I've had my '98 Savage since September.  It's got about 9500mi and about 1000 of those are mine.

It's run great since I've had it.

Today it started to lose power on the top end.

Starts fine, idles fine, accellerates OK but only at part throttle.  If I open the throttle up it still runs OK, but actually looses a little power.

In 4th and 5th gear, this phenomonon keeps me from exceeding 50 or 52 mph.  

I can tell that the fuel is feeding better at partial throttle.  If I open the throttle a little further I can feel the power drop slightly.  If I back off a fraction I can tell the power picks up very slightly until the bike accelerates a mph or two and then it hits the wall again.

Any ideas what could be causing this?

Still, this condition is

I tried that.  No change.

There is a little rust in the tank.  Do you think I've clogged the carb?

If so, what would the fix be?

Perhaps you may have to go beyond the fuel tank and pull the carb to get whatever is blocking the high side of your throttle. Since you can ride fine at 35 mph but goes flat at 50. This is my opinion, I agree if you have gunk in the tank get it out; but I think the gunk has traveled farther. My suggestion is follow the advise of someone who does know a carb better than I do. Just my .02.

One of the problems I had with my bike is the internal fuel screen (inside the tank) was PARTIALLY clogged, it had gunk and varnish all over it.  I have a 1997 that had 600 miles on it when I bought it.  I just removed the fuel screen and installed an inline filter, along with the petc0ck mod, those two things solved all my fuel feed problems.

At any rate, you may be getting restricted fuel flow because of a clogged fuel screen.  If that's the case, you'd go WAY lean when you open the throttle to WOT, but could be getting enough fuel at part throttle.  If you verify the screen (or anything else) is not restricting your fuel flow.  You likely have some sort of carb problem.  I'd suspect a clogged main jet from the symptoms then, but you'd have to start tearing into the carb to verify/fix.

Same problem last month.

Not only clean it but polish it! Use silver polish paste on both slide and carb walls; cut marker & rotate slide 1/4 if too worn. Works like a charm!

good luck