Your engine was made on 14 May 1996. Being a more modern engine than I expected, it is subject to emissions controls and does not have a Flo-Jet carburetor. Here is the illustrated parts list:
http://bsintek.basco.com/BriggsDocumentDisplay/Z6hrABVJ1DajI.pdfThe carburetor appears to be a Walbro. The mixture screw you have been adjusting, Item 94, is the idle mixture adjustment. The main jet, Item 142, is not adjustable. If your problem is lean mixture, it seems it will be due to an induction leak or dirt or water in the carburetor rather than maladjustment. Because there is no main jet mixture adjustment, it makes sense to check other items before attributing the rough running to a carburetor fault. The first question is what you mean by "rough running". Mixture seldom becomes rich due to a fault except in the case of float bowl flooding, and this usually causes hard starting and idle problems, not problems at high speed only. Lean mixture, on the other hand, can easily occur at high speed only, but it is usually indicated by hunting, or cyclic variation in engine speed, when it is a mild case. In more severe cases there will be pronounced hesitation when you advance the speed control. There may also be misfiring, and in extreme cases, spitting through the carburetor (sometimes misdiagnosed as backfiring, which is an explosion in the muffler).
Your engine has Magnetron electronic ignition - there are no points to require adjustment. It is possible, but fairly unlikely, that the Magnetron is in the process of failing and is causing erratic ignition at high speed. A fouled or faulty spark plug might also cause high speed ignition failure, though they more often cause hard starting.
The next thing we need is a more specific description of the rough running you mentioned.
