As you all know I have a ZR900 with Black Magic twins and am currently installing tempa flow. BM recomends 400 mains at 30 deg f. Tempa flow says to jet for the lowest temp and altitude you ride at. The lowest altitude is 500 feet and the coldest temp is -30 deg f.......what do you guys recomend for mains.........BM told me 420's but that seems a bit scary but if thats what you guys think also than I will go with it. Thanks in advance Dave

Sled Head Stock are 430's so 420 doesn't seem that bad, talk to BM again and tell them your concerns and get them to back it up!

For every 20 degrees in temp change=1 jet size. :hallo1:

there are a lot of things that make a diff when jetting, not just temp. like elevation, relative humidity and such. if i were you, i would put the bigger jets in first, see how that goes(watch the plugs and piston wash) and then go from there rich engines don't burn down and check them often, especailly with pipes, also seeing as how this is a trail sled, make sure you keep an eye on the mid-range jetting, you can burn an engine down just as quick even at half throttle