poor handling at top speed

Here is a blurb from wickipedia about tank slappers, it might be helpful.

Wobble, shimmy, tank-slapper, speed wobble, and death wobble are all words and phrases used to describe a rapid (4–10 Hz) oscillation of primarily just the front end (front wheel, fork, and handlebars). The rest of the bike remains essentially unaffected. This instability occurs mostly at high speed and is similar to that experienced by shopping cart wheels, airplane landing gear, and automobile front wheels.[7][8] While wobble or shimmy can be easily remedied by adjusting speed, position, or grip on the handlebar, it can be fatal if left uncontrolled.[41] This AVI movie shows wobble.

Wobble or shimmy begins when some otherwise minor irregularity, such as fork asymmetry,[42] accelerates the wheel to one side. The restoring force is applied in phase with the progress of the irregularity, and the wheel turns to the other side where the process is repeated. If there is insufficient damping in the steering the oscillation will increase until system failure occurs. The oscillation frequency can be changed by changing the forward speed, making the bike stiffer or lighter, or increasing the stiffness of the steering, of which the rider is a main component.[11]
 
Junster, you might be on to something about the fork. I will first try regular tubes to see if that fixes the problem. And as for the sprocket, I forgot to mention I am using andy's top hat adapter mounting the sprocket directly to the hub where the disc rotor would mount. So the sprocket is not on the spokes and it is true.

The only place I compromised on this build is the fork. It is a suntour low end fork. It is 90mm travel which is minimal. It seems pretty solid. Next time I go out when the shuttering starts I will glance at the fork.

BTW I still have the original rigid fork that came with it if need be. I put the suspension fork on since I plan on doing quite a bit of off roading. (I bought a 38T sprocket for the road and a 54T for off road)

Thanks for your help guys!
 
Toe, that is great info. Maybe I should tighten the head set to increase the stiffness of the steering or replace the fork. I am sitting here trying to think of what could be asymetrical and the only thing I can think of is the wheel could be "dished" a little to one side. I will check that too.
Thanks for your help.
 
A short travel fork is better than long if the bike wasn't designed for a fork. Please try putting the original non suspension fork back on and I think your problem will be over. Push down on the front and look at the angle of the neck change. As the fork colaspes the neck angle gets straighter up and down. That removes the trail of the front wheel and makes the steering to quick. Truck and cars went to tubeless because of heat buildup. Same as 10 ply tires. They aren't 10 plys anymore they are "10 ply rated" they really use 6 plys of thicker cords to get the same effect of the original 10 plys. I don't really know if at a bikes weight and speed it really makes any difference. Slime was invented to balance motorcycle tires. The flat protection was secondary. The problem I see in a bicycle tire is the size around of the tire and constant speed changes. There's no doubt many are making slime work for them. I get no flats either. Everybody should use what works for them. I run all on road, on pavement. For me using the best tires I can find on good wheels with regular tubes is enough to not have any flats. I agree with the benefits of slime just not in a bicyle you want to keep up with traffic on. I have to ride in the road with cars going 45+ in a 35 zone. I need all the speed and quickness I can get. The wheel dishing shouldn't make any difference. With only 2 wheels they will track the same as long as the front and rear tires have the same profile.
 
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That sprocket combo should be great for on then off roading. Maybe consider a second rear wheel with a offroad tire and the bigger sprocket to be a no pain changeover.
 
The chain will probably have to be changed also for the 38t-54t gears. I don't think the tensioner will take up that much slack.
 
I bought 10ft of #41 chain, thanks civilized. So here is an update. I didn't know the fork had it because it was so cheap but it has a control for stiffening and softening the fork. It was on a very soft setting. I put it on the firmest setting and went for a ride. The problem was much less noticable. So it looks like the problem is the fork. I mostly plan on riding at slow speeds off road so I might leave it but I am not sure.

I wonder if anyone else has this problem with suspension forks?

I have a brand new rockshox dart 3 fork laying around with a lock out feature. With the flip of a switch it becomes rigid. I was saving for a non motorized bike but I might pull it off the shelf for this bike. For on road high speeds lock it out and have nice suspension for slow off roading.
 
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