Personally I would leave the jetting alone, it just aint worth the agony for toasted pistons for a mear gain in performance.
If it's like 20 degree's out you could drop 1 and be safe. If your looking for better mileage drop the needle 1. I have allways found this a better gain [performance and mileage] than a jet size and still good for about 25 below. Think about where your run most of the time, heck, the MachZ has so much power you only need 1/4 throttle to do 80!
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