Further to all the other advice, how are you sitting on the bike at those speeds? If you are upright and bracing yourself against the wind blast, it could be as simple as tension in your arms, plus a death grip on the bars.

Next time you try it, crouch into the wind to reduce wind pressure on you and reduce your grip on the bars. If that cures it, you know the source of the problem. Next thing is to address it. Let us know what you find!

 

I was going to say steering head bearing out of adjustment, but like Sporty Pig said at 4200 miles I don't see it beaing an issue. My bike had a very bad speed wobble and after I have the steering head bearing adjusted it was gone and has not come back, still could be the bearing? check the air PSI in the fornt tire first, look the tire over ans see if it's cupped or chopped up. Don't go fast till you get it fixed LOL!

 

So what exactly is the steering head bearing and how do I adjust it? I assume thats the bolt that bolts the fork to the frame? I tightened the bottom nut on that bolt a month or so back bc I had a bolt on my AC, Handlebars, and an motor mount rattle loose so I went around and gave most everything I could get a wrench on an extra crank.

There is a youtube video I watched that talks about a slotted nut on the underside of the upper triple tree that can be tightened or loosened...but it says that the wobble would be from it being too loose, not too tight....?

 

Adjusting the fall away is not something that can be easily explained on a forum like this. It's not all that difficult, but it is a bit complicated. There are upper and lower steering head bearings inside the triple tree.

I highly doubt that is your problem. I didn't adjust the fall away on my bike until after 50,000 miles.

 

are you running a fork brace, that should eliminate some of the wobble.

 

I seem to get that same wobble usually at 80+ and accelerating. Sometimes if i back off the throttle itll smooth out. My buddy has the same thing with his nightster. I can also get that wobble at lower speeds if im laid into the throttle but once again back off the it usually smooths out.

 

There is a very specific set of circumstances that needs to occur to set the wobble off. On any bike this set of parameters is different. Often simply changing ANYTHING on the bike will make it disappear. Anything can be as simple as a few pounds of air in the tires, suspension stiffness, new tires, a windshield raised/lowered/removed/added, or even a different helmet (no kidding) - it is a very very exact set of things that sets it off. That is why it is truly difficult to diagnose, and why you're likely to find a "cure" that may not have even been the cause, because it's a combination of things, you'll just fix one and suddenly that perfect equation no longer exists - problem gone.

The same thing happens on bicycles, believe it or not but most cyclists will never see it as you need generally to get above 45-50mph for it to occur. (my top speed down a mountain is 62mph, btw - and I passed a guy on a Harley, honest!). I have worked with bike engineers to solve the problem on a couple of different framesets and only one time were we not able to solve the problem, but each time none of the engineers were doing anything other than guessing, eventually they make the "right" guess.

My sporty had such a wobble with my apes on it above 80 mph, and it was exactly due to the 16" apes, the tall bars flex back and forth, inducing the same action in the wheel.

The only other thing I have to add is that dude in the video experienced his tank slap at 144 mph!!!! OHHHHHFFFFFUUUUGGGGGG!!!!

 

Your investigation should include checking the fall away. It is not that uncommon that they have been loose or worked loose from the factory.

Never ever neglect checking all the basics to maintain a bike in top condition.

I should be part of the 1K, 5K, 10K etc. services.

 

are you running apes?

 

What year is your bike, what kind? It looked like a Yamaha to me. Maybe I am missing something but regardless type I would start with tires you might have seperation of the belts. Good luck.