaded.

Auke

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http://aukemarieke.blogspot.com

So I dug in further. I measured the side-to-side play at the big end and it was within spec, and the rod rocked back and forth without any difficulty. I put it through the rotation but that's meaningless with all the gears involved right now (haven't disassembled the lower end). Anything else I can do to check the lower end right now, without further disassembly?

However, I took a close look at the small end and here is what I found:













Manipulation of the piston on rod, and then pin in rod, suggested slightly more play in my XT than the comparison. I really don't like the burned oil deposits on the piston pin. Something made it run very hot which roasted the oil into a dark patina. The rod surfaces are also darker and splotchy; the darkness you see by the oil hole is not a shadow (as in the last picture). It is a circular area of darkened patina, which can be found around other oil feed holes. The piston bearing surfaces seem to show more wear on the XT in question. The carbon above the first ring seems very thick, and I can see a bit more blow-by past the first ring on this XT. I forgot to measure the clearance before I pulled the jug.

And sorry to beat a dead horse, but has anyone seen white deposits on a piston crown, like I have? Aren't those produced by detonation, which = heat? If so, that would explain the roasted oil on the piston pin.

I'm inclined to re-bore the cylinder, replace the piston and wrist-pin, and I'd probably be wise to swap out the crankshaft to get a fresh rod. Is this a reasonable course given what I can see so far? I will need to pull the case covers anyway to do this so that will allow me to check for loose bits in the clutch basket and other weirdness.

Incidentally, at least two of the fasteners--one threading into the crankcase and one into the jug--bound up and were very difficult to remove. One snapped clean off at the cylinder top, and the other came out with some aluminum filling in the threads from the crankcase. Something caused the fasteners to bind up with the aluminum. Heat-induced warpage? To run with it, that may have also caused the oil filter cover leak. Just some speculation.

Again, I submit the evidence for your review.

A steel bolt into an aluminium casing will corrode as the mix of metals works like a feeble battery, in a 17 year old bike thats to be expected. Think of it as natures loctite.

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Missing, presumed fed.
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