I just read your "101 Honda CRF Tips" (February 2006) and was confused about jetting recommendations for the '05 CRF 450X. My California bike came with a 45 pilot jet and a 142 main jet and those specs are confirmed in the owner's manual and shop manual, as well as what is actually in the bike. What should I use with an FMF Q exhaust and slight airbox mods?
Our recommended "best" jetting is correct in that story. And yes, it is leaner than stock. The one mistake is that the California and 49-state bike have the same jetting with the NCVU needle. Upon further investigation we found that the leaner setting was snappier and more crisp off idle for "higher skilled" riders, meaning ones that were pretty precise on the throttle pull. We've now found out it is a little too much for most guys, because if you back off the throttle and then get back on it, on a hill for instance, the leaner setting is much more likely to cough-stall. But I personally like this setting especially when riding at elevation and for the superior fuel mileage.
We asked a few riders who were happy with their setting and it seems the best, what we call "regular guy" jetting, is all stock brass with the NCYS needle in the 3rd position. You can be sloppy with the throttle and the bike is as reluctant to coughing as we've seen. It also seems to be smoother for the non-expert rider as well.
This jetting works with stock or aftermarket quiet mufflers. All of our jetting is done with stock air boxes for a number of reasons relating to sound and keeping the filter cleaner longer.
Where you've totally gone beyond our skill level is by cutting the air box. We haven't seen the need to do this on any of our X's and for this modification you'll need to consult higher sources on internet discussion boards.