Hello everyone!

So...I'm about 700 miles from home and my engine's started making a funny noise. I need to set off tomorrow after a trip to a garage (I've got a wedding to get to), but I was hoping to go armed with some idea of what might be happening...CAN YOU HELP?!!

I'm pretty sure the noise is pinking (I've heard it before on a car). Here's some extra info in no particular order that might be useful...

1) It started suddenly about 100 miles ago. It was initially only when uphill or at high acceleration. I thought for quite a while that I might be imagining it...

2) After the bike cooled over night it got mostly better, but then when the bike heated up it got worse, and then much worse (as in: happened at all speeds, all accelerations) over the course of about 15 miles, when I made it to a mechanic...

3) I've now changed the spark plug (was iridium, now stock, after reading this http://www.xt660.com/showthread.php?t=21527), which seemed to help a bit (but it's hard to be sure...the bike cooled in the mean time).

4) Since this started, there's been a big increase in back-firing under engine breaking...

5) I managed to drop the bike on its side about 200 miles before this all started (one side of the centre sand sank in the mud...), but I don't see any connection. It landed on a pannier and the handle bar, very minimal damage.

6) The bike's only done 5800 miles. I've checked the oil and so on, all seems fine.

7) It did have a badly fitted kev mod (the air intake temperature sensor variable resistor) which was causing a warning light which I decided to remove about 1000 miles ago, (I don't think it's a real one anyway, didn't look right...). It's also been fitted with a K&N air filter.

8) The mechanic's initial thought was a dry-running bottom end bearing, but I'm inclined to disagree...

So, any ideas?

The current best guess I've got is that it's running too lean. Apparently, this can happen it the K&N filter needs re-oiling? Or, someone the mechanic phoned suggested trying to restrict the air intake breather pipe slightly? Seems somewhat extreme...

Anyway...any suggestions would be greatly appreciated. I'll check this later tonight and in the morning.

Thanks!

Will.

Oh, and last bit on info: it was SERIOUSLY RAINY when all this started. Hmmm...

Bad gas, maybe?

Check the coil! It is a fairly common problem for the coils on XTs to cause problems in damp weather.

Dry it out, seal it with silicone grease, particularly the coil to HT lead joint. Also check the resistance of the primary and secondary windings with a multimeter. We have seen a few coils go bad over the years causing misfires, flat spots and rough running particularly at certain revs.

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I've checked the coil, and run another tank of petrol through it. No improvements. Anyone got any other suggestions?