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+1 for makemdan's gearing suggestion. There are lots of posts/threads on the forum debating the pros and cons of smaller front versus larger rear sprocket. I've got the 14 tooth front but will try 15/42 when I need to replace my chain.
One thing to watch with the larger rear sprocket is setting chain tension. With the Evo, (and, I think, all the single-sided swinging arm models), the suggested 61mm - 63mm is not the amount of up-and-down chain movement, it is the clearance between the chain run and the underside of the swinging arm.
Consequently, a larger rear sprocket will have the effect of moving the chain further from the swinging arm so, if you use the original 61mm - 63mm gap, the chain will be overtight so bear that in mind when tensioning. I don't know what the relative diameters of the 39 and 42 tooth sprockets are so I can't work out what the new clearance should be but I'd think another 5mm or so?
The best way to determine the new correct clearance, (and something you can do, Ian, if you're changing your shock), is, with the shock removed, pull the swinging arm up until the front and rear sprockets and swinging arm pivot are all in line. (It's at this point that the chain will be at its tightest.) If you set the chain tension in this position, then at any other point in the swinging arm's travel, the chain will be slacker.
Then, with the bike back together and on the side stand, you can measure the new clearance and use that as your reference for future chain adjustment.
If you do that, Ian, do let us know what the figure is, as it'll be useful for anyone else running the 15/42 combination.
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