So, I've got this 690 Enduro R with 17" wheels that the previous owner put on. Here's what it looked like on the lot before I bought it:
The thing is very unstable. On the highway I can just about induce a tank slapper by whacking the throttle. I've gone to pass someone and had the handlebars wiggling back and forth the whole way past (no point in letting off the gas, I figure). I feel like it calmed down a tiny bit with putting a smaller front fender on but I would like to calm it down a lot. I thought it was due to too little weight up front but it still wiggles even if I'm sitting way forward.
Any ideas what I can do with this thing to calm it down? I'm what you would call a tuning moron. So, even the most basic info might be useful.
Check to see if the tires are mounted correctly/balanced. If they are not balanced, you will feel it at speed. Colored dot on the tire mounted so it lines up with the stem? Street tires really need to be balanced. I would start there.
I had a highway crash in October on the 1090 induced by a tank slapper. I think I narrowed the cause down to a suspension issue, particularly the shock of all things. Once I reset the suspension to factory settings, almost all of that "too light in the front" feeling went away.
So my suggestion is set the suspension to factory settings and try from there. If that doesn't work, get the suspension tuned for you. If that doesn't work, then you need to whack the throttle more so it just picks the front wheel off the ground.
Ok, I'll check the balance on the tires. I definitely lined the dots up. I might not have balanced the rear... I haven't had a bike before that was sensitive to balancing. The tires that were on there to start behaved the same as the heidenaus I've got on there now. On a somewhat related note michelin doesn't seem to use dots on their tires any more (at least on the various pilot roads I've had).
I found the manual online so I'll get the thing back to factory suspension settings too. Original settings might be on the underside of the seat. I wonder where I put that... I have wondered if the rear was soft.
Sounds like the 1090 survived. Was that while commuting?
Putting smaller diameter wheels on a fork reduces trail. That makes the steering more 'nervous.' Notice the axle on the SMC version forks is inline w/ the fork vs out in front on the enduro fork to increase trail (on the SMC version.) The SMC triples are offset forward to counteract some of that but running a supermoto wheel on enduro forks will still result in less trail than the SMC. You can increase rake to help get some of the trail back. Theoretically, raise the forks as much as you can and increase the sag in the rear but I'm not sure how much of a difference you can actually make.
Check also the steering head bearings.
If they're worn - and they often are - that may worsen the problem.
I've never had a tank slapper, but from what i've read - loading the front end only makes it worse. So you better don't immediately let off the throttle or brake if you feel that it's going to get out of control.
I’m betting on smdub’s theory. That’s what I was going to suggest. My XRR came with an wheels and the front was way too low and twitchy with offroad sag numbers in the rear. I dropped the sag probably an inch more and it felt great.
Ok, looks like I've got some work to do. I'm not planning to sell the bike. I don't think I've had a street bike that I didn't put at least 45k miles on and I've still got a ways to go to get there with this one. That leaves out the 2 bikes that were stolen.
I tried out the easiest thing first. The fork caps are the only thing above the top triple now. I went out on the highway and that alone (10-15mm change) didn't feel too different. However, I did find that it was quite a bit different if I put my butt back as far as I can (I'm short with short arms so this isn't super far back). It was only a little bit wiggly there. Move my butt forward to the "natural" seating position and a sudden force on one side of the bars starts them oscillating. That would support the idea that the geometry is boogered up...
I found the seat and it does have the suspension settings on it. I assume standard would be where the settings were from the factory (it's also got "comfort" and "sport" settings). I'll set everything to standard next.
So, even the most basic info might be useful. 
I'm not planning to sell the bike. I don't think I've had a street bike that I didn't put at least 45k miles on and I've still got a ways to go to get there with this one. That leaves out the 2 bikes that were stolen. 