Mike, I have had experience with the Edelbrock
carb for past several years and have been very happy. They are easy to dial in. Start with the #7 needle in the middle of the adjustment range. Leave the "thumb" adjuster in the top of the carb, there is no reason to ever take it out. Not sure how easy the 450 is to reach the adjuster, but with my 400 I can stop and make quick adjustments on the trail if needed. If you follow the instructions from Edelbrock
about adjusting the needle, you can get it right on in a matter of minutes. You might still have a small closed throttle backfire, but not bad and it's easy to live with. Otherwise the carb is super easy to adjust for different conditions and never clogs a jet. The only complaint that I have is that every so often the float needle sticks in the "closed" position and won't fill the float bowl.
This happens after prolonged non-use and can usually be taken care of by tapping on the side of the float bowl with a stone or screwdriver handle. Good luck, the phone customer service at Edelbrock is also helpful. BTW, there is no "fuel screw", only the main needle and another adjustment for the pump delivery volume.
Edited by ncampion, April 02, 2011 - 04:31 PM.