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I don't know why they made it adjustable, but there is a very specific procedure published in Field Service Bulletin, #234, November 1955, for the Model '80', but no otherws. It basically explains that you will balance the cam gear position so as to provide identical lift between #1 and #2 exhaust valves, but it is several paragraphs long. You might want to purchase the FSB CD-ROM from Duane Larson's estate, as there nearly 2,000 pages of interesting information. And since it is copyrighted, I can't print it for you here.
Here's a link to order a CD:http://www.jd2cylservice.com/