Help me methodically diagnose a rattle...

[electric!sheep]

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:

Carbon-knock, according to the TSB, occurs on
a cold engine where accumilated carbon on the cyclinders
comes in contact with the head of the engine procuding a
knocking noise. This knock occurs until the carbon softens
due to the heat.

* 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